HVAC Clinic Helical Rotary & Centrifugal Water Chillers
Table Of Contents Introduction... 3 Chiller Types... 3 Chiller Components... 4 Refrigeration Cycle... 15 Application Considerations... 19
Introduction Two types of chiller technologies dominate the large chiller landscape. For the purposes of this discussion, large tonnage chillers will be chillers with a nominal capacity rating greater than 200 tons. The two dominant technologies utilized in large tonnage chillers are helical rotary chillers and centrifugal compression chillers. A third chiller technology, absorption refrigeration chillers, comprises a small percentage of the North American chiller market. Absorption chiller technologies will not be discussed in this clinic. Figure 1. Helical Rotary and Centrifugal Chillers Chiller Types Types Of Helical Rotary Chillers Helical rotary chillers can be either water cooled or air cooled (figure 2). Air cooled chillers are typically available from 150 to 500 ton at efficiencies from 1.0 kw/ton to 1.3 kw/ton. Water cooled helical rotary chillers are typically available from 100 to 675 tons at efficiencies from 0.55 kw/ton to 0.7 kw/ton at ARI conditions. Helical rotary chillers, either air or water cooled, are ideally suited for small to medium sized applied systems. Air cooled systems are less costly to install and require less maintenance, largely due to the lack of water treatment, compared to equivalent water cooled systems. However, air cooled systems operate at lower efficiencies compared to comparably sized water cooled systems. In addition, water cooled systems may employ a waterside economizers, a feature that is relatively expensive to employ with air cooled systems. Figure 2. Helical Rotary Chillers WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 3 of 25
Types Of Centrifugal Chillers Centrifugal chillers, due to their larger inherent size and lower head generating capabilities, are generally limited to water cooled and evaporative condensed applications (figure 3). However, with the advent of smaller magnetic levitation machines, some air cooled magnetic levitation centrifugal chillers are produced by a few select manufacturers. Water cooled centrifugal chillers are typically available from 100 tons to 6000 tons. Efficiencies, for water cooled machines, range from 0.45 kw/ton to 0.6 kw/ton. Magnetic levitation air cooled centrifugal chillers can operate at efficiencies as low as 0.9 kw/ton at full load and 0.45 kw/ton at 50% load. Figure 3. Centrifugal Chillers Chiller Components Helical Rotary Compressors Helical rotary chillers are generally comprised of an evaporator, one or multiple twin rotary compressors, a motor (either open or suction gas cooled), an oil separator, condenser, an electronic expansion device, evaporator, starter and control panel (figure 4). Each of these components will be explored in detail. Figure 4. Helical Rotary Chiller Components Two general types of vapor compression compressors are predominantly used in the HVAC industry. Those types are positive displacement compressors and dynamic compression compressors. Positive displacement compressors reduce the amount of refrigerant volume within the compression chamber, thus increasing its pressure. Dynamic compression cycles utilize the principle of static regain (as discussed in the duct clinic) to increase static pressure through changes in velocity pressure. Helical rotary compressors are positive displacement type compressors. A helical rotary compressor traps the refrigerant vapor between a male and female rotor and compresses it by decreasing the volume of the refrigerant (figure 5). WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 4 of 25
Figure 5. Male and Female Rotors The majority of helical rotary compressor designs use two mating screw rotors to perform the compression process. The male rotor is driven by the compressor motor. The lobes of the male rotor make contact and drive the female rotor, causing the two rotors to counter rotate. Refrigerant vapor enters the compressor housing through the intake port and fills the pockets formed by the lobes of the rotors (figure 6). As the rotors turn, they push these pockets of refrigerant toward the discharge end of the compressor. The continued rotation of the rotors forces the refrigerant to meshing point. It is at this point that the refrigerant begins to compress (point D). Finally, when the pockets of refrigerant reach the discharge port, the compressed vapor is released. Figure 6. Compression Process Centrifugal Compressors Unlike helical rotary compressors, which work on the principle of positive displacement compression, centrifugal compressors operate on the principle of dynamic compression. Dynamic compression involves the conversion of energy, from one form to another. In the case of a centrifugal compressor, kinetic energy is converted to static energy. The main component of a centrifugal compressor is the impeller. In a closed face impeller, relatively low pressure refrigerant vapor is drawn into the eye of the impeller. Within the impeller body are blades that are fitted between two enclosed surfaces (for closed faced impellers). The rotation of the impeller causes the refrigerant vapor to accelerate within the blades (impeller passages), thus increasing its velocity. As the refrigerant velocity is increased, its kinetic energy is increased. Figure 7. Centrifugal Impeller WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 5 of 25
After exiting the impeller, high velocity, high kinetic energy refrigerant enters the diffuser passages (figure 8). These passages initially match the outlet area of impeller outlet. However, as the refrigerant passes down the length of the diffuser, the passages gradually increase in size. As the passages increase in size, the refrigerant velocity decreases. Following the first law of thermodynamics, which states energy cannot be destroyed, only converted from one form to another; the refrigerant energy is converted from kinetic to static energy. This employs the same phenomenon employed with duct static regain, as discussed in previous clinics. Figure 8. Diffuser & Volute The higher pressure refrigerant, as it exits the diffuser, collects in the volute. Like the diffuser, the volute increases in volume as the refrigerant travels through its passages which further converts kinetic or velocity energy to static energy. Finally, the refrigerant exits the compressor and travels to the condenser (figure 9). Figure 9. Compression Cycle A centrifugal compressors part load capacity can be modulated with inlet guide vanes, a variable frequency drive, or a combination of both. Traditionally, inlet guide vanes were used to control chiller capacity. Inlet guide vanes prespin the refrigerant before it enters the impeller passages. This pre-spinning of refrigerant reduces the grab or dynamic energy the impeller can impart on the incoming refrigerant stream (figure 10). Figure 10. Inlet Guide Vanes WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 6 of 25
Newer centrifugal compressor designs have incorporated variable frequency drives (VFD) to modulate the compressor speed at part load. Centrifugal compressors utilize dynamic compression which lends itself very well to the use of VFD s. In the past, the cost of VFD s made them prohibitively expensive to utilize in centrifugal compressor designs. In recent years, the cost of VFD s has been reduced substantially. While variable frequency drives present a slight penalty at full load performance, they offer substantial energy savings at part load. At part load, the variable frequency drive is modulated in conjunction with the inlet guide vanes in order to optimize efficiency and chiller operation. A recent technological advancement to centrifugal compressors involves the use of magnetic levitation bearings. Within a magnetic levitation centrifugal, the main drive shaft and impeller are magnetically levitated through the use of specially designed magnetic bearings (figure 11). Much like a magnetic levitation high speed train, the shaft and impeller float within a magnetic cushion, eliminating any contact friction. Implementing magnetic bearings can dramatically improve efficiency, reduce noise and improve reliability. In addition, systems that utilize magnetic drives do not require oil. The oil in a system acts as an added barrier to heat transfer. Oil coats heat exchange surfaces, both the evaporator and condenser, decreasing efficiency. In addition, oil-less systems simplify the machine design (the oil management system is removed) and reduces maintenance. Inlet guide vanes are generally not used in magnetic bearing designs. Furthermore, magnetic bearing machines often operate at motor speeds well above 3600 RPM (generally between 20,000 and 45,000 RPM), requiring the use of frequency drives in order to achieve the required motor speed (figure 11). Figure 11. Magnetic Levitation Compressor Magnetic drive machines may use multiple smaller open faced impellers which rotate at very high speeds (>40,000 RPM) or a single larger closed faced impeller which rotates at a slower speed (20,000 RPM). Generally speaking, a larger closed face impeller will offer better aerodynamics compared to a similar capacity machine utilizing multiple open faced impellers. The aerodynamics of an impeller is directly related to the chillers efficiency. Oil Management Following the compressor, the refrigerant may be fed into an oil separator. Oil separators are utilized is most helical rotary and many centrifugal chiller designs. The oil separator is a centrifugal device that separates the oil from refrigerant vapor (figure 12). The centrifugal motion of the refrigerant vapor forces the oil outward to the walls of the cylinder. The oil collects on the walls, eventually draining to the bottom of the device and collects in the oil sump and is recycled through the system. Generally, the oil sump is heated in order to ensure proper lubrication and to minimize condensation. The oil free refrigerant discharges through the top of the oil separator and passes to the condenser. A chillers oil separator is generally capable of separating more than 99% of the systems oil from the refrigerant vapor. Figure 12. Oil Separator WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 7 of 25
The oil that discharges the compressor is at condensing pressure. At this higher pressure, the oil is always trying to flow to the lower pressure components of the system. After the oil discharges the oil separator, it passes through an oil management system. The first component of the oil management system is generally the oil reservoir. Here the oil collects before being distributed to the gears (if machine is gear driven), bearings and shaft seal (if open drive). The oil reservoir is often located above the compressor such that oil can be gravity fed to the system in case of power failure. After the oil is delivered to the critical components, it collects in the oil sump. Within the sump, some manufacturers utilize an oil pump to maintain oil pressure. Other manufacturers choose to use the difference in pressure between the condenser and evaporator to create the differential pressure required to adequately deliver oil to critical components. Generally speaking, chillers which employ oil pumps are able to run at lower condenser pressures as they do not require differential pressure to move oil. This enables these machines to run at lower entering condenser water temperatures, without the use of head pressure control. Finally, the oil passes through an oil filter before completing the cycle (figure 13). Figure 13. Oil Management System While oil separators are very efficient, they are not 100% efficient. Thus, the refrigeration system must be designed to adequately move the oil throughout the entire system. This includes the condenser, evaporator and refrigerant connections. Condensers After the refrigerant vapor discharges the oil separator, it is directed to the condenser. Both helical rotary and centrifugal water cooled chillers generally utilize a shell and tube condenser. In a shell and tube condenser, water is allowed to run through the interior of the tubes that form the heat exchanger while refrigerant vapor fills the shell space surrounding the tubes (figure 14). By allowing the condenser water to run through the tubes, maintenance personnel can clean the tubes in the event of scale formation. Alternately, if the refrigerant was to run through the tubes, the heat exchanger would not be cleanable. As heat is absorbed by the cooler condenser water, the refrigerant vapor condenses on the tube surfaces. The condensed refrigerant liquid then falls to the bottom on the shell. At the bottom on the shell, additional tubes called the subcooler further cool the liquid refrigerant. The subcooler prevents liquid refrigerant from flashing before it enters the expansion device. As the refrigerant vapor condenses, the condenser water temperature will be increased and returned to the heat rejection device. Figure 14. Condenser WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 8 of 25
Alternately, an air cooled chiller employs an air cooled condenser in lieu of a water cooled condenser. Air cooled condensers generally use propeller-type fans to draw outdoor air over a finned-tube heat transfer surface. The temperature difference between the hot refrigerant vapor that is flowing through the tubes and the cooler outdoor ambient air induces heat transfer. The resulting reduction in the heat content of the refrigerant vapor causes it to condense into liquid. Located in the bottom, last few rows of condenser tubing is the subcooler. In the subcooler, the refrigerant is cooled below the saturation temperature or subcooled (figure 15). Figure 15. Air Cooled Condenser Evaporative condensers provide many of the energy advantages of water cooled systems while simplifying the design of the system. An evaporative condensers efficiency is a function of the ambient wet bulb temperature. Thus, in drier climates, machines with evaporative condensers can provide tremendous energy efficiency gains (figure 16). In addition, due to the reduced condenser pressures experienced with evaporative condensing machines, centrifugal compressors (including magnetic levitation) can be utilized. Figure 16. Evaporative Condensed Magnetic Levitation Chiller An evaporative condenser functions by evaporating water as it is transferred across a warm heat exchanger (condenser). As discussed in previous clinics, the heat of vaporization is much higher than typical sensible only cooling. Hence, an evaporative condensed heat exchanger can function much more efficiently that a comparable dry heat exchanger. However, one of the inherent disadvantages to any evaporative process is the deposition of solids that occurs as water evaporates. Some manufacturers utilize a dry sensible pre-cooler to pre-cool the hot discharge refrigerant before it enters the wet evaporative condensed heat exchanger (figure 17). The formation of solids is decreased as the approach temperature between the two fluid streams is decreased. When utilized, a dry pre-cooled can virtually eliminate most scale formation. Water treatment may still however be required, depending on the quality of water being used in the evaporative condenser. WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 9 of 25
Figure 17. Evaporative Condenser Refrigerant Metering Device After the liquid refrigerant exits the condenser subcooler, it passes through some type of expansion device. The expansion device maintains the pressure difference between the low side and the high side of the machine. This device is commonly either an orifice type expansion device or an electronic expansion device. Both modulating orifice and electronic expansion devices are common to helical rotary chillers. Conversely, fixed orifice expansion devices are typically employed with centrifugal chillers. The expansion device controls the refrigerant flow in order to maintain a fixed amount of superheat exiting the evaporator. Without proper superheat, liquid may enter the compressor, causing the potential for premature failure. Figure 18. Expansion Device Evaporator Saturated refrigerant exits the expansion device and enters the evaporator. The evaporator in a helical rotary chiller may be either a direct expansion or flooded type evaporator. Centrifugal chillers, in contrast, generally utilize flooded evaporators. In a direct expansion type evaporators, the refrigerant flows through the tubes while the water flows over the outside of the tubes. Conversely, in a flooded type evaporator, the water flows through the tubes and the refrigerant is distributed in a uniform manner over the outside of the tubes (figure 19). WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 10 of 25
Figure 19. Evaporator Most contemporary water and air cooled helical rotary compressors utilize flooded type evaporators. Flooded evaporators offer the advantage of being both cleanable and offering a smaller capacity and efficiency degradation when operating with glycol. Air cooled chillers utilizing flooded evaporators becomes especially important when in colder climates. In colder climates, glycol is often an operating requirement in order to prevent freezing of components and ultimately damage to the system. Air cooled machines employing direct expansion type evaporators will experience a significant capacity and energy efficiency reduction when running glycol. A variation of the flooded evaporator is the falling film type evaporator (figure 20). In a typical flooded evaporator, the refrigerant enters at the bottom shell (figure 19). In contrast, in a falling film flooded evaporator, the refrigerant is sprayed over the top of the tube bundle rather than entering through the bottom of the heat exchanger. The evaporator contains two tube bundles. The first, the falling film tube bundle, contains the first tubes that make contact with the refrigerant which is distributed by means of the spray header. After the refrigerant passes over the falling film tube bundle, any remaining liquid refrigerant settles to the bottom of the heat exchanger and transfers heat to the flooded tube bundle. Refrigerant vapor passes through the suction slot and finally passes through the suction flange and on to the compressor. Figure 20. Falling Film Evaporator A falling film evaporator, compared to a traditional flooded evaporator, generally reduce the total refrigerant charge by as much as 40%. In addition, the improved heat exchanger characteristics of a falling film evaporator will generally improve the efficiency characteristics of the chiller. If air cooled machines with direct expansion type evaporators are used, the designer should consider locating the evaporator inside the conditioned building so as to avoid the use of glycol. This option, known as a remote evaporator, is available with several manufacturers of air cooled helical rotary chillers (figure 21). While this type of WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 11 of 25
installation may increase the installation cost, due to the cost of the added refrigerant piping, the payback on investment achieved by avoiding the use of glycol should be significant. Figure 21. Remote Evaporator Starters Chillers cause large electrical loads with a large instantaneous inrush current. Because of this instantaneous electrical load, helical rotary and centrifugal chillers cannot be started and stopped using a contact closure. A starter s primary function is to connect and disconnect the chiller from the incoming line. Three types of starters are common with helical rotary and centrifugal chillers. These are: Electro Mechanical Starters o Wye-Delta closed transition starter Electronic Starters o Solid State Starter o Variable Frequency Drive Figure 22. Starter Types In a typical across the line starter, the jobsite full load amps (FLA) are the same as the motor full load amps (figure 23). WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 12 of 25
Figure 23. Across The Line Starter The amp draw during startup, for the first few fractions of a second, is very large. This is due to the energy required to overcome static friction at startup. This large amp draw condition is referred to as locked rotor amps (figure 24). Figure 24. Amp Draw of Across the Line Starter A wye-delta starter is type of reduced voltage starter that makes use of the characteristics of wye and delta connections to reduce voltage without lowering current (figure 25). A wye-delta starter reduces inrush current by 66% compared to an across the line starter. Star-Delta starters require a special six lead motor which allows connection of the three phase windings in a Star connection or Delta connection. Figure 25. Star-Delta Starter WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 13 of 25
During starting, the motor is connected in a Star or Wye configuration. This reduces the voltage to the motor stator by a factor of three. This 1/3 voltage results in 1/3 current into the motor at start and 1/3 torque to the shaft. Centrifugal compressor starting torque requirements are low enough to allow the motor to start at 1/3 of full load torque. If line voltage is low as in a brown-out condition, the motor may not have sufficient starting torque to reach full speed until the delta transition occurs, resulting in a large current spike and mechanical torque shock to the driveline. Essentially, this condition causes the motor to start across the line after pulling 1/3 LRA from the line for 40 seconds. Obviously, this is less than desirable condition. Star-Delta starting creates stresses for the starter's switchgear, building electrical system, power grid, and chiller mechanical driveline. Although these stresses are 1/3 of the stresses generated by an ACL starter, they cause wear on the system. Star-Delta starters require periodic maintenance and are only available for low voltage motors. Solid state starters utilize silicone controlled rectifiers (SCR s) to control current levels at startup. This method is more expensive than across the line or wye-delta starters, but offers variable voltage and torque. Solid state starters reduce motor stress by softly ramping inrush current. The solid state starter provides a soft continuous current to the chiller motor during motor starting, limiting the inrush of current to a programmed starting value, by reducing the voltage to the motor during startup. This reduced voltage is accomplished when the silicone controlled rectifiers (SCRs) are turned on in a phased back mode during motor acceleration. As the motor accelerates and the inrush of current begins to drop, the SCR devices are fired with less delay time so that more of the AC main voltage is conducted. Once the motor is up to speed there is no longer any delay applied to the firing signals. The SCR devices are turned on full voltage is applied the motor. Figure 26. Solid State Starter Finally, variable frequency drives are becoming relatively prevalent in the design of helical rotary drives. Variable frequency drives modulate the incoming frequency in order to softly start the machine. Power is directly related to frequency in a linear fashion. For example, if we cut the frequency in half, the power delivered to the motor is cut in half. Thus, by modulating the frequency, a variable frequency drive can vary the power and torque delivered to a motor (figure 27). Figure 27. Variable Speed Drive WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 14 of 25
Variable frequency drives have the added benefit of being able to help control the part load capacity of the machine. Additionally, the starting current profile for a VSD shows a smooth increase in current as power is applied to the motor (this is similar to, but better than a solid state starter). Because the VSD supplies power at variable frequencies, starting power is applied at very low frequency yielding much higher torque than with a solid state starter. Therefore, the starting current does not exceed the full load value for the motor. This profile is the least stressful to the motor, drive line, and electrical system. Figure 28. VSD Starting Profile Control Panel The last component common to all helical rotary and centrifugal chillers is the unit control panel. The control panel is the brain that operates the chiller. The control panel governs the operation of the compressor in coordination with the expansion device and condenser fans (if air cooled) in order to maintain the leaving chilled water temperature. In addition, the control panel monitors the chillers operation and prevents it from operating outside of its limits. Finally, the chiller control panel should provide a simple means by which building operators can diagnose and troubleshoot the machine. Most modern chillers utilize a large easily read display, some of which utilize touchscreen technology, to present the information to the building operator (figure 28). Figure 29. Control Panel Refrigeration Cycle A typical vapor compression refrigeration cycle as it applies to a typical helical rotary or centrifugal chiller is demonstrated in figure 30. First, cool, low pressure saturated liquid (A) plus vapor enters the evaporator. The evaporator absorbs heat from the warmer the conditioned fluid, superheating the refrigerant. The superheated refrigerant vapor is then drawn in to the suction side of the helical rotary compressor (B). The compressor draws in the superheated refrigerant vapor (B) and compresses it to a higher pressure and temperature (C) such that it can reject heat to another fluid. This hot, high-pressure refrigerant vapor then travels to the condenser. As the refrigerant passes through the condenser, heat is transferred from the hot refrigerant vapor to relatively cool heat rejection fluid (water if water cooled or air if air cooled). This reduction in the heat content of the refrigerant vapor causes it to desuperheat, condense into a liquid, and finally subcool before leaving the condenser (D) for the expansion device. WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 15 of 25
Finally, the high-pressure liquid refrigerant (D) flows through the expansion device. The flow through the expansion devices causes a large pressure drop that reduces the pressure of the refrigerant to that of the evaporator. This pressure reduction causes a small portion of the liquid to boil off, or flash, cooling the remaining refrigerant to the desired evaporator temperature. The cooled mixture of liquid and vapor refrigerant then enters the evaporator (A) ad repeats the cycle (figure 30). Figure 30. Vapor Compression Refrigeration Cycle The vapor compression refrigeration cycle described in figure X is common to a large percentage of helical rotary chiller designs. However, adaptations to the cycle have been incorporated by some manufacturers in order to improve the efficiency of the cycle. A shortcoming to the vapor compression refrigeration cycle described in the pressure enthalpy chart of figure 30 is that some refrigeration effect is lost due to flashing of liquid refrigerant within the saturation dome. This flashing occurs due to the nature and slope of the saturated liquid line curve. Any degree of subcooling prior to the expansion device will cause an excess of saturated vapor prior to the evaporator. This shortcoming can be alleviated by installing a refrigeration economizer cycle (figure 31). Figure 31. Economizer Cycle Before refrigerant enters the expansion device, any refrigerant vapor is allowed to separate from liquid refrigerant in an economizer flash tank. As the chiller loads, the economizer feed valve opens, allowing the medium pressure vapor part way through the compressor rotors. Viewed on a pressure-enthalpy chart, subcooled liquid refrigerant exits the condenser (1). The subcooled liquid refrigerant experiences a pressure drop as it passes through the refrigerant piping and feed valve, causing some refrigerant to flash into vapor (point 1 to 2). Medium pressure vapor is fed to the compressor rotors (point 2 to 6) while liquid refrigerant is allowed fall to the bottom of the economizer flash tank (point 2 to 3). The liquid refrigerant passes WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 16 of 25
through the expansion device before entering the evaporator (point 3 to 4). The difference in refrigeration effect, due to the implementation of the economizer cycle, is shown by R. This added refrigeration effect increases the capacity and efficiency of the helical rotary chiller. Figure 32. Helical Rotary Economizer PH Cycle Similarly, an economizer cycle if often utilized with Centrifugal chiller applications. In order to utilize an economizer cycle within a centrifugal chiller, the chiller must either implement multiple impellers or multiple compressors. Much like a helical rotary machine, a centrifugal chiller economizer cycle will flash some degree of refrigerant vapor during the expansion process (point 1-2). This flashed refrigerant vapor is then fed to the compression cycle (figure 33). This lower energy vapor is fed at some intermediary point in the compression cycle, lowering the heat of compression during the compression cycle (point 6-7). Figure 33. Centrifugal Economizer PH Cycle This lowered heat of compression further increases the efficiency of the refrigerant cycle. It is at this point that economizer designs may vary slightly among the manufactures which employ centrifugal chillers with economizer cycles. Much like a helical rotary chiller, the vapor which is flashed during the expansion process WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 17 of 25
should be introduced at an equal pressure point during the compression cycle. In a helical rotary chiller, that involves introducing the refrigerant vapor at some intermediary point along the length of the rotary compressors. However, the inherent design of a centrifugal compressor does not allow for a means of introducing refrigerant vapor at an intermediary compression point. Thus, economizers utilized with centrifugal chiller designs require multiple stages of compression. This can be accomplished with either multiple impellers connected to a single motor or multiple impeller and motor combinations (figure 34). Figure 34. Centrifugal Economizer Cycle In either scenario, both the refrigeration effect is increased and the heat of compression is effectively decreased. This has the effect of both increasing the capacity and efficiency of the chiller. Thus, if multiple stages of compression are available, an economizer cycle is an effective means of increasing chiller capacity and efficiency. Helical Rotary Compressor Modulation Helical rotary compressor can be modulated with either a slide valve or a variable frequency drive. The position of the slide valve along the rotors controls the volume of refrigerant vapor delivered by the compressor. It does this by varying the amount of rotor length that can be used for compression (figure 35) by exposing the chamber that would otherwise be trapped by the refrigerant vapor. By changing the position of the slide valve, the compressor is able to load and unload. Some manufacturers utilize a variable position slide valve which can precisely match the system load. Other manufacturers use a stepped position slide valve. The stepped position slide valve operates much like the scroll staging example discussed earlier. Figure 35. Helical Rotary Slide Valve A more efficient method of controlling helical rotary compressors is to utilize a variable frequency drive to modulate compressor speed. A variable frequency drive, while more expensive than a slide valve, greatly increases compressor part load efficiency. In addition, as discussed earlier, the variable frequency drive can double as a soft starter for the compressor. Helical rotary machines, which utilize variable frequency drives, do not require a slide WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 18 of 25
valve. Eliminating the slide valve removes its associated inefficiencies and further decreases the number of moving parts by 50%. Application Considerations Single Versus Multiple Circuits Both helical rotary and centrifugal water chillers are available as single circuit machines or multiple circuit machines. Often, designers are lured by the appeal offered by multiple circuited machines. Multiple circuits offer redundancy. Each circuit contains its own compressor, expansion device and dedicated portions of the evaporator and condenser (figure 36). Figure 36. Multiple Circuits However, multiple circuited machines do not offer 100% redundancy. Multiple circuited machines share a single control system. Should the controls fail, the entire machine will fail. In addition, multiple circuited machines, which contain twice the number of moving parts, present double the potential for equipment failure. Equipment failure is directly related to the number of moving parts in a system. If we double the number of moving parts, we double the potential for failure. Finally, multiple circuited machines offer reduced part load efficiency. Consider a machine that operates with a single circuit compared to a machine that operates with dual circuits of the same capacity. A single circuited machine operating at 50% load utilizes 100% of its heat exchange surface area in the evaporator and condenser. A dual circuited machine, with one circuit off, operates at 50% load with 50% of its heat exchange surface area available for heat transfer. This decrease in heat transfer at lower loads equates to a significant drop in part load efficiency. If a designer requires redundancy, it is often better to design a plant with multiple chillers in lieu of a single chiller plant with multiple circuits. However, in applications where multiple machines exceed the design budget, multiple circuited machines may be a viable option. Variable Primary Flow Throughout the 21 st century, chillers were considered constant flow evaporator devices. However, the flow would always change slightly due to systems effects. These effects include the interaction of the pump with the system curve and variations of flow as chillers are enabled and disabled along with their associated pumps. Modern chilled water systems commonly employ variable flow through the evaporator. This further enables designers to achieve valuable pump energy savings and a somewhat simplified Hydronic design. WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 19 of 25
Figure 37. Variable Primary Chiller A variable primary chilled water system design often allows the designer to reduce the number of pumps required to operate the system (figure 38). Rather than utilizing dedicated primary and secondary pumps, a variable primary pumping system eliminates the need for secondary pumps. Figure 38. Variable Primary System In a variable primary system, the primary pumps are supplied with variable frequency drives. The secondary pumps are eliminated. As the system flow modulates with demand, the primary pumps modulate in order to satisfy the system demand. The flow through the chiller is allowed to modulate. As the system modulates below the minimum setpoint for the given chiller (generally around 30-40% of design flow), a two way valve located in a bypass pipe allows flow to bypass the system and satisfy the minimum flow required by the chillers. While variable primary pumping systems offer both energy savings and an opportunity for a simplified Hydronic design, several considerations must be taken into account when controlling the system. Those considerations are: Maintain minimum system flow (usually around 30-40% of design flow) Maintain maximum rate of change of flow (usually around 10% - 30% maximum change in flow per minute). Accurately control two way valve in bypass to maintain minimum chiller flow. WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 20 of 25
o Generally controlled with a flow sensor in return of production loop Condenser Water Temperature Control In virtually all design climates, chilled water plant energy savings can be realized by unloading the cooling tower as the ambient wet bulb temperature is depressed (for water cooled plants). The energy savings realized with condenser relief can be further amplified in drier climates. Furthermore, many chilled water systems are designed with plate and frame heat exchangers in order to provide a means of energy recovery at lower ambient wet bulbs. However, running a helical rotary or centrifugal chiller at lower condenser water temperatures can adversely affect the operation of the chiller. Virtually all helical rotary chillers and some centrifugal chillers rely on the pressure difference between the condenser and evaporator to transport oil. Without this pressure difference, the machine may be starved of much needed oil. In these instances, the system should be designed with some type of condenser head pressure control in order to maintain an adequate pressure differential within the machine. For water cooled systems with a cooling tower, three methods of condenser capacity control are recommended. The first method is to use a throttling valve at the condenser outlet. The throttling valve reduces the condenser water flow through the condenser (figure 39). The throttling method has the advantage of being simple and relatively inexpensive. In addition, the throttling method will save some pump energy. The system can be designed to either run on the pump curve or the pump can be controlled by a variable frequency drive. However, the throttling method has the potential to run into pump surge and may run below the tower minimum flow rate at low flows. Figure 39. Condenser Two Way Valve Control The second method of capacity control for water cooled systems is cooling tower bypass. Tower bypass mixes warm leaving condenser water with cool entering condenser water. Flow to the condenser is constant while flow to the cooling tower is variable (figure 40). The advantage to this system is that the flow through the condenser is constant. The disadvantage is that this system does not allow the system to start under low load conditions (such as coming out of plate and frame free cooling) and you may still run below the tower minimum flow. Figure 40. Condenser Three Way Mixing Valve WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 21 of 25
The third method of capacity control for water cooled system is chiller condenser bypass. Chiller condenser bypass varies the flow through the condenser by bypassing condenser water around the chiller condenser (figure 4`). The advantage to this system is that the pump and tower experience constant flow at all load conditions. The downside to this system is that we do not see any energy savings by unloading the pump. Figure 41. Condenser Three Way Diverting Valve In cold climates, either condenser throttling or condenser bypass are preferred. Both of these systems allow the chiller to start under low load, low ambient conditions or in the event of a system coming out of free cooling. Being that the condenser water flow is decreased with both methods; provisions need to be made to bypass the flow switch at low flow conditions. Often, a simple time delay switch may be used to prevent the chiller from disabling while attempting to start under cold condenser water conditions (i.e. coming out of plate & frame cooling). Heat Recovery Chillers Systems with simultaneous heating and cooling demands or that require domestic hot water can benefit from utilzing chillers with heat recovery. Heat recovery chillers utilze the heat energy produced by the condenser to produce warm/hot water. Two types of heat recovery chillers exist, those with Auxilary heat recovery condensers Heat pump chillers Auxilary heat recovery chillers utilize a second heat heat exchanger bundle (figure 42) in order create hot water. The auxiliary heat exchanger is preferentially loaded, receiving the hot gas directly discharged from the compressor, maximizing the hot water generation capability. After the hot, high pressure gas discharging the compressor generates hot water, the slightly cooler condenser refrigerant vapor is routed to the primary condenser. There is is condensed to a high pressure liquid and the refigeration cycle completes. Figure 42. Heat Recovery Chiller The chiller is controlled based on leaving chillled water temperature. The heat capacity generated from the auxiliary heat exchanger is not controlled. The heat generated to the auxiliary condenser is directly related to the chilled water capacity generated by the chiller. Auxilary heat recovery chillers are able to produce hot water in the rage of 95 o F to 115 o F. A heat pump chiller, like an auxilary heat recovery chiller, is capable of generating hot water. Unlike an auxilary heat recovery chiller, a heat pump chiller does not utilize a separate secondary heat exchagner in order to generate heat. WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 22 of 25
A heat pump chiller is optimized to generate high levels of compressor lift, utilzing its condenser bundle to produce heat. Heat pump chillers utilize 100% of the heat energy that would otherwise be rejected at the cooling tower, utilzing that heat to meet the building heating demand (figure 43). Heat pumnp chillers are able to produce hot water in the rage of 120 o F to 170 o F. However, the total lift generate by the compressor is generally limited to 65 o F to 100 o F. Thus, at higher hot water temperatures, the chilled water temperature produced by the chiller is limited. Figure 43. Heat Pump Chiller Heat pump chillers are controlled to maintain the hot water load. The chilled water capacity is not controlled. Being that the cooling load is the dependent variable, the chilled water capacity is directly related to the heating capacity required. Thus, a heat pump chiller is commonly one of multiple chillers in the plant. The heat pump chiller is base loaded. Any additional chilled water demand would be satisified by one or multiple dedicated cooling only chilers. If the heat pump chiller cannot maintain the heating load, a supplemental boiller may be installed to meet any additional heating demand. In order to determine the overall efficiency benefit of a heat pump chiller compared to a conventional chiller plus boiler plant, figure 44 depicts the efficiencies that could realistically be expected operating a heat pump chiller. Figure 44. Heat Pump System Example The conversion of COP to chiller kw/ton is: kw/ton = 3.516/COP WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 23 of 25
If a chiller has an efficiency of 1.0 kw/ton, the COP would be 3.5. Thus, assuming chiller with an efficiency in heat pump mode of 1.2 kw/ton, the cooling COP is 2.9. That equates to.35 units of input work to 1 unit of output cooling. A typical R-134 heat pump chiller can produce 150 o F to 180 o F hot water. In this example, assume 150 o F hot water and 44 o F chilled water with a cooling COP of 2.9 (1.2 kw/ton) and a heating COP of 3.8. For the purposes this example, this is a very conservative figure. Most modern R-134a heat pump chillers can operate at an efficiency greater than 1.2 kw/ton. Recall that COP is a unitless value relating output capacity to input work. The heat pump chiller will generate about 25-45% more to account for the heat of compression. In this example, we will assume the heat of compression is accounts for 35% of the heat generated by the chiller. That is to say for every 1.35 units of heating energy produced, the chiller will produce 1 unit of cooling energy. This give us a heating COP of 3.8. This results in a combined COP of the plant of 6.7 (2.9 + 3.8). A similar chiller plus boiler system may incorporate a chiller with a COP of 6.1 (0.57 kw/ton where k/w/ton = 3.516/COP) and a boiler operating at similar conditions at a.85 COP (85% efficient condensing boiler). The overall COP for this plant would be: Comparing the two systems (figure 45): Figure 45. Heat Pump COP System Efficiency the heat pump system uses 5 times less energy than the chiller plus boiler system (6.7 COP / 1.48 COP). Upon further anlysis of the two systems, it becomes readily apparent why heat pump systems can offer designers an opportunity to attain tremendous energy savings. In order to realize the energy savings potential afforded by a heat pump chiller, the system must have simultaneous heating and cooling loads. For example, larger buildings with a higher percentage of interior zones would generally create simultaneous loads. The ASHRAE 2008 Handbook reads: WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 24 of 25
Another common example of such a system would be a variable air volume operating during the warmer spring, summer and fall months. VAV systems are typically designed with some type of reheat at the air terminal units. These reheat terminal require some form of heat energy, even during the warmest months of the year. Figure 46. VAV Reheat The ratio of heat energy to cooling energy produced by a heat pump chiller is typically very close to 1.35 to 1. The difference in heating energy to cooling energy is a product of the heat produced by the refrigeration cycle (heat of compression). However, rarley will the building load profile follow the ratio of heating to cooling energy produced by the chiller. Thus, heat pump chillers are generally run in conjunction with supplemental boilers and chillers. WN Mechanical Systems Helical Rotary & Centrifugal Chillers Page 25 of 25