Sunday, July 12, 2009
Thursday, July 9, 2009
Wednesday, July 8, 2009
Chiller Controls
Chiller Controls
START-UP CONTROL
There are two basic methods used for controlling the start-up of a chilled water
system:
1. The most common method is “manual” initiation. With this method, the
facility operating staff makes the decision to start the system on the
basis of the time of year, outdoor temperature, and/or the number of
“hot” complaints received from the building occupants. This method is
widely used in northern climates that have more distinctly separate
heating and cooling seasons.
2. Outdoor temperature can be the trigger for starting a chilled water
system, particularly if air side economizer systems are used in the
facility. With economizers, outdoor air will satisfy the cooling
requirement with the temperature is at or below the air-handling units’
discharge temperature set point, usually about 558F. This temperature,
then, is set as the “change-over” temperature and the chilled water
system is started with the outdoor temperature rises above this set point.
Starting a chilled water system means, first of all, starting the chilled water
distribution pumps. After that, the chiller will start under its internal controls if
the chilled water supply temperature is above its set point condition. First, a
water-cooled chiller will start its condenser water pump. Then, if flow of both
chilled water and condenser water is “proven” by flow switches (preferably,
differential pressure type), the chiller will start.
Aside from the flow switches, every chiller is equipped with basic
operating safety controls to protect the refrigeration machine from damage:
1. Low refrigerant temperature
2. Low chilled water temperature
3. High condensing pressure
4. Low oil pressure
In the event that any of these conditions exist, the chiller will shut down and
require manual investigation and reset before operation can resume.
For rotary compressor chillers, there are typically two additional safety
cutouts:
1. High motor winding temperature
2. Motor overload (high amps)
CAPACITY CONTROL
Modern water chillers are normally provided with digital electronic controls
designed and integrated by the chiller manufacturer. In addition to the operating
safety elements outlined in the Section 4.1, these controls are used to provide
capacity control based on maintaining a supply chilled water temperature
set point.
Refrigerant Flow Control
For the vast majority of water chillers, capacity control means controlling the
refrigerant flow rate through the evaporator. Depending on the type of
compressor used, several methods are applied:
1. Reciprocating Compressors—Primary refrigerant flow is controlled by
the expansion valve that responds to the temperature of the refrigerant
gas leaving the evaporator to maintain set point. The throttling action of
the expansion valve causes a pressure increase in the system and the
compressor must then reduce its refrigerant flow rate to maintain its
discharge pressure set point.
The most common methods of reducing refrigerant flow rate and
capacity in reciprocating compressors are by opening suction valves,
bypassing refrigerant gas within the compressor, or bypassing
refrigerant gas flow externally to the compressor.
With the first method, called unloading, external actuators can
hold the suction valves, on one or more compressor cylinders, open.
Thus, no compression can take place in these cylinders and the gas flow
rate through them is reduced to zero.
With the other approach, called hot gas bypass, a solenoid valve
on the high pressure discharge on one or more cylinders can open and
divert the refrigerant flow back to the suction side of the cylinder. This
effectively reduces the pressure differential or “lift” produced by the
cylinder and reduces the refrigerant gas flow rate from the evaporator.
Unloading occurs in distinct steps based on the number of
cylinders in each compressor and number of compressors in each
chiller. For a four-cylinder compressor, there are four stages (or steps)
of capacity control, 25–50–75–100%. If there are two compressors in
the machine, this yields a total of eight steps of capacity control.
Obviously, the greater the number of compressors and the greater the
number of cylinders in each compressor, the “smoother” the capacity
control line.
2. Rotary Screw Compressors—Since the rotary screw compressor is a
positive displacement compressor, suction throttling can reduce the
refrigerant gas flow into the compressor. A modulating control method
is desirable in order to produce essentially infinite capacity adjustment
between the minimum and maximum flow rates and capacities.
A slide valve is a hot gas bypass control valve with a sliding action
arranged parallel to the rotor bores and located at the high pressure
discharge of the compressor. The valve is then modulated to return a
variable portion of the discharge gas back to the compressor suction.
This valve, in addition to controlling capacity, also adjusts the location
of the compressor discharge port as the load changes. This “axial
discharge port” then provides good part load performance without
reducing full-load efficiency.
3. Centrifugal Compressors—Refrigerant gas flow into a centrifugal
compressor can be controlled by adjustable inlet guide vanes, or
pigswill or peroration vanes, just as with a centrifugal fan. These
vanes are arranged radially at the inlet to the compressor impeller and
can be opened and closed by an external operator.
Since each vane rotates around an axial shaft, they affect the
direction of the flow entering the impeller. When the inlet vanes are
fully open, gas enters the impeller at 908 to the impeller. However, as
the inlet vanes begin to close, flow enters the impeller at an increasing
angle in the direction of the radial flow along the impeller blades. This
“pigswill” condition reduces the ability of the impeller to impart
kinetic energy to the refrigerant gas, thus reducing the flow rate.
Inlet vanes do not produce a pressure drop or “throttling” to
reduce refrigerant flow through the centrifugal compressor.
A minimum volumetric rate flow through a centrifugal
compressor is required for stable operation. If the volumetric flow
rates falls below this minimum, the compressor will become unstable
and surge. When this happens, the refrigerant flows alternatively
backwards and forwards through the compressor, producing noise and
poor operation. Extended operation under surge conditions will cause
mechanical damage to the compressor. The surge envelope will vary
from compressor to compressor but usually occurs when the volumetric
flow rate is reduced by 40–60%.
To prevent surge from occurring, internal hot gas bypass may be
used to allow capacity to be reduced while maintaining sufficient gas
flow through the compressor. (This condition, along with increasing
wind age losses and motor inefficiencies, accounts for the part load
performance characteristics.
In recent years, capacity control of centrifugal chillers by speed
control has been applied. Here, a large variable frequency drive is
applied to the chiller motor and the motor speed modulated to control
capacity. Generally, speed control improves efficiency over inlet vane
control down to about 55% of rated capacity, whole inlet vane control
is more efficient below 55% of rated capacity.
Compressor speed is directly related to capacity, but the pressure
(lift) produced by the compressor is a function of the square of the
speed. This may produce unsatisfactory operation and surge, requiring
the use of hot gas bypass, as the chiller unloads under speed control.
Adding speed control significantly increases the price of the
chiller and this option must be carefully evaluated to determine if this.
START-UP CONTROL
There are two basic methods used for controlling the start-up of a chilled water
system:
1. The most common method is “manual” initiation. With this method, the
facility operating staff makes the decision to start the system on the
basis of the time of year, outdoor temperature, and/or the number of
“hot” complaints received from the building occupants. This method is
widely used in northern climates that have more distinctly separate
heating and cooling seasons.
2. Outdoor temperature can be the trigger for starting a chilled water
system, particularly if air side economizer systems are used in the
facility. With economizers, outdoor air will satisfy the cooling
requirement with the temperature is at or below the air-handling units’
discharge temperature set point, usually about 558F. This temperature,
then, is set as the “change-over” temperature and the chilled water
system is started with the outdoor temperature rises above this set point.
Starting a chilled water system means, first of all, starting the chilled water
distribution pumps. After that, the chiller will start under its internal controls if
the chilled water supply temperature is above its set point condition. First, a
water-cooled chiller will start its condenser water pump. Then, if flow of both
chilled water and condenser water is “proven” by flow switches (preferably,
differential pressure type), the chiller will start.
Aside from the flow switches, every chiller is equipped with basic
operating safety controls to protect the refrigeration machine from damage:
1. Low refrigerant temperature
2. Low chilled water temperature
3. High condensing pressure
4. Low oil pressure
In the event that any of these conditions exist, the chiller will shut down and
require manual investigation and reset before operation can resume.
For rotary compressor chillers, there are typically two additional safety
cutouts:
1. High motor winding temperature
2. Motor overload (high amps)
CAPACITY CONTROL
Modern water chillers are normally provided with digital electronic controls
designed and integrated by the chiller manufacturer. In addition to the operating
safety elements outlined in the Section 4.1, these controls are used to provide
capacity control based on maintaining a supply chilled water temperature
set point.
Refrigerant Flow Control
For the vast majority of water chillers, capacity control means controlling the
refrigerant flow rate through the evaporator. Depending on the type of
compressor used, several methods are applied:
1. Reciprocating Compressors—Primary refrigerant flow is controlled by
the expansion valve that responds to the temperature of the refrigerant
gas leaving the evaporator to maintain set point. The throttling action of
the expansion valve causes a pressure increase in the system and the
compressor must then reduce its refrigerant flow rate to maintain its
discharge pressure set point.
The most common methods of reducing refrigerant flow rate and
capacity in reciprocating compressors are by opening suction valves,
bypassing refrigerant gas within the compressor, or bypassing
refrigerant gas flow externally to the compressor.
With the first method, called unloading, external actuators can
hold the suction valves, on one or more compressor cylinders, open.
Thus, no compression can take place in these cylinders and the gas flow
rate through them is reduced to zero.
With the other approach, called hot gas bypass, a solenoid valve
on the high pressure discharge on one or more cylinders can open and
divert the refrigerant flow back to the suction side of the cylinder. This
effectively reduces the pressure differential or “lift” produced by the
cylinder and reduces the refrigerant gas flow rate from the evaporator.
Unloading occurs in distinct steps based on the number of
cylinders in each compressor and number of compressors in each
chiller. For a four-cylinder compressor, there are four stages (or steps)
of capacity control, 25–50–75–100%. If there are two compressors in
the machine, this yields a total of eight steps of capacity control.
Obviously, the greater the number of compressors and the greater the
number of cylinders in each compressor, the “smoother” the capacity
control line.
2. Rotary Screw Compressors—Since the rotary screw compressor is a
positive displacement compressor, suction throttling can reduce the
refrigerant gas flow into the compressor. A modulating control method
is desirable in order to produce essentially infinite capacity adjustment
between the minimum and maximum flow rates and capacities.
A slide valve is a hot gas bypass control valve with a sliding action
arranged parallel to the rotor bores and located at the high pressure
discharge of the compressor. The valve is then modulated to return a
variable portion of the discharge gas back to the compressor suction.
This valve, in addition to controlling capacity, also adjusts the location
of the compressor discharge port as the load changes. This “axial
discharge port” then provides good part load performance without
reducing full-load efficiency.
3. Centrifugal Compressors—Refrigerant gas flow into a centrifugal
compressor can be controlled by adjustable inlet guide vanes, or
pigswill or peroration vanes, just as with a centrifugal fan. These
vanes are arranged radially at the inlet to the compressor impeller and
can be opened and closed by an external operator.
Since each vane rotates around an axial shaft, they affect the
direction of the flow entering the impeller. When the inlet vanes are
fully open, gas enters the impeller at 908 to the impeller. However, as
the inlet vanes begin to close, flow enters the impeller at an increasing
angle in the direction of the radial flow along the impeller blades. This
“pigswill” condition reduces the ability of the impeller to impart
kinetic energy to the refrigerant gas, thus reducing the flow rate.
Inlet vanes do not produce a pressure drop or “throttling” to
reduce refrigerant flow through the centrifugal compressor.
A minimum volumetric rate flow through a centrifugal
compressor is required for stable operation. If the volumetric flow
rates falls below this minimum, the compressor will become unstable
and surge. When this happens, the refrigerant flows alternatively
backwards and forwards through the compressor, producing noise and
poor operation. Extended operation under surge conditions will cause
mechanical damage to the compressor. The surge envelope will vary
from compressor to compressor but usually occurs when the volumetric
flow rate is reduced by 40–60%.
To prevent surge from occurring, internal hot gas bypass may be
used to allow capacity to be reduced while maintaining sufficient gas
flow through the compressor. (This condition, along with increasing
wind age losses and motor inefficiencies, accounts for the part load
performance characteristics.
In recent years, capacity control of centrifugal chillers by speed
control has been applied. Here, a large variable frequency drive is
applied to the chiller motor and the motor speed modulated to control
capacity. Generally, speed control improves efficiency over inlet vane
control down to about 55% of rated capacity, whole inlet vane control
is more efficient below 55% of rated capacity.
Compressor speed is directly related to capacity, but the pressure
(lift) produced by the compressor is a function of the square of the
speed. This may produce unsatisfactory operation and surge, requiring
the use of hot gas bypass, as the chiller unloads under speed control.
Adding speed control significantly increases the price of the
chiller and this option must be carefully evaluated to determine if this.
Tuesday, May 19, 2009
VAPOR COMPRESSION CYCLE CHILLERS
VAPOR COMPRESSION CYCLE CHILLERS
As introduced in Section , a secondary refrigerant is a substance that does not
change phase as it absorbs heat. The most common secondary refrigerant is water
and chilled water is used extensively in larger commercial, institutional, and
industrial facilities to make cooling available over a large area without
introducing a plethora of individual compressor systems. Chilled water has the
advantage that fully modulating control can be applied and, thus, closer
temperature tolerances can be maintained under almost any load condition.
For very low temperature applications, such as ice rinks, an antifreeze
component, most often ethylene or propylene glycol, is mixed with the water and
the term brine (left over from the days when salt was used as antifreeze) is used to
describe the secondary refrigerant.
In the HVAC industry, the refrigeration machine that produces chilled
water is generally referred to as a chiller and consists of the compressor(s),
evaporator, and condenser, all packaged as a single unit. The condensing medium
may be water or outdoor air.
The evaporator, called the cooler, consists of a shell-and-tube heat
exchanger with refrigerant in the shell and water in the tubes. Coolers are
designed for 3–11 fps water velocities when the chilled water flow rate is
selected for a 10–208F range.
For air-cooled chillers, the condenser consists of an air-to-refrigerant heat
exchanger and fans to provide the proper flow rate of outdoor air to transfer the
heat rejected by the refrigerant.
For water-cooled chillers, the condenser is a second shell-and-tube heat
exchanger with refrigerant in the shell and condenser water in the tubes.
Condenser water is typically supplied at 70–858F and the flow rate is selected for
a 10–158F range. A cooling tower is typically utilized to provide condenser water
cooling, but other cool water sources such as wells, ponds, and so on, can be used.
Thursday, April 16, 2009
Indoor environmental quality
Indoor environmental quality:
From the occupant point of view, the ideal situation is an indoor environment
that satisfies all occupants (i.e. they have no complaints) and does not
unnecessarily increase the risk or severity of illness or injury. Both the satisfaction
of people (comfort) and health status are influenced by numerous
factors: general well-being, mental drive, job satisfaction, technical competence,
career achievements, home/work interface, relationship with others,
personal circumstances, organizational matters, etc. and last but not least
environmental factors, such as
• IAQ: comprising odour, indoor air pollution, fresh air supply, etc.;
• thermal comfort: moisture, air velocity, temperature;
• acoustical quality: noise from outside, indoors, vibrations;
• visual or lighting quality: view, illuminance, luminance ratios, reflection;
• aesthetic quality.
Although there is rich scientific literature and the reports of several
national experiences on this subject, a uniform set of criteria for the countries
of Europe has not yet been defined.
Currently, in several standards and guidelines, human indoor environmental
requirements for spaces are expressed by physical and chemical
indicators (temperature, Decibel, Lux, CO concentration, etc.) (CEN, 1998;
ASHRAE, 2004a; ISO, 2005). Although required levels in those standards
and guidelines are met, it can be concluded from several studies that the
IEQ as experienced by occupants is not always acceptable and sometimes
is even unhealthy, causing health and comfort problems (Bluyssen et al.,
1995). This mismatch is due to several reasons:
• the relationship between objective measurement and human assessment
is not known for all physical/chemical parameters. No consensus model
is available for air quality. For light, recent findings show that brightness
of the surroundings is the key element and not only the illuminance
(Light & Health Research Foundation, 2002).
• even if established models for separate subjective issues exist [e.g. thermal
comfort (Fanger, 1972) and noise], the holistic effects of all separate
physical/chemical factors are still largely unknown.
Besides the physical/chemical indicators, other indicators are being
used such as the percentage of dissatisfied occupants, productivity numbers
(Clements-Croome, 2002), sick leave, estimated life expectations
(Carrothers et al., 1999) and even the number of deaths related to a BRI.
However, the determination and use of these indicators has not been documented
in guidelines or standards.
In several spaces (cars, space industry and buildings), health complaints
and comfort problems are strongly related to the available methods of ventilation.
This relation has been shown, for example, by the increased risk of
infectious disease transmission (recirculated air), sources in heating, ventilating
and air-conditioning (HVAC) systems causing an overall distribution
of unwanted pollutants (Bluyssen et al., 2003), and stagnant zones and
draught (insufficient ventilation effectiveness). Complaints are in general
related to air quality, thermal comfort and noise parameters.
From the occupant point of view, the ideal situation is an indoor environment
that satisfies all occupants (i.e. they have no complaints) and does not
unnecessarily increase the risk or severity of illness or injury. Both the satisfaction
of people (comfort) and health status are influenced by numerous
factors: general well-being, mental drive, job satisfaction, technical competence,
career achievements, home/work interface, relationship with others,
personal circumstances, organizational matters, etc. and last but not least
environmental factors, such as
• IAQ: comprising odour, indoor air pollution, fresh air supply, etc.;
• thermal comfort: moisture, air velocity, temperature;
• acoustical quality: noise from outside, indoors, vibrations;
• visual or lighting quality: view, illuminance, luminance ratios, reflection;
• aesthetic quality.
Although there is rich scientific literature and the reports of several
national experiences on this subject, a uniform set of criteria for the countries
of Europe has not yet been defined.
Currently, in several standards and guidelines, human indoor environmental
requirements for spaces are expressed by physical and chemical
indicators (temperature, Decibel, Lux, CO concentration, etc.) (CEN, 1998;
ASHRAE, 2004a; ISO, 2005). Although required levels in those standards
and guidelines are met, it can be concluded from several studies that the
IEQ as experienced by occupants is not always acceptable and sometimes
is even unhealthy, causing health and comfort problems (Bluyssen et al.,
1995). This mismatch is due to several reasons:
• the relationship between objective measurement and human assessment
is not known for all physical/chemical parameters. No consensus model
is available for air quality. For light, recent findings show that brightness
of the surroundings is the key element and not only the illuminance
(Light & Health Research Foundation, 2002).
• even if established models for separate subjective issues exist [e.g. thermal
comfort (Fanger, 1972) and noise], the holistic effects of all separate
physical/chemical factors are still largely unknown.
Besides the physical/chemical indicators, other indicators are being
used such as the percentage of dissatisfied occupants, productivity numbers
(Clements-Croome, 2002), sick leave, estimated life expectations
(Carrothers et al., 1999) and even the number of deaths related to a BRI.
However, the determination and use of these indicators has not been documented
in guidelines or standards.
In several spaces (cars, space industry and buildings), health complaints
and comfort problems are strongly related to the available methods of ventilation.
This relation has been shown, for example, by the increased risk of
infectious disease transmission (recirculated air), sources in heating, ventilating
and air-conditioning (HVAC) systems causing an overall distribution
of unwanted pollutants (Bluyssen et al., 2003), and stagnant zones and
draught (insufficient ventilation effectiveness). Complaints are in general
related to air quality, thermal comfort and noise parameters.
Tuesday, April 14, 2009
ASHRAE Comfort Zones
ASHRAE Comfort Zones
Based on results of research conducted at Kansas State University and at other institutions,
ANSI/ASHRAE Standard 55-1992 specified winter and summer comfort zones to provide for the
selection of the indoor parameters for thermal comfort . This chart is based upon an
occupant activity level of 1.2 met (69.8 W/m2). For summer, typical clothing insulation is 0.5 clo,
that is, light slacks and short-sleeve shirt or comparable ensemble; there is no minimum air speed
that is necessary for thermal comfort. Standard 55-1992 recommended a summer comfort zone with
an effective boundary temperature ET* 73 to 79°F (22.5 to 26°C) at 68°F (20°C) wet-bulb as its
upper-slanting boundary and dew-point temperature 36°F (2.2°C) as its bottom flat boundary. If the
clothing insulation is 0.1 clo higher, the boundary temperatures both should be shifted 1°F (0.6°C)
lower. Rohles et al. (1974) and Spain (1986) suggested that the upper boundary of the summer
comfort zone can be extended to 85 or 86°F (30°C) ET* if the air velocity of the indoor air
can be increased to 200 fpm (1 m/ s) by a ceiling fan or other means.
The winter comfort zone is based upon a 0.9-clo insulation including heavy slacks, long-sleeve
shirt, and sweater or jacket at an air velocity of less than 30 fpm (0.15 m/s). Standard 55-1992 recommended
a winter comfort zone with an effective boundary temperature ET* 68 to 74°F (20 to
23.3°C) at 64°F (17.8°C) wet-bulb as its slanting upper boundary and at dew-point 36°F (2.2°C) as
its bottom flat boundary.
Indoor air parameters should be fairly uniform in order to avoid local discomfort. According to
Holzle et al., 75 to 89 percent of the subjects tested found the environment within this summer
comfort zone to be thermally acceptable.
ASHRAE comfort zones recommend only the optimal and boundary ET* for the determination
of the winter and summer indoor parameters. For clothing insulation, activity levels, and indoor air
velocities close to the values specified in Standard 55-1992, a wide range of indoor design conditions
are available.
Based on results of research conducted at Kansas State University and at other institutions,
ANSI/ASHRAE Standard 55-1992 specified winter and summer comfort zones to provide for the
selection of the indoor parameters for thermal comfort . This chart is based upon an
occupant activity level of 1.2 met (69.8 W/m2). For summer, typical clothing insulation is 0.5 clo,
that is, light slacks and short-sleeve shirt or comparable ensemble; there is no minimum air speed
that is necessary for thermal comfort. Standard 55-1992 recommended a summer comfort zone with
an effective boundary temperature ET* 73 to 79°F (22.5 to 26°C) at 68°F (20°C) wet-bulb as its
upper-slanting boundary and dew-point temperature 36°F (2.2°C) as its bottom flat boundary. If the
clothing insulation is 0.1 clo higher, the boundary temperatures both should be shifted 1°F (0.6°C)
lower. Rohles et al. (1974) and Spain (1986) suggested that the upper boundary of the summer
comfort zone can be extended to 85 or 86°F (30°C) ET* if the air velocity of the indoor air
can be increased to 200 fpm (1 m/ s) by a ceiling fan or other means.
The winter comfort zone is based upon a 0.9-clo insulation including heavy slacks, long-sleeve
shirt, and sweater or jacket at an air velocity of less than 30 fpm (0.15 m/s). Standard 55-1992 recommended
a winter comfort zone with an effective boundary temperature ET* 68 to 74°F (20 to
23.3°C) at 64°F (17.8°C) wet-bulb as its slanting upper boundary and at dew-point 36°F (2.2°C) as
its bottom flat boundary.
Indoor air parameters should be fairly uniform in order to avoid local discomfort. According to
Holzle et al., 75 to 89 percent of the subjects tested found the environment within this summer
comfort zone to be thermally acceptable.
ASHRAE comfort zones recommend only the optimal and boundary ET* for the determination
of the winter and summer indoor parameters. For clothing insulation, activity levels, and indoor air
velocities close to the values specified in Standard 55-1992, a wide range of indoor design conditions
are available.
INDOOR DESIGN CONDITIONS
INDOOR DESIGN CONDITIONS
Indoor design parameters are those that the air conditioning system influences directly to produce a
required conditioned indoor environment in buildings. They are shown below and grouped as follows:
1. Basic design parameters
_ Indoor air temperature and air movements
_ Indoor relative humidity
2. Indoor air quality
_ Air contaminants
_ Outdoor ventilation rate provided
_ Air cleanliness for processing
3. Specific design parameters
_ Sound level
_ Pressure differential between the space and surroundings
The indoor design parameters to be maintained in an air conditioned space are specified in the
design document and become the targets to be achieved during operation. In specifying the indoor
design parameters, the following points need to be considered:
1. Not all the parameters already mentioned need to be specified in every design project. Except
for the indoor air temperature which is always an indoor design parameter in comfort air conditioning,
it is necessary to specify only the parameters which are essential to the particular situation
concerned.
2. Even for process air conditioning systems, the thermal comfort of the workers should also be
considered. Therefore, the indoor design parameters regarding health and thermal comfort for the
occupants form the basis of design criteria.
3. When one is specifying indoor design parameters, economic strategies of initial investment
and energy consumption of the HVAC&R systems must be carefully investigated. Design criteria
should not be set too high or too low. If the design criteria are too high, the result will be an excessively
high investment and energy cost. Design criteria that are too low may produce a poor indoor
air quality, resulting in complaints from the occupants, causing low-quality products, and possibly
leading to expensive system alternations.
4. Each specified indoor design parameter is usually associated with a tolerance indicated
as a _ sign, or as an upper or lower limit. Sometimes there is a traditional tolerance understood
by both the designers and the owners of the building. For instance, although the summer
indoor design temperature of a comfort air conditioning system is specified at 75 or 78°F
(23.9 or 25.6°C), in practice a tolerance of _2–3°F (_1.1 – 1.7°C) is often considered
acceptable.
5. In process air conditioning systems, sometimes a stable indoor environment is more important
than the absolute value of the indoor parameter to be maintained. For example, it may not be necessary
to maintain 68°F (20°C) for all areas in precision machinery manufacturing. More often, a
72°F (22.2°C) or even a still higher indoor temperature with appropriate tolerance will be more
suitable and economical.
AIR CONDITIONING
AIR CONDITIONING
Air conditioning is a combined process that performs many functions simultaneously. It conditions
the air, transports it, and introduces it to the conditioned space. It provides heating and cooling from
its central plant or rooftop units. It also controls and maintains the temperature, humidity, air
movement, air cleanliness, sound level, and pressure differential in a space within predetermined
The term HVAC&R is an abbreviation of heating, ventilating, air conditioning, and refrigerating.
The combination of processes in this commonly adopted term is equivalent to the current definition
of air conditioning. Because all these individual component processes were developed prior to the
more complete concept of air conditioning, the term HVAC&R is often used by the industry.
COMFORT AND PROCESSING AIR CONDITIONING
SYSTEMS
Air Conditioning Systems
An air conditioning, or HVAC&R, system is composed of components and equipment arranged in
sequence to condition the air, to transport it to the conditioned space, and to control the indoor environmental
parameters of a specific space within required limits.
Most air conditioning systems perform the following functions:
1. Provide the cooling and heating energy required
2. Condition the supply air, that is, heat or cool, humidify or dehumidify, clean and purify, and
attenuate any objectionable noise produced by the HVAC&R equipment
3. Distribute the conditioned air, containing sufficient outdoor air, to the conditioned space
4. Control and maintain the indoor environmental parameters–such as temperature, humidity,
cleanliness, air movement, sound level, and pressure differential between the conditioned space
and surroundings—within predetermined limits
Parameters such as the size and the occupancy of the conditioned space, the indoor environmental
parameters to be controlled, the quality and the effectiveness of control, and the cost involved determine
the various types and arrangements of components used to provide appropriate characteristics.
Air conditioning systems can be classified according to their applications as (1) comfort air
conditioning systems and (2) process air conditioning systems.
Comfort Air Conditioning Systems
Comfort air conditioning systems provide occupants with a comfortable and healthy indoor environment
in which to carry out their activities. The various sectors of the economy using comfort air
conditioning systems are as follows:
1. The commercial sector includes office buildings, supermarkets, department stores, shopping
centers, restaurants, and others. Many high-rise office buildings, including such structures as the
World Trade Center in New York City and the Sears Tower in Chicago, use complicated air conditioning
systems to satisfy multiple-tenant requirements. In light commercial buildings, the air conditioning
system serves the conditioned space of only a single-zone or comparatively smaller area.
For shopping malls and restaurants, air conditioning is necessary to attract customers.
2. The institutional sector includes such applications as schools, colleges, universities, libraries,
museums, indoor stadiums, cinemas, theaters, concert halls, and recreation centers. For example,
one of the large indoor stadiums, the Superdome in New Orleans, Louisiana, can seat 78,000 people.
3. The residential and lodging sector consists of hotels, motels, apartment houses, and private
homes. Many systems serving the lodging industry and apartment houses are operated continuously,
on a 24-hour, 7-day-a-week schedule, since they can be occupied at any time.
4. The health care sector encompasses hospitals, nursing homes, and convalescent care facilities.
Special air filters are generally used in hospitals to remove bacteria and particulates of submicrometer
size from areas such as operating rooms, nurseries, and intensive care units. The relative humidity in a
general clinical area is often maintained at a minimum of 30 percent in winter.
5. The transportation sector includes aircraft, automobiles, railroad cars, buses, and cruising
ships. Passengers increasingly demand ease and environmental comfort, especially for longdistance
travel. Modern airplanes flying at high altitudes may require a pressure differential of
about 5 psi between the cabin and the outside atmosphere. According to the Commercial Buildings
Characteristics (1994), in 1992 in the United States, among 4,806,000 commercial buildings having
67.876 billion ft2 (6.31 billion m2) of floor area, 84.0 percent were cooled, and 91.3 percent
were heated.
Process Air Conditioning Systems
Process air conditioning systems provide needed indoor environmental control for manufacturing,
product storage, or other research and development processes. The following areas are examples of
process air conditioning systems:
1. In textile mills, natural fibers and manufactured fibers are hygroscopic. Proper control of humidity
increases the strength of the yarn and fabric during processing. For many textile manufacturing
processes, too high a value for the space relative humidity can cause problems in the spinning
process. On the other hand, a lower relative humidity may induce static electricity that is harmful
for the production processes.
2. Many electronic products require clean rooms for manufacturing such things as integrated circuits,
since their quality is adversely affected by airborne particles. Relative-humidity control is
also needed to prevent corrosion and condensation and to eliminate static electricity. Temperature
control maintains materials and instruments at stable condition and is also required for workers who
wear dust-free garments. For example, a class 100 clean room in an electronic factory requires a
temperature of 72 _ 2°F (22.2 _ 1.1°C), a relative humidity at 45 _ 5 percent, and a count of dust
particles of 0.5-_m (1.97 _ 10_5 in.) diameter or larger not to exceed 100 particles/ ft3 (3531 particles
/m3).
3. Precision manufacturers always need precise temperature control during production of precision
instruments, tools, and equipment. Bausch and Lomb successfully constructed a constanttemperature
control room of 68 _ 0.1°F (20 _ 0.56°C) to produce light grating products in the
1950s.
4. Pharmaceutical products require temperature, humidity, and air cleanliness control. For instance,
liver extracts require a temperature of 75°F (23.9°C) and a relative humidity of 35 percent.
If the temperature exceeds 80°F (26.7°C), the extracts tend to deteriorate. High-efficiency air filters
must be installed for most of the areas in pharmaceutical factories to prevent contamination.
5. Modern refrigerated warehouses not only store commodities in coolers at temperatures of
27 to 32°F (_2.8 to 0°C) and frozen foods at _10 to _20°F (_23 to _29°C), but also provide
relative-humidity control for perishable foods between 90 and 100 percent. Refrigerated storage
is used to prevent deterioration. Temperature control can be performed by refrigeration systems
only, but the simultaneous control of both temperature and relative humidity in the space can only
be performed by process air conditioning systems.
Air conditioning is a combined process that performs many functions simultaneously. It conditions
the air, transports it, and introduces it to the conditioned space. It provides heating and cooling from
its central plant or rooftop units. It also controls and maintains the temperature, humidity, air
movement, air cleanliness, sound level, and pressure differential in a space within predetermined
The term HVAC&R is an abbreviation of heating, ventilating, air conditioning, and refrigerating.
The combination of processes in this commonly adopted term is equivalent to the current definition
of air conditioning. Because all these individual component processes were developed prior to the
more complete concept of air conditioning, the term HVAC&R is often used by the industry.
COMFORT AND PROCESSING AIR CONDITIONING
SYSTEMS
Air Conditioning Systems
An air conditioning, or HVAC&R, system is composed of components and equipment arranged in
sequence to condition the air, to transport it to the conditioned space, and to control the indoor environmental
parameters of a specific space within required limits.
Most air conditioning systems perform the following functions:
1. Provide the cooling and heating energy required
2. Condition the supply air, that is, heat or cool, humidify or dehumidify, clean and purify, and
attenuate any objectionable noise produced by the HVAC&R equipment
3. Distribute the conditioned air, containing sufficient outdoor air, to the conditioned space
4. Control and maintain the indoor environmental parameters–such as temperature, humidity,
cleanliness, air movement, sound level, and pressure differential between the conditioned space
and surroundings—within predetermined limits
Parameters such as the size and the occupancy of the conditioned space, the indoor environmental
parameters to be controlled, the quality and the effectiveness of control, and the cost involved determine
the various types and arrangements of components used to provide appropriate characteristics.
Air conditioning systems can be classified according to their applications as (1) comfort air
conditioning systems and (2) process air conditioning systems.
Comfort Air Conditioning Systems
Comfort air conditioning systems provide occupants with a comfortable and healthy indoor environment
in which to carry out their activities. The various sectors of the economy using comfort air
conditioning systems are as follows:
1. The commercial sector includes office buildings, supermarkets, department stores, shopping
centers, restaurants, and others. Many high-rise office buildings, including such structures as the
World Trade Center in New York City and the Sears Tower in Chicago, use complicated air conditioning
systems to satisfy multiple-tenant requirements. In light commercial buildings, the air conditioning
system serves the conditioned space of only a single-zone or comparatively smaller area.
For shopping malls and restaurants, air conditioning is necessary to attract customers.
2. The institutional sector includes such applications as schools, colleges, universities, libraries,
museums, indoor stadiums, cinemas, theaters, concert halls, and recreation centers. For example,
one of the large indoor stadiums, the Superdome in New Orleans, Louisiana, can seat 78,000 people.
3. The residential and lodging sector consists of hotels, motels, apartment houses, and private
homes. Many systems serving the lodging industry and apartment houses are operated continuously,
on a 24-hour, 7-day-a-week schedule, since they can be occupied at any time.
4. The health care sector encompasses hospitals, nursing homes, and convalescent care facilities.
Special air filters are generally used in hospitals to remove bacteria and particulates of submicrometer
size from areas such as operating rooms, nurseries, and intensive care units. The relative humidity in a
general clinical area is often maintained at a minimum of 30 percent in winter.
5. The transportation sector includes aircraft, automobiles, railroad cars, buses, and cruising
ships. Passengers increasingly demand ease and environmental comfort, especially for longdistance
travel. Modern airplanes flying at high altitudes may require a pressure differential of
about 5 psi between the cabin and the outside atmosphere. According to the Commercial Buildings
Characteristics (1994), in 1992 in the United States, among 4,806,000 commercial buildings having
67.876 billion ft2 (6.31 billion m2) of floor area, 84.0 percent were cooled, and 91.3 percent
were heated.
Process Air Conditioning Systems
Process air conditioning systems provide needed indoor environmental control for manufacturing,
product storage, or other research and development processes. The following areas are examples of
process air conditioning systems:
1. In textile mills, natural fibers and manufactured fibers are hygroscopic. Proper control of humidity
increases the strength of the yarn and fabric during processing. For many textile manufacturing
processes, too high a value for the space relative humidity can cause problems in the spinning
process. On the other hand, a lower relative humidity may induce static electricity that is harmful
for the production processes.
2. Many electronic products require clean rooms for manufacturing such things as integrated circuits,
since their quality is adversely affected by airborne particles. Relative-humidity control is
also needed to prevent corrosion and condensation and to eliminate static electricity. Temperature
control maintains materials and instruments at stable condition and is also required for workers who
wear dust-free garments. For example, a class 100 clean room in an electronic factory requires a
temperature of 72 _ 2°F (22.2 _ 1.1°C), a relative humidity at 45 _ 5 percent, and a count of dust
particles of 0.5-_m (1.97 _ 10_5 in.) diameter or larger not to exceed 100 particles/ ft3 (3531 particles
/m3).
3. Precision manufacturers always need precise temperature control during production of precision
instruments, tools, and equipment. Bausch and Lomb successfully constructed a constanttemperature
control room of 68 _ 0.1°F (20 _ 0.56°C) to produce light grating products in the
1950s.
4. Pharmaceutical products require temperature, humidity, and air cleanliness control. For instance,
liver extracts require a temperature of 75°F (23.9°C) and a relative humidity of 35 percent.
If the temperature exceeds 80°F (26.7°C), the extracts tend to deteriorate. High-efficiency air filters
must be installed for most of the areas in pharmaceutical factories to prevent contamination.
5. Modern refrigerated warehouses not only store commodities in coolers at temperatures of
27 to 32°F (_2.8 to 0°C) and frozen foods at _10 to _20°F (_23 to _29°C), but also provide
relative-humidity control for perishable foods between 90 and 100 percent. Refrigerated storage
is used to prevent deterioration. Temperature control can be performed by refrigeration systems
only, but the simultaneous control of both temperature and relative humidity in the space can only
be performed by process air conditioning systems.
Monday, April 13, 2009
HISTORICAL DEVELOPMENT
HISTORICAL DEVELOPMENT
The historical development of air conditioning can be summarized briefly.
Central Air Conditioning Systems
As part of a heating system using fans and coils, the first rudimentary ice system in the United
States, designed by McKin, Mead, and White, was installed in New York City’s Madison Square
Garden in 1880. The system delivered air at openings under the seats. In the 1890s, a leading consulting
engineer in New York City, Alfred R. Wolf, used ice at the outside air intake of the heating
and ventilating system in Carnegie Hall. Another central ice system in the 1890s was installed in
the Auditorium Hotel in Chicago by Buffalo Forge Company of Buffalo, New York. Early central
heating and ventilating systems used steam-engine-driven fans. The mixture of outdoor air and return
air was discharged into a chamber. In the top part of the chamber, pipe coils heat the mixture
with steam. In the bottom part is a bypass passage with damper to mix conditioned air and bypass
air according to the requirements.
Air conditioning was first systematically developed by Willis H. Carrier, who is recognized as
the father of air conditioning. In 1902, Carrier discovered the relationship between temperature and
humidity and how to control them. In 1904, he developed the air washer, a chamber installed with
several banks of water sprays for air humidification and cleaning. His method of temperature and
humidity regulation, achieved by controlling the dew point of supply air, is still used in many industrial
applications, such as lithographic printing plants and textile mills.
Perhaps the first air-conditioned office was the Larkin Administration Building, designed by
Frank L. Wright and completed in 1906. Ducts handled air that was drawn in and exhausted at roof
level. Wright specified a refrigeration plant which distributed 10°C cooling water to air-cooling
coils in air-handling systems.
The U.S. Capitol was air-conditioned by 1929. Conditioned air was supplied from overhead
diffusers to maintain a temperature of 75°F (23.9°C) and a relative humidity of 40 percent during
summer, and 80°F (26.7°C) and 50 percent during winter. The volume of supply air was controlled
by a pressure regulator to prevent cold drafts in the occupied zone.
Perhaps the first fully air conditioned office building was the Milan Building in San Antonio,
Texas, which was designed by George Willis in 1928. This air conditioning system consisted of one
centralized plant to serve the lower floors and many small units to serve the top office floors.
In 1937, Carrier developed the conduit induction system for multiroom buildings, in which recirculation
of space air is induced through a heating/cooling coil by a high-velocity discharging
airstream. This system supplies only a limited amount of outdoor air for the occupants.
The variable-air-volume (VAV) systems reduce the volume flow rate of supply air at reduced loads
instead of varying the supply air temperature as in constant-volume systems. These systems were introduced
in the early 1950s and gained wide acceptance after the energy crisis of 1973 as a result of
their lower energy consumption in comparison with constant-volume systems. With many variations,
VAV systems are in common use for new high-rise office buildings in the United States today.
Because of the rapid development of space technology after the 1960s, air conditioning systems
for clean rooms were developed into sophisticated arrangements with extremely effective air
filters. Central air conditioning systems always will provide a more precisely controlled, healthy,
and safe indoor environment for high-rise buildings, large commercial complexes, and precisionmanufacturing
areas.
Unitary Packaged Systems
The first room cooler developed by Frigidaire was installed about in 1928 or 1929, and the “Atmospheric
Cabinet” developed by the Carrier Engineering Company was first installed in May 1931.
The first self-contained room air conditioner was developed by General Electric in 1930. It was a
console-type unit with a hermetically sealed motor-compressor (an arrangement in which the motor
and compressor are encased together to reduce the leakage of refrigerant) and water-cooled condenser,
using sulfur dioxide as the refrigerant. Thirty of this type of room air conditioner, were built
and sold in 1931.
Early room air conditioners were rather bulky and heavy. They also required a drainage connection
for the municipal water used for condensing. During the postwar period the air-cooled model
was developed. It used outdoor air to absorb condensing heat, and the size and weight were greatly
reduced. Annual sales of room air conditioners have exceeded 100,000 units since 1950.
Self-contained unitary packages for commercial applications, initially called store coolers, were
introduced by the Airtemp Division of Chrysler Corporation in 1936. The early models had a refrigeration
capacity of 3 to 5 tons and used a water-cooled condenser. Air-cooled unitary packages
gained wide acceptance in the 1950s, and many were split systems incorporating an indoor air handler
and an outdoor condensing unit.
Packaged units have been developed since the 1950s, from indoor to rooftops, from constantvolume
to variable-air-volume, and from few to many functions. Currently, packaged units enjoy
better performance and efficiency with better control of capacity to match the space load. Computerized
direct digital control, one of the important reasons for this improvement, places unitary packaged
systems in a better position to compete with central hydronic systems.
Refrigeration Systems
In 1844, Dr. John Gorrie designed the first commercial reciprocating refrigerating machine in the
United States. The hermetically sealed motor-compressor was first developed by General Electric
Company for domestic refrigerators and sold in 1924.
Carrier invented the first open-type gear-driven factory-assembled, packaged centrifugal chiller
in 1922 in which the compressor was manufactured in Germany; and the hermetic centrifugal
chiller, with a hermetically sealed motor-compressor assembly, in 1934. The direct-driven hermetic
centrifugal chiller was introduced in 1938 by The Trane Company. Up to 1937, the capacity of centrifugal
chillers had increased to 700 tons.
During the 1930s, one of the outstanding developments in refrigeration was the discovery by
Midgely and Hene of the nontoxic, nonflammable, fluorinated hydrocarbon refrigerant family
called Freon in 1931. Refrigerant-11 and refrigerant-12, the chlorofluorocarbons (CFCs), became
widely adopted commercial products in reciprocating and centrifugal compressors. Now, new refrigerants
have been developed by chemical manufacturers such as DuPont to replace CFCs, so as
to prevent the depletion of the ozone layer.
The first aqueous-ammonia absorption refrigeration system was invented in 1815 in Europe. In
1940, Servel introduced a unit using water as refrigerant and lithium bromide as the absorbing solution.
The capacities of these units ranged from 15 to 35 tons (52 to 123 kW). Not until 1945 did
Carrier introduce the first large commercial lithium bromide absorption chillers. These units were
developed with 100 to 700 tons (352 to 2460 kW) of capacity, using low-pressure steam as the heat
source.
Positive-displacement screw compressors have been developed in the United States since the
1950s and scroll compressors since the 1970s because of their higher efficiency and smoother rotary
motion than reciprocating compressors. Now, the scroll compressors gradually replace the reciprocating
compressors in small and medium-size refrigeration systems. Another trend is the development of
more energy-efficient centrifugal and absorption chillers for energy conservation. The energy consumption
per ton of refrigeration of a new centrifugal chiller dropped from 0.80 kW/ton (4.4 COPref)
in the late 1970s to 0.50 kW/ton (7.0 COPref) in the 1990s. A series of rotary motion refrigeration
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