hvac systems - compressors

HVAC Systems3. HVAC Processes

Refrigeration System

Basic refrigeration system could hypothetically be constructed using a drum of liquid refrigerant atatmospheric pressure, a coil, a collecting drum, and a valve to regulate the flow of refrigerant into thecoil. Opening the valve allows the liquid refrigerant to flow into the coil by gravity. As warm air is blownover the surface of the coil, the liquid refrigerant inside the coil will absorb heat from the air, eventuallycausing the refrigerant to boil while the air is cooled. Adjustment of the valve makes it possible tosupply just enough liquid refrigerant to the coil so that all the refrigerant evaporates before it reachesthe end of the coil.


• One disadvantage of this system is that after the liquid refrigerantpasses through the coil and collects in the drum as a vapor, it cannotbe reused. The cost and environmental impacts of chemicalrefrigerants require the refrigeration process to continue without lossof refrigerant.

• Additionally, the boiling temperature of R-22 at atmospheric pressureis -40.8°C. At this unnecessarily low temperature, the moisturecontained in the air passing through the coil freezes on the coilsurface, ultimately blocking it completely.


Closing the CycleTo solve the first problem, a system is needed to collect this used refrigerant and return it to the liquid phase.Then the refrigerant can be passed through the coil again. This is exactly what happens in a typical mechanicalrefrigeration system. Liquid refrigerant absorbs heat and evaporates within a device called an evaporator.

In this example system, air is cooled when it passes through the evaporator, while the heat is transferred to therefrigerant, causing it to boil and change into a vapor. As discussed in the previous period, a refrigerant canabsorb a large amount of heat when it changes phase. Because of the refrigerant changing phase, the systemrequires far less refrigerant than if the refrigerant was just increasing in temperature.

The refrigerant vapor must then be transformed back into a liquid in order to return to the evaporator andrepeat the process.


The liquid refrigerant absorbed heat from the air while it was inside the evaporator, and was transformed into a vapor in the process of doing useful cooling. if the heat is then removed from this vapor, it will transform (condense) back to its original liquid phase. Heat flows from a higher temperature substance to a lower temperature substance. In order to remove heat from the refrigerant vapor, it must transfer this heat to a substance that is at a lower temperature. Assume that the refrigerant evaporated at -40.8°C. To condense back into liquid, the refrigerant vapor must transfer heat to a substance that has a temperature less than -40.8°C]. If a substance were readily available at this cooler temperature, however, the refrigerant would not be required in the first place. The cooler substance could accomplish the cooling by itself.How can heat be removed from this cool refrigerant vapor, to condense it, using a readily-available substance that is already too warm for use as the cooling medium.


Boiling Point of WaterAt higher pressures, refrigerant boils and condenses at higher temperatures. This can be explained byexamining the properties of water. At atmospheric pressure (14.7 psia [0.10 MPa]), water boils andevaporates at 212°F [100°C]. When pressure is increased, however, water does not boil until itreaches a much higher temperature. At a higher pressure there is a greater force pushing against thewater molecules, keeping them together in a liquid phase.Recall that, at a given pressure, the temperature at which a liquid will boil into a vapor is the sametemperature at which the vapor will condense back into a liquid.


This curve illustrates the pressures and corresponding temperatures at which R-22 boils andcondenses. At a pressure of 85 psia [0.59 MPa], the liquid R-22 will boil at 5.1°C. As an example,assume that a compressor is used to increase the pressure of the resulting refrigerant vapor to 280 psia[1.93 MPa]. This increase in pressure raises the temperature at which the vapor would condense backinto liquid to 49.7°C. In order to condense the refrigerant vapor at this higher temperature, a substanceat a temperature less than 49.7°C is needed. Ambient air or water is generally available attemperatures less than this.


A compressor, condenser, and expansion device form the rest of the system thatreturns the refrigerant vapor to a low-temperature liquid, which can again be used

to produce useful cooling. This cycle is called the vapor-compressionrefrigeration cycle.


In this cycle, a compressor is used to pump the low-pressure refrigerant vapor from theevaporator and compress it to a higher pressure. This hot, high-pressure refrigerant vaporis then discharged into a condenser. Because heat flows from a substance at a highertemperature to a substance at a lower temperature, heat is transferred from the hotrefrigerant vapor to a cooler condensing media, which, in this example, is ambient air. Asheat is removed from the refrigerant, it condenses, returning to the liquid phase. Thisliquid refrigerant is, however, still at a high temperature.


Finally, an expansion device is used to create a large pressure drop that lowers thepressure, and correspondingly the temperature, of the liquid refrigerant. Thetemperature is lowered to a point where it is again cool enough to absorb heat in theevaporator.


This diagram illustrates a basic vapor-compression refrigeration system that contains thedescribed components. First, notice that this is a closed system. The individualcomponents are connected by refrigerant piping. The suction line connects theevaporator to the compressor, the discharge line connects the compressor to thecondenser, and the liquid line connects the condenser to the evaporator. The expansiondevice is located in the liquid line.


• Recall that the temperature at which refrigerant evaporates and condenses isrelated to its pressure. Therefore, regulating the pressures throughout this closedsystem can control the temperatures at which the refrigerant evaporates andthen condenses. These pressures are obtained by selecting system componentsthat will produce the desired balance. For example, select a compressor with apumping rate that matches the rate at which refrigerant vapor is boiled off in theevaporator. Similarly, select a condenser that will condense this volume ofrefrigerant vapor at the desired temperature and pressure.


• Evaporator

At the inlet to the evaporator, the refrigerant exists as a cool, low-pressure mixture ofliquid and vapor. In this example, the evaporator is a finned-tube coil used to cool air.Other types of evaporators are used to cool water.The relatively warm air flows across this finned-tube arrangement and the coldrefrigerant flows through the tubes. The refrigerant enters the evaporator and absorbsheat from the warmer air, causing the liquid refrigerant to boil. The resulting refrigerantvapor (%) is drawn to the compressor.


• Compressor

The compressor raises the pressure of the refrigerant vapor (%) to a pressure andtemperature high enough (&) so that it can reject heat to another fluid, such asambient air or water. There are several types of compressors. The type shown in thisfigure is a reciprocating compressor. This hot, high-pressure refrigerant vapor thentravels to the condenser.


• Condenser

The condenser is a heat exchanger used to reject the heat of the refrigerant to another medium. Theexample shown is an air-cooled condenser that rejects heat to the ambient air. Other types of condensersare used to reject heat to water. The hot, high-pressure refrigerant vapor (&) flows through the tubes of thiscondenser and rejects heat from the cooler ambient air that passes through the condenser coil. As the heatcontent of the refrigerant vapor is reduced, it condenses into liquid ('). From the condenser, the high-pressure liquid refrigerant travels to the expansion device.


The primary purpose of the expansion device is to drop the pressure of the liquid refrigerant to equal the pressure in the evaporator. Several types of expansion devices can be used. The device shown is an expansion valve.The high-pressure liquid refrigerant (') flows through the expansion device, causing a large pressure drop. This pressure drop reduces the refrigerant pressure, and, therefore, its temperature, to that of the evaporator. At the lower pressure, the temperature of the refrigerant is higher than its boiling point. This causes a small portion of the liquid to boil, or flash. Because heat is required to boil this small portion of refrigerant, the boiling refrigerant absorbs heat from the remaining liquid refrigerant, cooling it to the desired evaporator temperature. The cool mixture of liquid and vapor refrigerant then enters the evaporator ($) to repeat the cycle.

• Expansion device


Placing each component in its proper sequence within the system, the compressor andexpansion device maintain a pressure difference between the high-pressure side of thesystem (condenser) and the low-pressure side of the system (evaporator). This pressuredifference allows two things to happen simultaneously. The evaporator can be at apressure and temperature low enough to absorb heat from the air or water to be cooled,and the condenser can be at a temperature high enough to permit heat rejection toambient air or water that is at normally available temperatures.These major components are discussed in further detail in the “RefrigerationCompressors” and “Refrigeration System Components” clinics.

Refrigeration System Pressure–Enthalpy (Ph)chart

The pressure–enthalpy (Ph) chart plots the properties of a refrigerant— refrigerant pressure on the vertical axis andenthalpy on the horizontal axis. Enthalpy is a measure of heat quantity, both sensible and latent, per kg of refrigerant. Itis typically expressed in terms of kJ/kg.The right-hand side of the chart indicates the conditions at which the refrigerant will be in the vapor phase. The left-hand side of the chart indicates the conditions at which the refrigerant will be in the liquid phase. In the middle of thechart is an envelope (curve). The left-hand boundary of the envelope indicates the saturated liquid condition. The right-hand boundary indicates the saturated vapor condition. If the enthalpy of the refrigerant lies inside the


envelope, the refrigerant exists as a mixture of liquid and vapor. If the enthalpy of therefrigerant lies to the right of the envelope, the vapor is superheated. Similarly, if theenthalpy of the refrigerant lies to the left of the envelope, the liquid is subcooled. Linesof constant temperature cross the P–h chart as shown.

P-h Chart for Water

To further demonstrate the use of the P–h chart, let us look at the process of heating and boiling water, at aconstant pressure, on a P–h chart for water. As discussed earlier, at atmospheric pressure (14.7 psia [0.10 MPa])water boils at 212°F [100°C]. At $, the water temperature is 180°F [82.2°C]. As we add heat to the water, thetemperature and enthalpy of the water increase as they move toward %. When the water reaches its saturatedcondition (%), at 212°F [100°C], it starts to boil and transform into vapor. As more heat is added to the water, itcontinues to boil while the temperature remains constant. A greater percentage of the water is transforminginto vapor as it moves toward &. When the water reaches & on the saturation vapor line, it has completelytransformed into vapor. Now, as more heat is added to the vapor, its temperature begins to increase againtoward D, 240°F [115.6°C].

Heat of Vaporization of Water

The distance between the edges of the envelope indicates the quantity of heat required to transformsaturated liquid into saturated vapor at a given pressure. This is called the heat of vaporization. Forexample, B represents the enthalpy of saturated liquid water at 14.7 psia [0.10 MPa] and C represents theenthalpy of saturated water vapor at the same pressure. The difference in enthalpy between B and C—970 Btu/lb [2256.3 kJ/kg]—is the heat of vaporization for water at this pressure.

Evaporator

The Ph chart can be used to analyze the vapor-compression refrigeration cycle and determine theconditions of the refrigerant at any point in the cycle. The chart in this example is for R-22. Because therefrigeration cycle is a continuous process, defining the cycle can start at any point. This example begins inthe lower left-hand portion of the Ph chart, where the refrigerant enters the evaporator.

Evaporator

• At the inlet to the evaporator, the refrigerant is at a pressure of 85 psia [0.59 MPa] and a temperature of 41.2°F [5.1°C], and is a mixture of liquid and vapor (mostly liquid). This cool, low-pressure refrigerant enters the evaporator ($) where it absorbs heat from the relatively warm air that is being cooled. This transfer of heat boils the liquid refrigerant inside the evaporator and superheated refrigerant vapor is drawn to the compressor (&).

• The change in enthalpy from A to C that occurs inside the evaporator is called the refrigeration effect. This is the amount of heat that each pound [kg] of liquid refrigerant will absorb when it evaporates.

Super Heat

Compressors are designed to compress vapor. Liquid refrigerant can cause damage if drawn into thecompressor. In some refrigeration systems additional heat is added to the saturated vapor (%) in theevaporator to ensure that no liquid is present at the compressor inlet. This additional amount of heat, abovesaturation, is called superheat. This superheated vapor (&) is generally 8°F to 12°F [4.4°C to 6.7°C] above thesaturated vapor condition when it enters the compressor. In this example, the refrigerant vapor issuperheated 10°F [5.6°C], from 41.2°F [5.1°C] to 51.2°F [10.7°C].

Compressor

The compressor draws in the superheated refrigerant vapor (&) and compresses it to a pressure andtemperature (') high enough that it can reject heat to another fluid. As the volume of the refrigerant isreduced by the compressor, its pressure is increased. Additionally, the mechanical energy used by thecompressor to accomplish this task is converted to heat energy. This causes the temperature of therefrigerant to also rise as its pressure is increased.

Heat Compression

When the refrigerant vapor is discharged from the compressor, its temperature is substantially higherthan its saturation temperature (the temperature at which the refrigerant would condense). The increasein enthalpy from & to ' is due to heat added by the compressor, or the heat of compression. In thisexample, the refrigerant leaves the compressor at 280 psia [1.93 MPa] and 191.5°F [88.6°C]. At this higherpressure, the corresponding saturation temperature is 121.5°F [49.7°C]. The refrigerant vapor leaving thecompressor is therefore 70°F [38.9°C] above its saturation temperature. This hot, high-pressurerefrigerant vapor then travels to the condenser.

Condenser

Inside of the condenser, heat is transferred from the hot, high-pressure refrigerant vapor (') to relatively coolambient air. This reduction in the enthalpy of the refrigerant vapor causes it to de-superheat. It becomessaturated vapor, condenses into saturated liquid, and further sub-cools before leaving the condenser (*) to goto the expansion device. First, the refrigerant vapor is cooled (the line from ' to () to its saturation temperatureof 49.7°C. Next, as additional heat is removed by the condenser, the refrigerant vapor condenses to itssaturated liquid condition (the line from ( to )). This saturated liquid refrigerant now passes through the area ofthe condenser called the sub-cooler. Here, the liquid refrigerant is further cooled (the line from ) to *), in thisexample, to 43.3°C. Because the saturation temperature at the condensing pressure is 49.7°C, the refrigeranthas been subcooled 6.4°C. With the temperature of the refrigerant in the condenser this high, air at normalambient conditions can be used to absorb the heat from the refrigerant. From the condenser, the high-pressure, subcooled liquid refrigerant (*) travels to the expansion device.

Expansion Device

The primary purpose of the expansion device is to drop the pressure of the liquid refrigerant to equal theevaporator pressure. At this lower pressure, the refrigerant is now inside the saturation envelope where itexists as a mixture of liquid and vapor. The high-pressure liquid refrigerant (*) flows through the expansiondevice, causing a large pressure drop. This pressure drop reduces pressure and temperature of therefrigerant to that of the evaporator ($). At the lower pressure, the temperature of the refrigerant is higherthan its boiling point. This causes a small portion of the liquid to boil, or flash. Because heat is required toboil this small portion of refrigerant, boiling refrigerant absorbs heat from the remaining liquid refrigerant,cooling it to the evaporator temperature. Notice that there is no change in enthalpy during the expansionprocess. The purpose of sub-cooling the liquid refrigerant in the condenser is to avoid flashing the refrigerantbefore it reaches the expansion device. If a valve is used as the expansion device, the presence of refrigerantvapor can cause improper operation and premature failure.

Expansion Device

The temperature of the refrigerant entering the expansion device (*) is 110°F [43.3°C] and its pressure is280 psia [1.93 MPa]. (The refrigerant condensed at 121.5°F [49.7°C] and was subcooled to 110°F [43.3°C].)The enthalpy of the refrigerant at this condition is 42.4 Btu/lb [98.6 kJ/kg]. As mentioned previously, thereis no change in enthalpy during the expansion process—it is the same at both * and $. The refrigerantleaves the expansion device ($) at evaporator conditions, 85 psia [0.59 MPa] and 41.2°F [5.1°C]. At thispressure, the enthalpy of saturated liquid is 21.8 Btu/lb [50.7 kJ/kg] and the enthalpy of saturated vapor is108.2 Btu/lb [251.7 kJ/kg]. Because there is no change of enthalpy during the expansion process, themixture of liquid and vapor leaving the expansion device must have the same enthalpy as the liquidentering the expansion device. This is true if 76.2% of the refrigerant is liquid and 23.8% of the refrigerantis vapor. This is determined as shown below:

Expansion Device

Refrigeration Cycle

This cool mixture of liquid and vapor refrigerant leaving the expansion device then enters the evaporator ($) to repeat the cycle. The vapor-compression refrigeration cycle has successfully recovered the refrigerant that boiled in the evaporator and converted it back into a cool liquid to be used again.

ExampleA - 6.9°C, 0.62 MPa, 96.8 kJ/kgB - 6.9°C, 0.62 MPa, 252.4 kJ/kgC - 12.5°C, 0.62 MPa, 256.6 kJ/kgD - 87.8°C, 1.93 MPa, 298.7 kJ/kgE - 49.7°C, 1.93 MPa, 262.4 kJ/kgF - 49.7°C, 1.93 MPa, 107.5 kJ/kgG - 41.9°C, 1.93 MPa, 96.8 kJ/kgH - 6.9°C, 0.62 MPa, 52.8 kJ/kg

a. How much superheat is in this system?b. How much sub-cooling is in this system?c. What is the refrigeration effect of this system?d. At the inlet to the evaporator, what percentage of the refrigerant exists as a vapor?

HVAC Systems

4. HVAC plants

Compressor Types

The traditional reciprocating compressor has been used in the industry for decades. It containscylinders, pistons, rods, a crankshaft, and valves, similar to an automobile engine. Refrigerant isdrawn into the cylinders on the down-stroke of the piston and compressed on the upstroke.

Compressor Types

• Scroll and helical-rotary (or screw) compressors have become more common, replacing thereciprocating compressor in most applications due to their improved reliability and efficiency.

• These three types of compressors (reciprocating, scroll, and helical-rotary) all work on theprinciple of trapping the refrigerant vapor and compressing it by gradually shrinking the volumeof the refrigerant. Thus, they are called positive-displacement compressors.

• In contrast, centrifugal compressors use the principle of dynamic compression, which involvesconverting energy from one form to another in order to increase the pressure and temperature ofthe refrigerant. The centrifugal compressor uses centrifugal force, generated by a rotatingimpeller, to compress the refrigerant vapor.

Scroll Compressor

Similar to the reciprocating compressor, the scroll compressor works on the principle of trapping therefrigerant vapor and compressing it by gradually shrinking the volume of the refrigerant. The scrollcompressor uses two scroll configurations, mated face-to-face, to perform this compression process. Thetips of the scrolls are fitted with seals that, along with a fine layer of oil, prevent the compressedrefrigerant vapor from escaping through the mating surfaces.The upper scroll, called the stationary scroll, contains a discharge port. The lower scroll, called the drivenscroll, is connected to a motor by a shaft and bearing assembly. The refrigerant vapor enters through theouter edge of the scroll assembly and discharges through the port at the center of the stationary scroll.

Scroll Compressor

Scroll Compressor

• During the first full revolution of the shaft, or the intake phase, the edges of the scrolls separate,allowing the refrigerant vapor to enter the space between the two scrolls. By the completion offirst revolution, the edges of the scrolls meet again, forming two closed pockets of refrigerant.

• During the second full revolution, or the compression phase, the volume of each pocket isprogressively reduced, increasing the pressure of the trapped refrigerant vapor. Completion of thesecond revolution produces near maximum compression.

• During the third full revolution, or the discharge phase, the interior edges of the scrolls separate,releasing the compressed refrigerant through the discharge port. At the completion of therevolution, the volume of each pocket is reduced to zero, forcing the remaining refrigerant vaporout of the scrolls.

• Looking at the complete cycle, notice that these three phases—intake, compression, anddischarge—occur simultaneously in an ongoing sequence. While one pair of these pockets isbeing formed, another pair is being compressed and a third pair is being discharged.

Scroll Compressor

In this example scroll compressor, refrigerant vapor enters throughthe suction opening. The refrigerant then passes through a gap inthe motor, cooling the motor, before entering the compressorhousing. The refrigerant vapor is drawn into the scroll assemblywhere it is compressed, discharged into the dome, and finallydischarged out of the compressor through the discharge opening. Inthe air-conditioning industry, scroll compressors are widely used inheat pumps, rooftop units, split systems, self-contained units, andeven small water chillers.

Helical-Rotary (Screw) Compressor

Similar to the scroll compressor, the helical-rotarycompressor traps the refrigerant vapor and compressesit by gradually shrinking the volume of the refrigerant.This particular helical-rotary compressor design uses twomating screw-like rotors to perform the compressionprocess.

The rotors are meshed and fit, with very closetolerances, within the compressor housing. The gapbetween the two rotors is sealed with oil, preventing thecompressed refrigerant vapor from escaping through themating surfaces. Only the male rotor is driven by thecompressor motor. The lobes of the male rotor engageand drive the female rotor, causing the two parts tocounter rotate.


Refrigerant vapor enters the compressor housing through the intake portand fills the pockets formed by the lobes of the rotors. As the rotors turn,they push these pockets of refrigerant toward the discharge end of thecompressor. After the pockets of refrigerant travel past the intake portarea, the vapor, still at suction pressure, is confined within the pockets bythe compressor housing.

Viewing the compressor from the opposite side showsthat continued rotation of the meshed rotor lobes drivesthe trapped refrigerant vapor (to the right), toward thedischarge end of the compressor, ahead of the meshingpoint. This action progressively reduces the volume ofthe pockets, compressing the refrigerant. Finally, whenthe pockets of refrigerant reach the discharge port, thecompressed vapor is released and the rotors force theremaining refrigerant from the pockets.


In this example helical-rotary compressor, refrigerantvapor is drawn into the compressor through the suctionopening and passes through the motor, cooling it. Therefrigerant vapor is drawn into the compressor rotorswhere it is compressed and discharged out of thecompressor. In the air-conditioning industry, helical-rotary compressors are most commonly used in waterchillers ranging from 70 to 450 tons [200 to 1,500 kW].

Centrifugal Compressor

The centrifugal compressor uses the principle of dynamic compression, which involves converting energy from one form to another, to increase the pressure and temperature of the refrigerant. It converts kinetic energy (velocity) to static energy (pressure). The core component of a centrifugal compressor is the rotating impeller.


The center, or eye, of the impeller is fitted with blades that drawrefrigerant vapor into radial passages that are internal to the impellerbody. The rotation of the impeller causes the refrigerant vapor toaccelerate within these passages, increasing its velocity and kineticenergy.The accelerated refrigerant vapor leaves the impeller and enters thediffuser passages. These passages start out small and become largeras the refrigerant travels through them. As the size of the diffuserpassage increases, the velocity, and therefore the kinetic energy, of therefrigerant decreases. The first law of thermodynamics states thatenergy is not destroyed—only converted from one form to another.Thus, the refrigerant’s kinetic energy (velocity) is converted to staticenergy (or static pressure). Refrigerant, now at a higher pressure,collects in a larger space around the perimeter of the compressorcalled the volute. The volute also becomes larger as the refrigeranttravels through it. Again, as the size of the volute increases, the kineticenergy is converted to static pressure.


This chart plots the conversion of energy that takes place as the refrigerant passes through thecentrifugal compressor. In the radial passages of the rotating impeller, the refrigerant vapor accelerates,increasing its velocity and kinetic energy. As the area increases in the diffuser passages, the velocity, andtherefore the kinetic energy, of the refrigerant decreases. This reduction in kinetic energy (velocity) isoffset by an increase in the refrigerant’s static energy or static pressure. Finally, the high-pressurerefrigerant collects in the volute around the perimeter of the compressor, where further energyconversion takes place.


In this example centrifugal compressor, refrigerant vapor is drawn into the compressor and enters thecenter of impeller. This particular centrifugal compressor uses multiple impellers to perform thecompression process in stages. The impellers rotate on a common shaft that is connected to the motor. Inthe air-conditioning industry, centrifugal compressors are most commonly used in prefabricated waterchillers ranging from 100 to 3,000 tons [350 to 10,500 kW]. They are also used in field-assembled waterchillers up to 8,500 tons [30,000 kW].

Open compressor

In addition to the different methods of compression, compressorscan be classified as open, hermetic, and semi hermetic. Areciprocating compressor will be used to explain these terms.An open compressor is driven by an external power source, suchas an electric motor, an engine, or a turbine. The motor is coupledto the compressor crankshaft by a flexible coupling. Since theshaft protrudes through the compressor housing, a seal is used toprevent refrigerant from leaking out of the compressor housing.This motor is cooled by air that is drawn in from the surroundingspace. The heat removed from the motor must still be rejectedfrom the space, either by mechanical ventilation or, if the space isconditioned, by the building’s cooling system.

Hermetic compressor

A hermetic compressor, on the other hand, seals the motor withinthe compressor housing. This motor is cooled by the refrigerant,either by refrigerant vapor that is being drawn into the compressorfrom the suction line or by liquid refrigerant that is being drawn fromthe liquid line. The heat from the motor is then rejected by thecondenser. Hermetic compressors eliminate the need for the shaftcouplings and external shaft seals that are associated with openmotors. The coupling needs precise alignment, and these seals are aprime source of oil and refrigerant leaks. On the other hand, if amotor burns out, a system with a hermetic compressor will requirethorough cleaning, while a system with an open compressor will not.

Semi-hermetic compressor

Similarly, the motor for a semi-hermetic compressor isalso contained within the compressor housing and iscooled by the refrigerant. The term “semi-hermetic”means that the sealed housing is designed to be openedto repair or overhaul the compressor or motor.

Compressor Capacity Control

• The capacity of a compressor is defined by the volume of evaporated refrigerant that can be compressed within a given time period. The compressor needs a method of capacity control in order to match the ever-changing load on the system.

Compressor Capacity Control

• Capacity control is commonly accomplished by unloading the compressor. The method used for unloading generally depends on the type of compressor. Many reciprocating compressors use cylinder unloaders. Scroll compressors generally cycle on and off. Helical-rotary compressors use a slide valve or a similar unloading device. Centrifugal compressors typically use inlet vanes or a variable-speed drive in combination with inlet vanes. In addition, all four types of compressors could use variable speed to control their capacity.

Scroll Compressor – Cycle On & Off

• Scroll compressors do not have valves or unloaders. A piece of equipment that uses scroll compressors generally unloads by using multiple compressors and turning them on and off, as needed, to satisfy the evaporator load.

Cycling Scroll Compressors

• Cycling multiple scroll compressors is very similar to the use of cylinder unloaders on a singlereciprocating compressor. As an example, a large 40-ton [140.6-kW] reciprocating compressormay have eight cylinders with unloaders on six of them, allowing it to unload in equal steps of 10tons [35.2 kW] each, with a minimum nominal capacity of 10 tons [35.2 kW]. A similar 40-ton[140.6-kW] unit using scroll compressors would include four separate 10-ton [35.2-kW] scrollcompressors. Just as the reciprocating compressor unloads in equal intervals by unloading a pairof cylinders, the scroll compressor unit unloads in the same 10-ton [35.2-kW] intervals by shuttingoff individual compressors.


At design conditions, the capacities of the evaporator and thisfour-compressor unit balance at a suction temperature of 43°F[6.1°C] and a capacity of 44 tons [154.7 kW] (A). As the coolingload decreases below this balance point, assuming a constantcondensing pressure, the capacity of the unit decreases withthe falling suction temperature along the four-compressorcurve until it reaches B. Here, the first scroll compressor is shutoff and the capacity of the unit decreases immediately to 30tons [105.5 kW] (C) along the three compressor curve.

As the load continues to decrease, the individual compressorsshut off in a similar manner until the suction temperaturereaches a minimum set point and the final compressor is shutoff. The minimum capacity of the four-compressor unit in thisexample is 8 tons [28.1 kW].


• Excessive starting and stopping of scroll compressors is not a concern. The reciprocatingcompressor system on Figure 35 includes a single large compressor with a single large motor. Incontrast, the scroll compressor system has four small compressors, each with its own small motor.These small motors are designed to cycle, just like those used with small reciprocatingcompressors.

Screw Chillers - Side Valve

The helical-rotary compressor used as the example in this clinic isunloaded using a slide valve that is an integral part of the compressorhousing. Other helical-rotary compressor designs may use a variety ofmethods to vary capacity. Some of these methods are similar infunction to the slide valve presented in this clinic. One majordetermining factor is whether the compressor is designed to unloadin steps, like a reciprocating compressor, or if it has variableunloading. The position of the slide valve along the rotors controlsthe volume of refrigerant vapor delivered by the compressor, byvarying the amount of rotor length actually used for compression. Bychanging the position of the slide valve, the compressor is able tounload to exactly match the evaporator load, instead of unloading insteps like the reciprocating compressor discussed earlier.

Screw Chillers - Side ValveAt full load, the slide valve is closed. The compressor pumps itsmaximum volume of refrigerant, discharging it through thedischarge port. As the load on the compressor decreases, theslide valve modulates toward the open position. The openingcreated by the valve movement allows refrigerant vapor tobypass from the rotor pockets back to the suction side of thecompressor. This reduces the volume of vapor available for thecompression process. It also reduces the amount of rotorlength available for compression. In this manner, the volume ofrefrigerant that is pumped by the compressor is varied,unloading it to balance the existing load.

Centrifugal Chillers - Inlet Vanes

A common method of modulating the capacity of a centrifugalcompressor is to use a set of vanes installed at the inlet of thecompressor impeller. While a survey of other centrifugalcompressor designs shows that there are other methods ofcapacity control, many of them function in a manner similar tothe inlet vanes presented in this section of the clinic. Inlet vanes“pre-swirl” the refrigerant before it enters the impeller. Bychanging the refrigerant’s angle of entry, these vanes lessen theability of the impeller to take in the refrigerant. As a result, thecompressor’s refrigerant pumping capacity decreases tobalance with the evaporator load.

Inlet VaneThese curves represent the performance of a typical centrifugalcompressor over a range of inlet vane positions. The pressuredifference between the compressor inlet (evaporator) andoutlet (condenser) is on the vertical axis and compressorcapacity is on the horizontal axis. The surge region representsthe conditions that cause unstable compressor operation. Asthe load on the compressor decreases from the full-loadoperating point (A), the inlet vanes partially close, reducing theflow rate of refrigerant vapor and balancing the compressorcapacity with the new load (B).

Less refrigerant, and therefore less heat, are transferred to the condenser. Since the available heatrejection capacity of the condenser is now greater than required, the refrigerant condenses at a lowertemperature and pressure. This reduces the pressure difference between the evaporator and thecondenser. Continuing along the unloading line, the compressor remains within its stable operating rangeuntil it reaches C. Inlet vanes on a centrifugal compressor allow it to unload over a broad capacity rangewhile preventing the compressor from operating in the surge region.

Variable Speed

Alternatively, the capacity of a compressor can be controlled by varying the rotational speed of the compressormotor. This is accomplished using a device called an adjustable-frequency drive (AFD) or variable-speed drive.On a reciprocating compressor, this would vary the speed at which the crankshaft rotates, thus controlling therate at which the piston travels back and forth inside the cylinder. On a scroll compressor, this would vary thespeed at which the driven scroll rotates. If applied to a helical-rotary compressor, this would vary the speed atwhich the rotors rotate. Applied to a centrifugal compressor, this would vary the speed at which the impellerrotates.Although variable-speed capacity control could be applied to all four types of compressors discussed in thisclinic, it is most often applied to centrifugal compressors. Because speed variation reduces both the flow rate ofrefrigerant through the compressor and the pressure differential created by the compressor, it is used inconjunction with inlet vanes. This requires fairly complex control strategies to balance refrigerant flow rate,pressure differential, and load.