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.
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