The evaporator is the low-temperature cooling side of the system and the condenser is the high- temperature heat-rejection side of the system. Reciprocating: Similar to a car engine with multiple pistons, a crankshaft is turned by an electric motor, the pistons compress the gas, heating it in the process.
The hot gas is discharged to the condenser instead of being exhausted out a tailpipe. The pistons have intake and exhaust valves that can be opened on demand to allow the piston to idle, which reduces the chiller capacity as the demand for chilled water is reduced. This unloading allows a single compressor to provide a range of capacities to better match the system load. This is more efficient than using a hot-gas bypass to provide the same capacity variation with all pistons working. Some units use both methods, unloading pistons to a minimum number, then using hot-gas bypass to further reduce capacity stably.
Capacities range from 20 to tons. As the helical rotors rotate, the gas is compressed by direct volume reduction between the two rotors. Capacity is controlled by a sliding inlet valve or variable-speed drive VSD on the motor. Screw compressor Centrifugal: The centrifugal compressor operates much like a centrifugal water pump, with an impeller compressing the refrigerant.
Centrifugal chillers provide high cooling capacity with a compact design. They can be equipped with both inlet vanes and variable-speed drives to regulate control chilled water capacity control. Capacities are tons and up. The compressor requires no lubricant and has a variable-speed DC motor with direct-drive for the centrifugal compressor. Capacities range from 60 to tons. Because there are few absorption machines in the Northwest U.
You can learn more about absorption chillers at the Energy Solutions Center. Key Components of Mechanical Compression Chillers Evaporator Chillers produce chilled water in the evaporator where cold refrigerant flows over the evaporator tube bundle. The refrigerant evaporates changes into vapor as the heat is transferred from the water to the refrigerant. The chilled water passes through coils in the air-handler to remove heat from the air used to condition spaces throughout the building.
The warm water warmed by the heat transferred from the building ventilation air returns to the evaporator and the cycle starts over. Compressor Vaporized refrigerant leaves the evaporator and travels to the compressor where it is mechanically compressed, and changed into a high-pressure, high-temperature vapor.
Upon leaving the compressor, the refrigerant enters the condenser side of the chiller. Condenser Inside the water-cooled condenser, hot refrigerant flows around the tubes containing the condenser-loop water.
The heat transfers to the water, causing the refrigerant to condense into liquid form. The condenser water is pumped from the condenser bundle to the cooling tower where heat is transferred from the water to the atmosphere. The liquid refrigerant then travels to the expansion valve. Expansion valve The refrigerant flows into the evaporator through the expansion valve or metering device.
This valve controls the rate of cooling. Once through the valve, the refrigerant expands to a lower pressure and a much lower temperature. Controls Newer chillers are controlled by sophisticated, on-board microprocessors. Chiller control systems include safety and operating controls. If the equipment malfunctions, the safety control shuts the chiller down to prevent serious damage to the machine.
Operating controls allow adjustments to some chiller operating parameters. Safety Issues Chillers are typically located in a mechanical equipment rooms.
Each type of refrigerant used in a chiller compressor has specific safety requirements for leak detection and emergency ventilation. Consult your local mechanical code or the International Mechanical Code for details. The EPA has enacted regulations regarding the use and handling of refrigerants to comply with the Clean Air Act of All personnel working with refrigerants covered by this act must be appropriately licensed. Best Practices for Efficient Operation The following best practices can improve chiller performance and reduce operating costs: Operate multiple chillers for peak efficiency: Plants with two or more chillers can save energy by matching the building loads to the most efficient combination of one or more chillers.
In general, the most efficient chiller should be first one used. Establish a chilled-water reset schedule. A reset schedule can typically adjust the chilled-water temperature as the outside-air temperature changes.
The actual leaving tower water temperature may be limited by the ambient wet bulb temperature. Purge air from refrigerant: Air trapped in the refrigerant loop increases pressure at the compressor discharge. This increases the work required from the compressor. Newer chillers have automatic air purgers that have run-time meters. Daily or weekly tracking of run time will show if a leak has developed that permits air to enter the system.
Optimize free cooling: If your system has a chiller bypass and heat exchanger, known as a water-side economizer, it should be used to serve process loads during the winter season. The water-side economizer produces chilled water without running the chiller.
Condenser water circulates through the cooling tower to reject heat, and then goes to a heat exchanger bypassing the chiller where the water is cooled sufficiently to meet the cooling loads. Verify Performance of hot-gas bypass and unloader: These are most commonly found on reciprocating compressors to control capacity. Make sure they operate properly. Both low-level and high-level refrigerant conditions can be detected this way. Taken together, these readings serve as a valuable baseline reference for operating the system and troubleshooting problems.
Many newer chillers automatically save logs of these measurements in their on-board control system, which may be able to communicate directly with the DDC system. In some designs, this air is simply drawn into the motor housing by the rotating motor shaft. The vent passages tend to get dirty and clog, resulting in higher operating temperatures and hot spots that adversely affect motor efficiency and reliability.
Other designs, such as totally-enclosed fan- coold TEFC and totally-enclosed air-over TEAO , use a separate fan with a protective housing to cool the motor.
They approach the efficiency and reliability of hermetic motors. Hermetic compressor motors eliminate the need for the shaft couplings and external shaft seals that are associated with open motors.
The coupling needs precise alignment, and these seals are a prime source of oil and refrigerant leaks. On the other hand, if a motor burns out, a hermetic chiller will require thorough cleaning, while an open motor will not.
Controls and Starter A microprocessor-based control panel is provided on the chiller to provide accurate chilled-water control as well as monitoring, protection, and adaptive limit functions. These controls monitor chiller operation and prevent the chiller from operating outside its limits.
They can compensate for unusual operating conditions, keeping the chiller running by modulating system components rather than simply shutting it down when a safety setting is violated. When serious problems occur, diagnostic messages aid troubleshooting. Modern control systems not only provide accurate, optimized control and protection for the chiller, but can also interface with a building automation system for integrated system control.
In a chilled water system, optimal performance is a system-wide issue, not just a matter of chiller design and control. A starter links the chiller motor and the electrical distribution system. Its primary function is to connect start and disconnect stop the chiller from line power—similar to what a switch does for a light bulb. The starter, however, handles much more current and must have the appropriate interlocks to work with the chiller control panel and oil pump.
Every electrically driven chiller requires a starter. It must be compatible with the characteristics of both the compressor motor and the electrical circuitry of the chiller. There are many types of starters, including star-delta, across-the-line, auto-transformer, primary reactor, and solid state. A variable- speed drive, which is used to modulate the speed of the motor during normal operation, also serves as a starter.
Important characteristics to consider when selecting a starter include first cost, reliability, line voltage, and available current. The starter may be mounted on, or remotely from, the chiller. Use of a unit-mounted starter reduces electrical installation costs. It may also improve reliability and save system design time, since all of the components are pre-engineered and factory-mounted.
Depending on the type of starter selected, there are several options that can simplify installation. Disconnects allow the starter to be isolated from the electrical distribution system, and short-circuit protection can be provided using fuses or a circuit breaker. A pressure—enthalpy p-h chart illustrates the refrigeration cycle of the centrifugal water chiller.
Refrigerant vapor leaves the evaporator and flows to the compressor, where it is compressed to a higher pressure and temperature. High-pressure refrigerant vapor then travels to the condenser where it rejects heat to water, and then leaves as a saturated liquid. The pressure drop created by the first expansion device causes part of the liquid refrigerant to evaporate and the resulting mixture of liquid and vapor enters the economizer.
The remaining saturated liquid refrigerant enters the second expansion device. The pressure drop created by the second expansion device lowers the pressure and temperature of the refrigerant to evaporator conditions, causing a portion of the liquid refrigerant to evaporate. The resulting mixture of liquid and vapor enters the evaporator. In the evaporator, the liquid refrigerant boils as it absorbs heat from water and the resulting vapor is drawn back to the compressor to repeat the cycle.
The pressure-enthalpy chart plots the properties of a refrigerant — refrigerant pressure vertical axis versus enthalpy horizontal axis.
Enthalpy is a measure of the heat content, both sensible and latent, per pound [kg] of refrigerant. B represents the heat content of saturated vapor HCFC refrigerant at the same pressure and temperature. The difference in heat content, or enthalpy, between A and B — that is, If the heat content of the refrigerant at any pressure falls to the right of the curve, the vapor is superheated.
Similarly, if the heat content of the refrigerant falls to the left of the curve, the liquid is subcooled. Finally, when the heat content of the refrigerant falls inside the curve, the refrigerant exists as a mixture of liquid and vapor. Pe 1 9 evaporator. Refrigerant leaves the evaporator as saturated vapor c and flows to the first-stage impeller of the compressor. There, the refrigerant vapor is compressed to a higher pressure P1 and temperature d. Cooler refrigerant vapor that flashed within the economizer is mixed with the refrigerant discharged from the first-stage impeller, reducing the heat content of the mixture e.
The second stage of compression further elevates the pressure Pc and temperature of the refrigerant f. Energy provided to the compressor is imparted to the refrigerant as an increase in pressure and superheat. Superheated refrigerant vapor leaves the compressor and enters the condenser.
Water flowing through the condenser absorbs heat from the hot, high-pressure refrigerant vapor, causing it to desuperheat g and condense into saturated liquid h before leaving the condenser to travel to the first expansion device. The first expansion device reduces the pressure h to i of the refrigerant to the second-stage impeller inlet pressure P1.
This pressure drop causes a portion of the liquid refrigerant to evaporate, or flash. The evaporating refrigerant absorbs heat from the remaining liquid refrigerant, reducing its enthalpy from i to j. The resulting mixture of liquid and vapor enters the economizer i. Here, the vapor is separated from the mixture and routed directly to the second-stage impeller inlet e and the remaining liquid travels on to the second expansion device j. Just before it enters the evaporator, the liquid refrigerant flows through a second expansion device that reduces its pressure Pe and temperature to evaporator conditions k.
The cool, low-pressure mixture of liquid and vapor enters the distribution system in the evaporator shell and absorbs heat from water that flows through the tubes. This transfer of heat boils the liquid refrigerant, and the resulting saturated refrigerant vapor is drawn back to the compressor c to repeat the cycle. The change in enthalpy from C to A that occurs during the refrigeration cycle is called the refrigeration effect.
This is the amount of heat that each pound [kg] of liquid refrigerant will absorb when it evaporates. The benefit of the economizer can be demonstrated by comparing the refrigeration cycles with and without an economizer. Without an economizer, refrigerant from the condenser h expands directly to evaporator conditions l, producing a smaller refrigeration effect B to A.
Some chiller designs may subcool the liquid refrigerant in the condenser h moves to the left to increase this refrigeration effect. Also, in a chiller without an economizer, all of the refrigerant vapor must go through both stages of compression to return to condensing conditions. In a chiller with an economizer, refrigerant vapor that flashes in the economizer bypasses the first stage of compression, resulting in an overall energy savings of 3 to 4 percent.
Refrigerants When selecting which refrigerant to use in a centrifugal water chiller, the manufacturer considers efficiency, operating pressures, compatibility with materials, heat transfer properties, stability, toxicity, flammability, cost, availability, and environmental impact. Refrigerants commonly used in centrifugal chillers can be classified as low, medium, or high pressure based on the normal operating pressures in the refrigeration cycle.
Chillers using a high-pressure refrigerant like HCFC, or a medium-pressure refrigerant like HFCa, operate at pressures that are well above atmospheric pressure.
As we are about to see, some sections of chillers that use a low-pressure refrigerant such as HCFC- operate at below-atmospheric pressure. In chillers designed to use a low-pressure refrigerant, the evaporator and the piping leading to the suction side of the compressor operate at pressures that are lower than atmospheric pressure.
Therefore, if small leaks exist in either of these sections, air will leak into the chiller instead of refrigerant leaking out. Air inside a chiller reduces the surface area available for heat transfer.
It also increases the refrigerant pressure in the condenser, which increases the pressure difference required across the compressor and causes more power to be consumed. Finally, infiltration of moist air can cause corrosion and other harmful chemical reactions inside the chiller. Purge System Low-pressure chillers typically include a purge system to remove air and moisture that may leak in, while minimizing the emission of refrigerant.
The purge consists of a small refrigeration system, a pump-out system, controls, and a filter drier. The tank with the evaporator coil separates condensable refrigerant from noncondensable air. Because the purge evaporator operates at a lower temperature and pressure than the chiller condenser, a mixture of refrigerant vapor and air is drawn from the chiller condenser, just above the level of the liquid refrigerant.
This is where air typically concentrates in a low-pressure chiller. The mixture enters the purge tank, and the refrigerant condenses on the cold evaporator tubes and returns to the chiller condenser as liquid.
The air does not condense but instead accumulates in the top of the tank. Eventually, enough air accumulates to cover a large portion of the coil. The air insulates the coil, reducing the amount of heat transferred and the temperature of the refrigerant leaving the purge evaporator coil.
This temperature is called the purge suction temperature. The drop in purge suction temperature signals the need for a pump-out sequence. When the purge suction temperature drops below the set point, a controller turns on the pump-out compressor and opens the isolation valves.
Since the air contains a very small amount of refrigerant, it is pumped from the purge tank into a filtration canister. This canister adsorbs nearly all of the remaining refrigerant, and the air is then piped to the chiller vent line. When the purge suction temperature rises again, the controls close the valves and turn off the pump-out compressor. A filter drier is located in the refrigerant drain line, between the purge tank and the chiller condenser.
The filter drier removes moisture, acid, and dirt from the liquid refrigerant before it returns to the condenser. The purge controls can also be used to track and record how often pump-out occurs. Leaks can be detected early by comparing pump-out activity over the last 24 hours to the day average.
The capacity of most centrifugal compressors is controlled by vanes at the inlet of the compressor impeller. While a survey of other centrifugal compressor designs shows that there are various methods of capacity control, many of them function in a manner similar to the inlet vanes presented in this period. Inlet vanes, installed ahead of the impeller, are a common method of modulating the capacity of the compressor over a wide range of load conditions refrigerant flow rates.
As a result, each inlet vane position creates a new compressor performance characteristic without changing the rotational speed of the impeller. This example shows 2 impellers in series. These impellers share the task of compressing the refrigerant. Centrifugal water chillers are generally available with 1, 2, or 3 impellers. The forces that act on the refrigerant vapor within the centrifugal compressor impeller can be broken down into 2 components. One component acts to move the refrigerant away from the impeller in a radial direction.
This component is called radial velocity Vr. The second component acts to move the refrigerant in the direction of impeller rotation. This component is called tangential velocity Vt.
Together, these components generate the resultant velocity vector R , the length of which is proportional to the amount of kinetic energy in the refrigerant. Recall that kinetic energy is converted to static energy, or static pressure. The tangential velocity Vt is proportional to the product of impeller rotational speed and impeller diameter. Therefore, the static-pressure-producing capacity of a compressor can be adjusted by changing the flow rate of refrigerant, the impeller speed, or the diameter of the impeller.
Consider a given-diameter compressor impeller that rotates at a constant speed. As the load on the chiller decreases, the inlet vanes partially close and the flow rate of refrigerant through the compressor drops. Radial velocity Vr , which is proportional to refrigerant flow, decreases as well. Even though the speed of rotation and diameter of the impeller are constant, the tangential velocity Vt which is proportional to the product of impeller rotational speed and impeller diameter drops because of the pre-swirling of the refrigerant caused by the inlet vanes.
The result is a shorter resultant velocity vector R , which means that less static pressure is generated. As the load and the corresponding refrigerant flow rate continue to fall, the radial velocity force drops, too. At some point, the radial force becomes smaller than the generated static pressure, letting the pressurized refrigerant vapor flow backward from the diffuser passages into the impeller. This instantaneously reduces the pressure within the passages below the radial force and the compressor is able to re-establish the proper direction of refrigerant flow.
This condition is known as surge. So long as this unstable load condition exists, the refrigerant alternately flows backward and forward through the compressor impeller, generating noise and vibration. These curves represent the performance of a typical 2-stage compressor over a range of inlet vane positions.
The pressure difference between the compressor inlet evaporator and outlet condenser is on the vertical axis and the refrigerant flow rate is on the horizontal axis.
The dashed line represents the conditions that cause the compressor to surge. Any operating point that falls to the right of this line is satisfactory for stable operation. To balance the load on the chiller, the compressor must pump a certain quantity of refrigerant vapor at evaporator pressure and elevate it to the pressure dictated by the condensing conditions.
The intersection of the refrigerant flow rate and the pressure difference between the inlet and outlet of the compressor identifies the compressor operating point. Superimposing the operating point on the previous compressor performance curves establishes the point at which the compressor will balance the load.
The starting point A is the full-load operating point. As the load on the chiller decreases, the inlet vanes partially close, reducing the flow rate of refrigerant vapor produced within the evaporator and balancing the chiller capacity with the new load B. Less refrigerant, and therefore less heat, is transferred to the condenser. Since the heat rejection capacity of the condenser is now greater than required, the refrigerant condenses at a lower temperature and pressure.
This reduces the pressure difference between the evaporator and the condenser. Continuing along the unloading line, the compressor remains within its stable operating range until it reaches the surge region at C. An adjustable-frequency drive AFD , or variable-speed drive, is another device used to vary the capacity of a centrifugal compressor.
AFDs are widely used with fans and pumps, and with the advancement of microprocessor-based controls for chillers, they are now being applied to centrifugal water chillers. It will, however, offer energy savings by reducing motor speed at low-load conditions when cooler condenser water is available.
An AFD also controls the inrush current at start-up, reducing stress on the compressor motor. Depending on the application, it may make sense to take the additional money needed to purchase an AFD and use it to purchase a more efficient chiller instead. This period discusses general maintenance requirements of centrifugal water chillers.
Although some of the information applies specifically to the design presented in this clinic, requirements for other centrifugal chiller designs are also included. Once a centrifugal chiller is installed and put into operation, it usually continues to function without a full-time attendant. In many cases, the machine starts and stops on a schedule controlled by a building automation system or a simple time clock. The only daily maintenance requirement is to complete and review the operating log.
Water chillers are designed for maximum reliability with a minimum amount of maintenance. Like all large mechanical systems, however, certain routine maintenance procedures are either required or recommended.
This guideline includes a list of recommended data points to be logged daily for each chiller. Much of this data may be available from the display on the chiller control panel. The hermetic motor eliminates the need for external shaft seals associated with open motors. These seals are a prime source of oil and refrigerant leaks and should be inspected on a regular basis.
Hermetic motor designs also eliminate the annual coupling and seal inspections, alignment, and shaft seal replacement associated with open motors. With the advent of microprocessor-based controls, the control panel and auxiliary controllers require no recalibration or maintenance. Remotely-mounted electronic sensors send information to the unit controller, which can be connected to a building automation system to communicate information and allow system-level optimization.
These systems can notify the operator with an alarm or diagnostic message when a problem occurs. As for any mechanical equipment, a daily visual inspection of the chiller is recommended to look for oil leaks, condensation, loosened electrical or control wiring, or signs of corrosion. Special attention should be given to safety controls and electrical components.
A qualified service technician should check the chiller annually for leaks. The United States Environmental Protection Agency EPA mandates refrigerant recovery whenever a refrigeration circuit is opened during the normal service of any air conditioning system.
Some centrifugal compressor designs do require periodic maintenance of mechanical system components. This includes oil and refrigerant filter changes, oil strainer changes, and a compressor inspection. Open-motor compressor designs require shaft alignment, coupling inspection, bearing lubrication, and cleaning of the motor windings on a quarterly or annual basis. In all cases, strictly follow the maintenance requirements and recommendations published by the manufacturer. To ensure optimum heat transfer performance, the heat transfer surfaces must be kept free of scale and sludge.
Even a thin deposit of scale can substantially reduce heat transfer capacity. Engage the services of a qualified water treatment specialist to determine the level of water treatment required to remove contaminants from the cooling tower water. Scale deposits are best removed by chemical means. During this process, the water- cooled condenser is commonly isolated from the rest of the cooling-tower-water circuit by valves, while a pump circulates cleaning solution through the condenser tubes.
Sludge is removed mechanically. This typically involves removing the water boxes from the condenser and loosening the deposits with a stiff-bristled brush.
The loosened material is then flushed from the tubes with clear water. As part of this procedure, the strainers in both the chilled-water and cooling-tower-water circuits should be cleaned every year. Every 3 years more frequently in process or critical applications , a qualified service organization should perform nondestructive inspections of the evaporator and condenser tubes.
The eddy-current tube test is a common method. Rarely, problems may arise that cause refrigerant or water leaks.
These must be repaired immediately. Oil analysis is an important annual maintenance task required for centrifugal water chillers. It may be conducted more frequently for chillers that run continuously or more often than normal. This test, performed by a qualified laboratory, verifies the integrity of the refrigeration system by testing the concentrations of moisture, acidity, and metal. This analysis can determine where problems exist or could potentially develop. By taking oil samples on a regular basis, normal operating trends for the compressor and bearing metals can be analyzed.
Refrigerant analysis measures contamination levels and determines suitability for continued use. It can also determine if recycled refrigerant is suitable for reuse. Refrigerant analysis helps extend the life of the existing charge and ensures that the chiller is operating at peak efficiency.
Regularly logging oil and refrigerant charges, and examining the trends of this data, can help identify potential problems before they occur. An oil analysis is a key preventive maintenance measure and should be conducted at least annually. It will help the compressor last longer while maintaining chiller efficiency and reducing refrigerant emissions. A certified chemical laboratory can be contracted to perform the analysis for all types of compressors.
Often the chiller manufacturer can provide this service. Condensing Temperature Control To achieve stable compressor unloading over a wide range of conditions, a reduction in condensing pressure condenser relief must accompany a reduction in load. As the chiller load decreases, the flow rate of refrigerant vapor through the compressor also decreases. In turn, the pressure difference between the evaporator and the condenser moves the operating point downward toward B.
If the condenser pressure had been controlled to a constant value instead, the compressor would have unloaded along a nearly constant pressure line toward C. This would result in a greatly reduced range of operation. Condenser relief, however, is only beneficial to a certain point. ALL chillers require a minimum pressure difference between the evaporator and condenser to ensure proper management of oil and refrigerant.
The most common method of maintaining this pressure difference at various load conditions is to control the condensing temperature by varying the temperature or flow rate of water through the condenser. By controlling condensing temperature, most centrifugal water chillers can start and operate over a wide range of conditions.
Controlling condensing temperature: 1 maintains chiller efficiency, 2 maintains the required pressure differential between the evaporator and condenser for controlled flow through the refrigerant metering system, and 3 prevents the pressure imbalance that could cause oil loss problems.
Controlling the refrigerant pressure difference between the evaporator and condenser of a water-cooled chiller is accomplished by varying the temperature or flow rate of the water flowing through the condenser. The following are 5 common methods used to control condensing temperature: 1 Cycling or varying the speed of the cooling tower fans to control the temperature of the water leaving the cooling tower 2 Using a cooling tower bypass pipe to mix warmer leaving-condenser water with the colder tower water and control the temperature entering the condenser, as illustrated here 3 Modulating a throttling valve to restrict the flow of water through the condenser 4 Using a chiller bypass pipe to vary the flow rate of water through the condenser 5 Using a variable-speed drive on the condenser water pump to vary the water flow rate through the condenser.
Each of these strategies has its advantages and disadvantages. Selecting the appropriate condensing temperature control scheme will depend on the specific requirements of the application.
The water flow rate through the chiller condenser must stay between the minimum and maximum condenser bundle flow rates specified by the chiller manufacturer. Constant or Variable Evaporator Water Flow Previous chiller designs required that a constant flow rate of water be maintained through the evaporator. This requirement has changed due to advances in chiller controls. Increased sensing and control capabilities now allow chiller manufacturers to design controls that monitor, and respond faster to, fluctuating conditions.
While the chiller may be able to handle variable water flow through the evaporator, the specific application of the chilled water system may not warrant variable flow.
As always, each application should be analyzed to determine if variable evaporator water flow is warranted. The controls on many current chiller designs can properly control the chiller in response to varying evaporator flow rates, with the following limitations:. These limits depend on the specific design variables of the actual evaporator bundle such as the number of tubes, number of passes, and geometry.
Implementation of a method for sensing evaporator water flow through each chiller is the only way to make sure that the water flow rate stays within these limits. For example, the maximum rate of change to maintain the chilled water set point is more stringent than the maximum rate of change to keep the chiller on line.
There are 3 common levels of protection desired: maintaining chilled water set point control, keeping the chiller on line, and protecting the chiller from damage. The limits for these different levels of protection should be obtained from the chiller manufacturer. Short Evaporator-Water Loops Proper chilled water temperature control requires that the temperature of the chilled water returning to the evaporator not change any faster than the chiller controls can respond.
The volume of water in the evaporator loop acts as a buffer, ensuring that the return water temperature changes slowly and, therefore, providing stable temperature control. If there is not a sufficient volume of water in the loop to provide an adequate buffer, temperature control can be lost, resulting in erratic system operation.
The chiller manufacturer should be consulted for volume requirements of the evaporator- water loop. Short water loops may be unavoidable in close-coupled or very small applications, particularly in systems where the load consists of only a few air handlers or processes. To prevent the effect of a short water loop, a storage tank or large header pipe can be added to the system to increase the volume of water in the loop and ensure a slowly changing return water temperature.
A second solution is to reduce the water flow rate in the chilled water loop while using the same size pipes. This also increases the loop time—the time it takes a particle of water to travel through the chilled water loop—and ensures that the return water temperature changes slowly. This solution has the added benefit of reduced pumping energy requirements.
Heat Recovery Salvaging usable heat from the refrigeration cycle—heat that would normally be rejected to the atmosphere—can significantly reduce the operating costs of many buildings. Heat recovery is most commonly accomplished using 2 condensers and the fact that hot refrigerant vapor migrates to the area with the lowest temperature. Raising the refrigerant condensing temperature in the standard condenser prompts the refrigerant to flow instead to the second condenser, where it rejects its heat to the water flowing through the tubes.
The condensing temperature in the standard condenser is controlled by varying the temperature or the flow rate of the cooling tower water. Typical uses for the hot water from the second condenser include: heat for spaces around the perimeter of the building, reheat coils in air conditioning systems, and bathroom, laundry, or kitchen requirements.
Any building with a simultaneous heating and cooling load is a potential candidate for heat recovery.
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