Refrigeration CyclesRefrigeration Cycles

Chapter 11Chapter 11

Refrigerators & Heat PumpRefrigerators & Heat PumpRefrigeration:Refrigeration: The transfer of heat from The transfer of heat from lower temperature regions to higher lower temperature regions to higher temperature is called refrigeration.temperature is called refrigeration.

Refrigerator:Refrigerator: Devices that produces Devices that produces refrigeration are called refrigerators.refrigeration are called refrigerators.

Refrigerant:Refrigerant: The working fluid used in The working fluid used in refrigerators are called refrigerant.refrigerators are called refrigerant.

Heat Pump:Heat Pump: Refrigerator used for the Refrigerator used for the purpose of heating a space by transferring purpose of heating a space by transferring heat from a cooler medium are called heat heat from a cooler medium are called heat pump.pump.

COP (coefficient of performance):COP (coefficient of performance): The The performance of refrigeration and heat performance of refrigeration and heat pumps are expressed in terms of COP.pumps are expressed in terms of COP.

innet

LR W

QCOP

,

input Work

effect Cooling

Input Required

Output Desired

innet

LHP W

QCOP

,

input Work

effect Heating

Input Required

Output Desired

Reversed Carnot CycleReversed Carnot Cycle All four processes that comprise the All four processes that comprise the Carnot cycle can be reversed. Carnot cycle can be reversed.

Reversing the cycle will also reverse the Reversing the cycle will also reverse the directions of any heat and work directions of any heat and work interactions. interactions.

The result is a cycle that operates in the The result is a cycle that operates in the counterclockwise direction, which is called counterclockwise direction, which is called the the reversed Carnot cyclereversed Carnot cycle..

A refrigerator or heat pump that operates A refrigerator or heat pump that operates on the reversed Carnot cycle is called a on the reversed Carnot cycle is called a Carnot refrigeratorCarnot refrigerator or a or a Carnot heat pumpCarnot heat pump..

The standard of comparison for refrigeration The standard of comparison for refrigeration cycle is the reversed Carnot cycle.cycle is the reversed Carnot cycle.

The coefficient of performance e of Carnot The coefficient of performance e of Carnot refrigerators and heat pumps were determined refrigerators and heat pumps were determined to be to be

1

1,

LH

CarnotR TTCOP

HLCarnotHP TT

COP

1

1,

The Ideal Vapor Compression The Ideal Vapor Compression Refrigeration CycleRefrigeration Cycle

In an In an ideal vapor compressionideal vapor compression refrigeration refrigeration cycle, the refrigerant enters the cycle, the refrigerant enters the compressor as a saturated vapor and is compressor as a saturated vapor and is cooled to the saturated liquid state in the cooled to the saturated liquid state in the condenser. It is then throttled to the condenser. It is then throttled to the evaporator pressure and vaporizes as it evaporator pressure and vaporizes as it absorbs heat from refrigerated space.absorbs heat from refrigerated space.

The ideal vapor compression refrigeration The ideal vapor compression refrigeration cycle consists of following four processes.cycle consists of following four processes.

1 – 2:1 – 2: Isentropic compression in a compressor. Isentropic compression in a compressor.

2 – 3:2 – 3: Constant pressure heat rejected in Constant pressure heat rejected in condenser.condenser.

3 – 4:3 – 4: Throttling in an expansion device (same Throttling in an expansion device (same enthalpy remains constant)enthalpy remains constant)

4 – 1:4 – 1: Constant pressure heat absorption in an Constant pressure heat absorption in an evaporator.evaporator.

In a household refrigerator, the freezer In a household refrigerator, the freezer compartment where heat is absorbed by compartment where heat is absorbed by the refrigerant serves as the evaporator. the refrigerant serves as the evaporator. The coils behind the refrigerator, where The coils behind the refrigerator, where heat is dissipated to the kitchen air serve heat is dissipated to the kitchen air serve as the condenser. as the condenser.

The area under the process curve on a T-The area under the process curve on a T-s s diagram represents the heat transfer.diagram represents the heat transfer.

Another diagram frequently used in the Another diagram frequently used in the analysis of vapor-compression analysis of vapor-compression refrigeration cycle is P-refrigeration cycle is P-hh diagram. diagram.

Actual Vapor-Compression Actual Vapor-Compression Refrigeration CyclesRefrigeration Cycles

There are many irreversibilities that occurs There are many irreversibilities that occurs in various components. Two common in various components. Two common sources of irreversibilites are fluid friction sources of irreversibilites are fluid friction (causes pressure drop) and heat transfer (causes pressure drop) and heat transfer to or from surrounding.to or from surrounding.

Cascade Refrigeration SystemsCascade Refrigeration Systems

For applications that require large temperature and pressure For applications that require large temperature and pressure ranges, refrigeration is performed in stages(2 or more).ranges, refrigeration is performed in stages(2 or more).

Large pressure range means poor compressor Large pressure range means poor compressor performance.performance.

Performing refrigeration in stages is achieved by Cascade Performing refrigeration in stages is achieved by Cascade Refrigeration Cycles (that is more than a refrigeration cycle Refrigeration Cycles (that is more than a refrigeration cycle operating in series).operating in series).

Cascading improves the COP of a refrigeration system.Cascading improves the COP of a refrigeration system.

The refrigerant in both cycles could be the same or different.The refrigerant in both cycles could be the same or different.

Using the following figure ,write expressions for mass flow Using the following figure ,write expressions for mass flow rates ratio and COP?rates ratio and COP?

See Example 10.3See Example 10.3

Cascade Refrigeration SystemsCascade Refrigeration Systems

Multistage Compression Multistage Compression Refrigeration SystemsRefrigeration Systems

The heat exchanger in Cascade Refrigeration System can be The heat exchanger in Cascade Refrigeration System can be replaced by a mixing chamber if the refrigerant in the two cycles replaced by a mixing chamber if the refrigerant in the two cycles is the same.is the same.

Such system is called Multistage Compression Refrigeration Such system is called Multistage Compression Refrigeration System.System.

Liquid refrigerant (exit of condenser) expands to the mixing Liquid refrigerant (exit of condenser) expands to the mixing (flash) chamber pressure where part of it vaporizes ( see Fig.)(flash) chamber pressure where part of it vaporizes ( see Fig.)

The saturated vapor mixes with the superheated vapor (point 3) The saturated vapor mixes with the superheated vapor (point 3) from the exit of the low pressure compressor. from the exit of the low pressure compressor.

Hence, two-stage compression with inter-cooling.Hence, two-stage compression with inter-cooling.

Multistage Compression decreases the work of the compressorMultistage Compression decreases the work of the compressor

See Example 10.4See Example 10.4

Multistage Compression Multistage Compression Refrigeration SystemsRefrigeration Systems

Gas Refrigeration CyclesGas Refrigeration Cycles

Gas Refrigeration Cycle is reversed Brayton cycle (see Fig.).Gas Refrigeration Cycle is reversed Brayton cycle (see Fig.).Note, the expansion process is performed in a turbine rather Note, the expansion process is performed in a turbine rather than a throttling valve as in vapor compression refrigeration than a throttling valve as in vapor compression refrigeration systems (Why?).systems (Why?).The heat transfer processes donot take place at constant The heat transfer processes donot take place at constant temperatures. Hence, it differs from Carnot Cycle.temperatures. Hence, it differs from Carnot Cycle.Hence, Gas Refrigeration Cycle do have lower COPs relative to Hence, Gas Refrigeration Cycle do have lower COPs relative to vapor–compression refrigeration cycles. Illustrate by a T-s vapor–compression refrigeration cycles. Illustrate by a T-s diag.?diag.?Gas Refrigeration Cycles involve simple lighter components Gas Refrigeration Cycles involve simple lighter components (Aircraft cooling) and can incorporate regeneration (suitable for (Aircraft cooling) and can incorporate regeneration (suitable for liquidation of gases)liquidation of gases)Multistage Compression decreases the work of the compressorMultistage Compression decreases the work of the compressorSee Example 10.5See Example 10.5

Gas Refrigeration CyclesGas Refrigeration Cycles

Gas Refrigeration Cycle with Gas Refrigeration Cycle with RegenerationRegeneration

Absorption Refrigeration SystemsAbsorption Refrigeration SystemsRefrigeration in which there is a source of Refrigeration in which there is a source of inexpensive thermal energy at a temperature of inexpensive thermal energy at a temperature of 100 to 200OC is absorption refrigeration100 to 200OC is absorption refrigeration

The refrigerant is absorbed by a transport The refrigerant is absorbed by a transport medium and compressed in liquid form.medium and compressed in liquid form.

The most widely used absorption refrigeration The most widely used absorption refrigeration system is the ammonia – water system where system is the ammonia – water system where ammonia serves as the refrigerant and water as ammonia serves as the refrigerant and water as the transport mediumthe transport medium

Other absorption refrigeration systems include Other absorption refrigeration systems include water-Lithium bromide where water serves as a water-Lithium bromide where water serves as a refrigerant (limited applications-Why?).refrigerant (limited applications-Why?).

Absorption Refrigeration SystemsAbsorption Refrigeration SystemsThe basic principles can be discussed by the The basic principles can be discussed by the Ammonia absorption refrigeration cycle shown in Ammonia absorption refrigeration cycle shown in Fig.Fig.

ARS are: ARS are: complex, occupy more space and less complex, occupy more space and less efficient (hence,efficient (hence, expensive compared to vapor expensive compared to vapor compression systems).compression systems).

In ARS liquid is compressed instead of vapor, In ARS liquid is compressed instead of vapor, thus the work input is very small compared to thus the work input is very small compared to vapor compression systems.vapor compression systems.

Write an expression for the COP of an ARS?Write an expression for the COP of an ARS?Derive an expression for the maximum COP of an Derive an expression for the maximum COP of an

absorption refrigeration system and comment?absorption refrigeration system and comment?

Absorption chillersAbsorption chillersAbsorption chillers are Absorption chillers are air-conditioning air-conditioning

systemssystems based on absorption refrigeration. based on absorption refrigeration.

Absorption chillers cooling capacity decreases sharply Absorption chillers cooling capacity decreases sharply with decrease in with decrease in source temperaturesource temperature..

The COP is affected less by decrease in The COP is affected less by decrease in source source temperature.temperature.

Read more about absorption chillersRead more about absorption chillers..