Engineering Manual of Automatic Controls
Chiller, Boiler, and Distribution System Control Applications


CHILLER SYSTEM CONTROL

Introduction

A chilled water system consists of a refrigeration system (water chiller), a chilled water distribution system, loads cooled by the chilled water and a means of dissipating the heat collected by the system. The refrigeration system cools water pumped through it by a chilled water pump. The chilled water flows through the distribution system to coils in air handling units or terminal units. Heat is removed from the refrigeration system using water or air. For chilled water control within AHU systems, see the Air Handling System Control Applications section.

Chilled water systems are used in many buildings for cooling because of their flexibility and operating cost compared with direct expansion (DX) cooling coil systems. Typically chilled water is generated at a central location by one or more chillers and distributed to coils in air handling system (Fig. 1). The quantity and temperature of the water supplied must be sufficient to meet the needs of all fan systems. Since the chilled water system is the major user of energy in many buildings, energy costs should be a consideration in chilled water plant configuration.

Fig. 1. Typical Water Chilling System.

Fig. 1. Typical Water Chilling System.

A chilled water system can provide hot water for a heating load when a simultaneous heating and cooling load exists. It can be used with a chilled water, ice tank, or phase change material thermal storage system to lower the peak load demand and allow use of a smaller chiller. It can use the system cooling tower during light load conditions to supply cool water to the system without running the chiller, if the OA WB temperature is low enough.

Chiller capacity controls are usually factory installed by the chiller manufacturer. The BMCS usually stages chillers on and off, provides chiller controls with a chilled water temperature setpoint, and controls the condenser water system. Chillers are usually controlled from their leaving water temperature; except that chillers using reciprocating compressors are often controlled from their entering water temperature, since staging and loading in steps causes steps in the leaving water temperature.

Chiller types are classified by type of refrigeration cycle: vapor-compression or absorption. In addition, those using the vapor-compression cycle are referred to by the type of compressor: centrifugal or positive displacement. A positive displacement compressor can be either reciprocating or screw for this discussion. See related ASHRAE and chiller manufacturers manuals for detailed information of chiller cycles.

Vapor-Compression Refrigeration

Vapor-Compression Cycle

The vapor-compression cycle is the most common type of refrigeration system. When the compressor (Fig. 2) starts, the increased pressure on the high side and the decreased pressure on the low side causes liquid refrigerant to flow from the receiver to the ;expansion valve. The expansion valve is a restriction in the liquid line which meters the refrigerant into the evaporator. It establishes a boundary between the low (pressure) side, including the evaporator and the high (pressure) side, including the condenser and the receiver. The compressor is the other boundary. The liquid refrigerant in the evaporator boils as it absorbs heat from the chilled water. The refrigerant leaves the evaporator and enters the compressor as a cold low-pressure gas. The refrigerant leaves the compressor as a hot high-pressure gas and passes through the condenser where it is cooled by the condenser water until it condenses and returns to the receiver as a liquid. The cycle is the same regardless of the compressor type or refrigerant used.

Two common types of expansion valves are constant pressure and thermostatic. The constant pressure valve is suitable only when the load is constant. It is essentially a pressure regulator which maintains a constant pressure in the evaporator.

The thermostatic expansion valve is used for varying cooling loads, such as those found in HVAC systems. It has a remote temperature sensing element which is usually installed on the suction line between the evaporator and the compressor. It is set to adjust the expansion valve so there is a small amount of superheat in the suction line refrigerant. Superheat means that all of the liquid has evaporated and the vapor has been heated above the evaporation temperature by the water or air being cooled. This prevents liquid from entering the compressor.

A flooded shell and tube chiller evaporator (Fig. 3) is usually used with centrifugal compressors while, a direct expansion chiller evaporator (Fig. 4) is used with positive displacement compressors. In both cases the condenser is a large pressure cylinder (shell) with tubes connected to inlet and outlet headers. In the flooded shell and tube type evaporator, the shell is about 80 percent filled with refrigerant and the chilled water flows through the tubes. Heat from the water evaporates the refrigerant surrounding the tubes which cools the water. The refrigerant vapor rises to the top of the shell and into the refrigerant suction line.

Fig. 2. Typical Vapor-Compression Cycle Water Chiller.

Fig. 2. Typical Vapor-Compression Cycle Water Chiller.


Fig. 3. Flooded Shell and Tube Chiller Evaporator.

Fig. 3. Flooded Shell and Tube Chiller Evaporator.


Fig. 4. Direct Expansion Chiller Evaporator.

The direct expansion chiller evaporator is the reverse of the flooded shell and tube chiller evaporator, water in the shell and the refrigerant in the tubes.

The compressor can be reciprocating, centrifugal, or screw type. The centrifugal and screw types are generally found on the larger systems.

The chiller condenser is usually water cooled but may be air cooled or evaporative cooled. The most common water cooled condenser is the shell and tube type (similar to Figure 3). The cooling (condenser) water flows through the tubes and the refrigerant vapor condenses on the cool tube surface and drops to the bottom of the shell where it flows into the liquid line to the receiver or evaporator.

An air cooled condenser is a series of finned tubes (coils) through which the refrigerant vapor flows. Air is blown over the coils to cool and condense the refrigerant vapor.

An evaporative condenser is similar to the air cooled condenser where the refrigerant flows through a coil. Water is sprayed over the coil and then air is blown over the coil to evaporate the water and condense the refrigerant. Evaporative condensers are rarely used because of the additional maintenance compared with an air cooled condenser.

Centrifugal Compressor

Centrifugal compressors are available in a wide range of sizes. Compressor capacity can be modulated from maximum to relatively low values. Centrifugal chiller systems can be designed to meet a wide range of chilled liquid (evaporator) and cooling fluid (condenser) temperatures.

Operation of the compressor is similar to a centrifugal fan or pump. Gaseous refrigerant enters the inlet (Fig. 5) and passes through inlet vanes into the chambers or blades radiating from the center of the impeller. The impeller, rotating at a high rate of speed, throws the gas to the outer circumference of the impeller by centrifugal force. This increases the velocity and pressure of the gas. The gas is then thrown from the impeller into the volute where most of the velocity (kinetic energy) is converted to pressure.

Use of a larger evaporator and condenser decreases the energy needed by the compressor for a given cooling load. Typical single stage high speed compressor construction is shown in Figure 5. The prerotation vanes (inlet guide vanes), located in the suction side of the compressor, control the gaseous refrigerant flow by restricting flow. As the vanes vary the flow, the compressor pumping capacity varies. In the open position the vanes give a rotating motion to the refrigerant in a direction opposite to the impeller rotation. This allows the chambers or blades to pick up a larger amount of gas.

Fig. 5. Cutaway of Single Stage Centrifugal Compressor.

Reprinted by permission: The Trane Company, LaCrosse, WI 54601

Fig. 5. Cutaway of Single Stage Centrifugal Compressor.

Centrifugal compressors are driven by turbines, electric motors, or internal combustion engines. Inlet vane control or speed control varies the capacity. Each method has different performance characteristics. A combination of speed and inlet vane control provides the highest operating efficiency. Multiple stage direct drive type compressors are available in many configurations.

Refrigerant head is the pressure difference between the compressor inlet and outlet and is the primary factor affecting chiller efficiency. For a given load, reducing refrigerant head improves efficiency. Evaporation and condensation temperatures establish these pressures and are determined by chilled water temperature and condenser water temperature. Refrigerant head is reduced by the following:

- Reducing condenser water temperature.
- Raising chilled water temperature.
- Reducing load.
- Decreasing design differential temperature of evaporator and condenser heat exchangers by increasing the size of the heat exchangers.

The load for maximum chiller efficiency varies with chillers and chiller manufacturers, but is often 70 to 80 percent.

Reciprocating Compressor

The reciprocating compressor is a positive displacement device consisting of several cylinders and pistons. The crankshaft is driven by a motor or engine. Spring loaded valves allow low pressure refrigerant vapor to enter the cylinder on the downstroke and high pressure refrigerant vapor to exit on the upstroke. Because the compressor is a positive displacement device its capacity is not greatly influenced by refrigerant head. However, power required per unit of cooling is directly related to refrigerant head. Keeping condenser temperature as low as possible also reduces energy requirements, therefore, compressors with water cooled condensers use less power than air cooled condensers. However, condenser water temperature must not be allowed to go too low or there will not be enough pressure difference to circulate the refrigerant.

Reciprocating chiller capacity is controlled in stages (steps). Methods of capacity control include the following:

- Unloading cylinders
- On-off cycling of multiple compressors
- Hot-gas bypass
- Hot-gas through evaporator

Cylinder unloading or multiple compressor on-off cycling is sequenced by automatic controls. The cylinder inlet valves are held open so no compression takes place during cylinder unloading. Capacity control mechanisms and controls are usually packaged with the chiller.

Screw Compressor

A screw compressor is a positive displacement device which uses two meshed helical rotors to provide compression. It is also known as a helical rotary compressor. Basic construction of a helical rotary twin screw compressor is shown in Figure 6. The capacity of a screw compressor can be modulated by speed control or a sliding valve that varies the length of compression area of the helical screws and bypasses some gas back to the inlet of the compressor.

Fig. 6. Helical Rotary Twin Screw Compressor.

Fig. 6. Helical Rotary Twin Screw Compressor.

Absorption Refrigeration

Absorption Cycle

The absorption cycle uses a fluid called an absorbent to absorb evaporated refrigerant vapor in an 'absorber' section. The resulting combination of fluid and refrigerant is moved into a 'generator' section where heat is used to evaporate the refrigerant from the absorbent.

In the absorber (Fig. 7) the absorbent, also called strong absorbent at this point, assimilates the refrigerant vapor when sprayed through it. The resulting weak absorbent is pumped by the generator pump through the heat exchanger, where it picks up some of the heat of the strong absorbent, then into the generator. In the generator the weak absorbent is heated to drive (evaporate) the refrigerant out of the absorbent and restore the strong absorbent. The strong absorbent then passes through the heat exchanger, where it gives up some heat to the weak absorbent, and then returns to the spray heads in the absorber completing the cycle for the absorbent.

NOTE: Industry standards reverse the definitions of strong absorbent and weak absorbent when ammonia is the refrigerant and water the absorbent.

The refrigerant vapor migrates from the generator to the condenser where it is cooled until it condenses to a liquid. The liquid refrigerant flows to the evaporator where the refrigerant pump sprays the liquid over the chilled water coils. The heat from the chilled water evaporates the liquid. The resulting vapor migrates to the absorber where it is absorbed by the strong absorbent and pumped to the generator to complete the refrigerant cycle.

Fig. 7. Absorption Chiller Operating Cycle Schematic.

Fig. 7. Absorption Chiller Operating Cycle Schematic.

Figure 8 is a typical water-lithium bromide absorption cycle chiller. Lithium bromide is the absorbent and water is the refrigerant. Use of water as a refrigerant requires that the system be sealed, all the air removed, and an absolute pressure of 0.25 in. Hg be maintained. Under these conditions the refrigerant (water) boils at 40F which allows the refrigerant to cool the chilled water to 44F.

Fig. 8. Diagram of Two-Shell Lithium Bromide Cycle Water Chiller.

Reprinted by permission from the ASHRAE Handbook - 1994 Refrigeration

Fig. 8. Diagram of Two-Shell Lithium Bromide Cycle Water Chiller.

Absorption Chiller

Capacity control of a water-lithium bromide absorption chiller is modulated by changing the concentration of the strong absorbent by varying the heat input to the process in the generator, controlling condenser water flow, or controlling flow of the strong absorber. Heat sources may be hot water, high temperature hot water, steam, or a gas flame (direct fired). Light loads require a reduced concentration of strong absorbent (absorbent retains more refrigerant) or less flow of the strong absorbent. The amount of heat required for a given cooling load is proportional to the temperature difference between condensing water and chilled water (refrigerant head). It is also proportional to temperature lift (chilled water temperature difference).

Some absorption chillers require the condensing water be kept constant at the design temperature. To improve seasonal operating efficiency some designs accept condensing water temperatures below design down to 45F. This requires an internal control that transfers liquid from refrigerant circuit to absorbent circuit, transfers liquid from absorbent circuit to refrigerant circuit, limits heat input, or a combination. Low condenser water temperature decreases energy usage and increases unit capacity.

When the condenser water temperature is too low, the absorbent contains too much refrigerant and the solution crystallizes. A safety control used by some absorption units senses when the lithium bromide concentration is getting too high with a low cooling water temperature and takes action to avoid crystallization.

Absorption chillers are normally used where heat energy is available at very low cost such as the exhaust from a steam turbine. They also are used to reduce electric load and therefore peak electric demand.

Chiller Control Requirements

Basic Chiller Control

Basic chiller control is a sensor in the chilled water supply or return and a controller to provide a control signal to a capacity control actuator. Capacity control is unique to each compressor type. Summarized, the controls for each compressor type are:

1. Centrifugal - Controller output operates a pneumatic or electric actuator to position inlet vanes as a function of the controlled temperature. If speed control is available, the controller sequences motor or rotor speed with inlet vanes.

2. Reciprocating - Controller provides a stepped output to sequence refrigerant solenoid valves, valve unloading, hot gas bypass, or multiple compressors as a function of controlled temperature.

3. Screw - Controller operates speed control or a pneumatic or electric actuator to position sliding bypass valve in response to temperature input.

4. Absorption - Controller output operates a valve to modulate the steam, hot water, or gas supply to maintain controlled temperature.

Capacity high limit controls are used on centrifugal and screw compressors to limit electrical demand during high load periods such as morning cool-down. A load limiting control reduces motor current draw to an adjustable maximum. Capacity of some chillers can be reduced to as low as 10 percent.

Most chillers with modulating capacity control use proportional-integral control, and often receive their chilled water setpoint from a BMCS to optimize building energy efficiency.

System Controls which Influence the Chiller

Whatever the configuration of a chilled water system, proper control is necessary to meet the overall system requirements. Condenser and chilled water temperatures establish refrigerant head and energy needed per unit of cooling. Minimum condenser temperature limits vary for different chiller designs. Condenser temperatures should be maintained as close to the minimum limits as possible to minimize refrigerant head. Actual condenser water temperature is dependent on outdoor wet bulb temperatures. Chilled water temperature is dependent on system design and building load.

Safety Controls

When an unsafe condition exists, the compressor should stop automatically. Safety cutout controls may have automatic or manual reset and include the following:

1. High condenser pressure.
2. Low refrigerant pressure or temperature.
3. Back-up for the low chilled water temperature controller (on some reciprocating chillers).
4. High motor temperature.
5. Motor overload.
6. Low oil pressure.
7. Low oil sump temperature.
8. High oil sump temperature.
9. Chilled water flow interlock.
10. Condenser water flow interlock.

The preceding are all two-position (on-off) controls. In addition, modulating limit controls sensing high condenser pressure or low evaporator pressure-temperature reduce compressor capacity to avoid the safety cutout conditions of Items 1 and 2.

Chiller/BMCS Interface

Most chillers are supplied with micro-processor controllers with a significant database of safety, operating, monitoring, and setpoint status and values. The BMCS normally provides control of the chilled water pump, the cooling tower fans, and the chiller system (AUTO/OFF commands to the chiller controller). The chilled water temperature setpoint and on occasions the maximum load setpoint are also dictated by the BMCS.

It is desirable for the BMCS to have access to the chiller-controller database, but due to the cost and complexity of a custom interface to convert the data to a format acceptable to the BMCS, it is seldom done. Adoption of open communication standard protocols as the ASHRAE BACnet and the Echelon LonMark¬ will replace the expensive interfaces with direct interfaces.

Chilled Water Systems

Central Cooling Plants

The central cooling system generates chilled water for distribution to a building or group of buildings. It consists of one or more chillers. Multiple chillers may all be the same or different capacities and/or different types. The energy may be provided by electricity or a fuel-combustion source. Central chiller system optimization is an important control function to minimize energy use, especially in multiple chiller plants. The control program must be dynamic and constantly check current conditions and adjust chiller system operations accordingly. A control program must select the most efficient loading and chiller combinations then, sequence pumps and control cooling towers to match the current load condition. Built-in safeguards prevent short cycling and exceeding demand limits. Strategies for total chiller system optimization include:

1. Supplying chilled water at a temperature that minimizes chiller and pump energy while satisfying the current demand load.
2. Selecting the chiller or chiller combination in multiple chiller plants to satisfy the current load at minimum operating cost. The influence of refrigerant head pressures and chiller efficiency curves must be considered.
3. Using rejected heat when a heating load exists at the same time as a cooling load.
4. Using thermal storage to store day time rejected heat and/or night time cooling. Thermal storage can also reduce the size of chiller equipment.

Single Centrifugal Chiller Control

Capacity control is the primary method used to control a single chiller to meet the cooling load. Typically centrifugal chiller capacity control is accomplished by a chiller discharge water temperature controller. Discharge control responds quickly to load changes to maintain the chilled water temperature. The chilled water supply temperature may be reset from chilled water return temperature or from the zone with the greatest load. To ensure that all loads are met, resetting based on zone demand requires monitoring all the chilled water valves on the fan systems. Resetting from return water temperature recognizes the average temperature only and not the individual loads.

Where chilled water constant speed pumping horsepower is more than 25 to 33 percent of the compressor horsepower, increases in chilled water temperature could force the use of more pumping energy than can be saved in reduced compressor energy. This is because chilled water control valves open wider due to the increased water temperature. The increased flow requires the pump(s) to use more power. In these cases, chilled water reset should not be used or should be limited to reset only when flow is below the break even point of compressor versus pump energy.

Single Centrifugal Chiller Control Application

Functional Description

Fig. 9. Single Chiller Control Graphic.

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Fig. 9. Single Chiller Control Graphic.

Item No. Function
1 Indicates when chiller system is required by fan system.
2 Chilled water pump ON-OFF-AUTO function. In AUTO pump runs when fan systems need chilled water.
3 Chiller ON-OFF-AUTO function (In ON and AUTO chilled water flow required for chiller to run).
4 Condenser pump status. Pump started by chiller controls when chilled water needed.

5, 6

Chiller leaving water temperature and setpoint.
7 Icon to select chiller control dynamic sequence display (Fig. 10).
8 BMCS commandable load limiting function.
9-14 Operator information.
15 Control program coordinates chiller control.
16 Icon to select cooling tower control displays.

Fig. 10. Single Chiller Control Dynamic Sequence Display.

Fig. 10. Single Chiller Control Dynamic Sequence Display.

Features

1. Automatic start-stop and setpoint optimization of chiller.
2. User friendly monitoring and adjustment.
3. Optimized unoccupied operation.
4. Chiller cannot start late shortly before the unoccupied period begins.

Conditions For Successful Operation

1. Control network, software, and programming advises chiller controller of AHU chilled water demands.

2. Interlock and control wiring coordinated with the chiller manufacturer.

3. Appropriate cooling tower and control.

4. For single-chiller systems without primary-secondary pumping, three-way air handling unit valves may be used for 80 to 85 percent of the chilled water flow (Small valves, up to 15 to 20 percent total flow, may be two-way, which are simpler to pipe.) If all two-way valves are provided on single pump systems, chilled water flow or pressure controls (See Dual Centrifugal Chillers) are provided to maintain the required flow (varies with chiller manufacturers) through the chiller. Do not use three-way valves when diversity is used in the chiller system design.

NOTE: Little pumping energy can be saved on a single-pump single-chiller system by using two-way AHU control valves since the chiller usually requires high flow anyway.

5. During the unoccupied period the 80 percent load limiting parameter (see Specification following) is based on the assumption that AHUs are VAV and are operating under a reduced maximum cfm setpoint during all unoccupied cooling modes (see the Air Handling System Control Applications) to save fan energy and place the chiller operation in the maximum efficiency range.

6. Chilled water temperature reset from AHU chilled water valve position requires:
a. No valve always full open.
b. Maximum of 30 to 40 valves. With too many valves the probability that one valve will always be open is very great.
c. Zone setpoint discipline is maintained. Lowering setpoints to resolve complaints may result in one or more valves being always open.

7. Chilled water temperature reset timing increments are compatible with valve control response. If the temperature reset is too fast, the valve cannot adjust to the new temperature, resulting in instability.

Specification

The chiller system operation shall be enabled anytime the time of day is less than 1545 and any AHU chilled water valve is open greater than twenty percent for more than three minutes. Anytime the chiller system is enabled, the chilled water pump shall run.

Anytime chilled water flow is proven via a chilled water pump current sensing relay, the chiller controls shall be enabled to operate under factory controls, subject to a chiller software ON-OFF-AUTO function (chilled water flow must still be proven in the 'ON' mode). Provide control and interlock wiring per the chiller manufacturers recommendation.

Upon a call for chilled water, the chiller controls shall start the condenser water pump and energize the cooling tower fan controls.

When condenser water flow is proven via a condenser water pump current sensing relay, the chiller shall start, operate, and load under chiller factory controls to maintain the chilled water temperature setpoint, 46F at start-up.

Anytime all chilled water valves are less than 85 percent open, the chilled water temperature setpoint shall be incremented at a rate of 0.3F every 10 minutes up to a maximum of 52F.

Anytime any chilled water valve is full open, the chilled water temperature setpoint shall be decremented at a rate of 0.3F degrees every 10 minutes down to a minimum of 45F.

The maximum allowable percentage of chiller full load electrical current shall be commandable from the BMCS, and shall be 80 percent during all unoccupied periods of operation.

Multiple Chiller System Control Applications

Multiple chiller systems offer standby capacity and improved economy at partial loads. Multiple chiller systems may be piped for either parallel or series chilled water flow.

In the parallel piped arrangement (Fig. 11), return chilled water is divided among the chillers then recombined after chilling. Two methods of operation at light loads are depicted. One uses a pump and a check valve for each chiller. The other uses a common pump with an isolation valve for each chiller. Multiple pumps with check valves allow one chiller and the associated pump to be shut down during light load conditions to save energy and require that the system be able to operate with the reduced flow. The check valves prevent reverse flow through the shut down chiller. Use of a common pump and isolation valves require that the operating chiller be able to withstand full system flow. The isolation valves allow the operating chiller to supply only the chilled water temperature required to meet system demands. Without the isolation valves, half of the water flows through the chiller which is shut down and is not cooled. When the uncooled water is mixed with the cooled water, the temperature will be the average of the water temperatures. As a result, the on-line chiller must supply water cool enough so that the average will satisfy the primary sensor and thus the system. To meet this requirement the on-line chiller may need to supply water close to the freezing point.

The temperature sensor in the common chilled water supply is the primary capacity control. The temperature low limit control prevents the outlet temperature of each chiller from going too low. A return water temperature sensor can be used in conjunction with a supply water temperature sensor to turn off one chiller in light load conditions.

Fig. 11. Parallel Piped Chillers.

Fig. 11. Parallel Piped Chillers.

In the series arrangement (Fig. 12) chilled water pressure drop is higher if the chillers are not designed for higher flow. At partial loads, compressor power consumption is lower than for the parallel arrangement.

When the condensers of series units are water cooled, they are piped in series counterflow to balance loading. When piped series-counterflow, Chiller 1 receives warmer condenser and chilled water while Chiller 2 receives colder entering condenser and chilled water. This makes refrigerant head approximately the same for each chiller. The controls may be set to shutdown either chiller at partial loads.

Fig. 12. Series Piped Chillers.

Fig. 12. Series Piped Chillers.

When two chillers of equal size and similar characteristics are used, the point at which the second chiller is activated is usually when the first chiller reaches 100 percent load. This is demonstrated in Figure 13 which plots kW per ton versus percent load for one chiller and two chillers at various temperature differences between condenser and chilled water temperatures. Curves vary slightly for different temperature differences so a microprocessor-based control system is used for maximum efficiency. The microprocessor checks chilled and condenser water temperature, looks up chiller efficiency at those temperatures, and calculates the optimum changeover point.

Curves A and B in Figure 13 illustrate that for the chillers operating at design condition with a 43F temperature differential (DT) between chilled and condenser water the second chiller must be added when the first chiller reaches 100 percent load (50 percent of chiller system capacity). The next set of curves (C and D) show the chiller is more efficient because of the smaller DT (31F) and that the first chiller can be loaded to 110 percent (55 percent of system load) before the second chiller is added. The third set of curves shows the extremely efficient operation with a 19F DT.

Fig. 13. Efficiency - Two Equal Size Chillers.

Fig. 13. Efficiency - Two Equal Size Chillers.

Dual Centrifugal Chillers Control Application

Functional Description

Fig. 14. Dual Centrifugal Chiller Control Graphic.

Fig. 14. Dual Centrifugal Chiller Control Graphic.

Item No. Function
1 Indicates when chiller system is required by fan system.
2 Chilled water pump ON-OFF-AUTO function. In AUTO pump runs when fan systems need chilled water.
3 Chiller ON-OFF-AUTO function (In ON and AUTO chilled water flow required for chiller to run).
4 Condenser pump status. Pump started by chiller controls when chilled water needed.

5, 6

Chiller leaving water temperature and setpoint.
7 Icon to select chiller control dynamic sequence display (Fig. 15).
8 BMCS commandable load limiting function.
9 Lead chiller selector function.
10-17 Operator information.
18 Control program coordinates chiller staging and control.
19 Icon to select cooling tower control displays.
20 Icon to select chilled water flow and pressure control displays.

Fig. 15. Dynamic Chiller Control Display.

Fig. 15. Dynamic Chiller Control Display.

Features

1. Automatic start-stop, staging, and setpoint optimization of chillers.
2. User friendly monitoring and adjustment.
3. Optimized unoccupied operation.
4. Chiller that has run longest since last start is first to stop.
5. Chillers cannot start late (shortly before the unoccupied period begins).

Conditions For Successful Operation

1. Control network, software, and programming advises chiller controller of AHU chilled water demands.

2. Interlock and control wiring coordinated with the chiller manufacturer.

3. Appropriate cooling tower and control is required. See Cooling Tower and Condenser Water Control.

4. Two-way AHU control valves. This allows good single-chiller, single pump operation.

5. Appropriate chilled water flow and differential pressure controlled bypass valve to keep the minimum required flow through the chillers (varies with chiller manufacturers).

6. The unoccupied period 80 percent load limiting parameters are based on the assumption that VAV AHUs are operating under a reduced maximum cfm setpoint during all unoccupied cooling modes (see the Air Handling System Control Applications section) to save fan energy while simultaneously placing the chiller operation in its maximum efficiency range.

NOTE: When using two-way AHU valves with this coupled chiller configuration, exercise care in optimizing the chilled water temperature. With both chillers running, raising the chilled water temperature results in greater flow and a smaller DT which must be considered in the chiller shedding strategy. For an example of a pressure bypass system, see Dual Pumps, Dual Chillers, Pressure Bypass, 90 Percent Chiller Flow, Direct Return.

Specification

The chiller system operation shall be enabled anytime the time of day is before 1545 and any AHU chilled water valve is open greater than twenty percent for greater than three minutes. Anytime the chiller system is enabled, the lead chilled water pump shall run.

Anytime the lead chiller has run longer than 90 minutes, the chilled water temperature has been greater than 1 degree above the chilled water temperature setpoint for greater than 4 minutes, and the time is less than 1545, the off chilled water pump shall start.

Anytime both chillers are running and the chiller plant water differential temperature has been less than 4.4F for greater than 4 minutes, the chilled water pump with the longest 'on' duration since the last start shall stop and remain off at least 30 minutes.

Anytime chilled water flow is proven via a chilled water pump current sensing relay, the respective chiller controls shall be enabled to operate under factory controls, subject to a chiller software ON-OFF-AUTO function (chilled water flow must still be proven in the 'ON' mode). Provide control and interlock wiring per the chiller manufacturers recommendation.

Upon a call for chilled water, the chiller controls shall start the condenser water pump and energize the cooling tower fan controls.

When condenser water flow is proven via a condenser water pump current sensing relay, the chiller shall start, operate, and load under chiller factory controls to maintain the chilled water temperature setpoint, 44F at start-up.

Anytime all chilled water valves are less than 80 percent open, the chilled water temperature setpoint shall be incremented at a rate of 0.3F every 5 minutes up to a maximum of 50F.

Anytime any chilled water valve is full open, the chilled water temperature setpoint shall be decremented at a rate of 0.3F every 5 minutes down to a minimum of 42F.

The maximum allowable percentage of chiller full load electrical current shall be commandable from the BMCS, and shall be 80 percent during all unoccupied periods of operation.

Similar Multiple Centrifugal Chillers Control Applications

- Equal Sized Centrifugal Chillers Control

Fig. 16. Multiple Equal Sized Chillers Control Graphic.

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Fig. 16. Multiple Equal Sized Chillers Control Graphic.  

System Description

Figure 16 shows a typical 'decoupled' multiple chiller system. Each chiller has a (primary) dedicated constant speed pump selected to produce the chiller design flow through the primary loop, including the 'decoupler line'. The decoupler line isolates the primary and secondary pumping systems and handles any imbalance between the two flow loops. The decoupler line is typically sized to handle the flow of the largest primary pump at a negligible pressure drop, should be at least 6 to 10 pipe diameters in length, and the tees at each end should be configured as shown to oppose any undesirable induced flow forces. Decoupler flow should always be forward, not to exceed the flow of one chiller. Any backward decoupler flow will dilute the secondary chilled water supply with secondary return water thus raising the secondary supply temperature above design.

The secondary pumping system is variable volume and may contain many varieties of pumping loops.

Control, the staging of chillers on and off, is normally:

- Start a chiller anytime the decoupler has a backward flow.
- Stop a chiller anytime the decoupler forward flow exceeds that of the next chiller to be shed.

Software Partitioning

From an operational and control perspective, the physical configuration of chiller plant digital controllers is usually transparent. The configuration varies, depending upon:

- Chiller staging algorithm.
- Redundant/backup control requirements.
- Condenser water system configuration.

NOTE: Where water leaving cooling towers becomes common before being extended to the chiller plant, a single cooling tower isolating, staging, and loading algorithm is usually preferred.

- Chiller monitoring requirements.
- Controller capacity for monitoring and control.
- Other project-unique requirements.

Figure 17 is a schematic of a digital system configuration. Each chiller has a dedicated cooling tower and a dedicated controller for chiller, cooling tower, and condenser water monitoring and control. Figure 18 shows a variation of Figure 17 where condenser water is common to all chillers and the cooling towers are staged in response to condenser water demand.

Fig. 17. Typical Digital Controller Configuration for Multiple Chillers.

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Fig. 17. Typical Digital Controller Configuration for Multiple Chillers. 


Fig. 18. Digital Control of Sequenced Cooling Towers.

Fig. 18. Digital Control of Sequenced Cooling Towers.

- Multiple Centrifugal Chiller Sequencing

Functional Description

Fig. 19. Control Graphic for Multiple Similar Chillers.

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Fig. 19. Control Graphic for Multiple Similar Chillers.

Item No. Function
1, 2 Secondary pump speeds.
3 Icon for selection of secondary control details.
4 Secondary pump leaving water temperature (operator information).
5 Four chiller pump status indicators (green = on, yellow = off, red = alarm) (typical).

6

Four Icons for selection of chiller detail graphic.
7 Four chiller status indicators (typical), operator information.
8 BMCS commandable AUTO-OFF functions for each chiller and ON-OFF-AUTO functions for each chiller pump.
9 Decoupler temperatureĐindicates direction of flow.
10 Primary flow indicates primary loop loading.
11 Secondary flow indicates secondary loop loading.
12 Decoupler flow the difference between primary and secondary flows.
13-16 Temperatures for calculating secondary flow.
17 Status of optional AUTO-MANUAL toggle switch.
18 Four chiller CHWS temperature indicators.
19 Operator information (from secondary system).
20 Icon for selection of chilled water setpoint details display.
21 Icon for selection of chiller sequencing display (Fig. 20).

Fig. 20. Multiple Chiller Sequencing.

Fig. 20. Multiple Chiller Sequencing.

Features

1. Automatic start-stop sequencing of multiple decoupled chillers.
2. User friendly monitoring and adjustment.
3. Flow calculations without costly and maintenance-prone flow meters (see Specification).
4. Chiller that has been off longest is next to start.
5. Constant flow through chillers with a variable flow secondary water system.

Conditions For Successful Operation

1. Control network, software, and programming to advise chiller plant controller of AHU chilled water demands.
2. Interlock and control wiring coordinated with the chiller manufacturer.
3. Appropriate cooling tower and control.
4. Two way AHU control valves provide variable flow secondary operation.
5. Precise and matched temperature sensors for accurate flow calculation. Refer to the flow equation in Figure 16.
6. Proper and precise positioning of primary return water well and sensor to get accurate measurement of mixed water temperature.
7. Digital controller configuration to suit cost and reliability requirements.

Specification

Chiller Plant Start-Up:

Anytime any secondary pump starts, the chiller plant controls shall be enabled, and the chiller pump that has been off longest shall start, subject to its software ON-OFF-AUTO function and its respective chiller software AUTO-OFF function. Pump/chiller combinations with either function OFF shall be removed from the control sequence.

When any chiller pump flow is proven, its respective chiller controls shall be energized. Upon a call for cooling by the chiller controls, the chiller controls shall enable the condenser water system controls and, upon proof of condenser water flow, the chiller shall start. Starting, loading, and interlock wiring shall be as recommended by the chiller manufacturer.

Chiller On-staging:

Anytime a chiller has operated greater than 50 minutes and the decoupler line temperature is greater than the chiller leaving water temperature setpoint by greater than 1.0F for greater than 5 minutes, the off chiller pump that has been off longest shall start.

Chiller Off-staging:

Anytime more than one chiller is operating and the decoupler has a supply chilled water flow in excess of the capacity of one chilled water pump for greater than 3.0 minutes, the chiller that has been running longest shall stop.

Chilled water flow calculations:

The primary supply water flow shall be calculated by summing the design water flow for all operating chiller pumps (each pump shall have a commandable value for its design flow).

FP = Flow (P1,P2,....Pn)

The secondary water flow shall be calculated by dividing the product of the primary flow times the primary water differential temperature by the secondary water differential temperature.

The decoupler flow shall be calculated by subtracting the secondary return water flow from the primary supply water flow.

FD = FP - FS

The temperature sensors for flow calculation shall be platinum and software field-matched to within 0.1 degree at 50F. The primary return water sensor shall be in a stainless steel well extended at least 50 percent of the distance across the pipe and positioned as far away from the decoupler/secondary return mixing tee as possible.

CHILLER SYSTEM CONTROL - Continued

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