Refrigeration Cycles Chapter 11

Refrigerators & Heat Pump Refrigeration: The transfer of heat from lower temperature regions to higher temperature is called refrigeration. Refrigerator: Devices that produces refrigeration are called refrigerators. Refrigerant: The working fluid used in refrigerators are called refrigerant. Heat Pump: Refrigerator used for the purpose of heating a space by transferring heat from a cooler medium are called heat pump.

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

Reversed Carnot Cycle All four processes that comprise the Carnot cycle can be reversed. Reversing the cycle will also reverse the directions of any heat and work interactions. The result is a cycle that operates in the counterclockwise direction, which is called the reversed Carnot cycle. A refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator or a Carnot heat pump.

The standard of comparison for refrigeration cycle is the reversed Carnot cycle. The coefficient of performance e of Carnot refrigerators and heat pumps were determined to be

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

The ideal vapor compression refrigeration cycle consists of following four processes. 1 – 2: Isentropic compression in a compressor. 2 – 3: Constant pressure heat rejected in condenser. 3 – 4: Throttling in an expansion device (same enthalpy remains constant) 4 – 1: Constant pressure heat absorption in an evaporator.

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

The area under the process curve on a T-s diagram represents the heat transfer. Another diagram frequently used in the analysis of vapor-compression refrigeration cycle is P-h diagram.

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

Cascade Refrigeration Systems For applications that require large temperature and pressure ranges, refrigeration is performed in stages(2 or more). Large pressure range means poor compressor performance. Performing refrigeration in stages is achieved by Cascade Refrigeration Cycles (that is more than a refrigeration cycle operating in series). Cascading improves the COP of a refrigeration system. The refrigerant in both cycles could be the same or different. Using the following figure ,write expressions for mass flow rates ratio and COP? See Example 10.3

Cascade Refrigeration Systems

Multistage Compression Refrigeration Systems The heat exchanger in Cascade Refrigeration System can be replaced by a mixing chamber if the refrigerant in the two cycles is the same. Such system is called Multistage Compression Refrigeration System. Liquid refrigerant (exit of condenser) expands to the mixing (flash) chamber pressure where part of it vaporizes ( see Fig.) The saturated vapor mixes with the superheated vapor (point 3) from the exit of the low pressure compressor. Hence, two-stage compression with inter-cooling. Multistage Compression decreases the work of the compressor See Example 10.4

Multistage Compression Refrigeration Systems

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

Gas Refrigeration Cycles

Gas Refrigeration Cycle with Regeneration

Absorption Refrigeration Systems Refrigeration in which there is a source of inexpensive thermal energy at a temperature of 100 to 200OC is absorption refrigeration The refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia – water system where ammonia serves as the refrigerant and water as the transport medium Other absorption refrigeration systems include water-Lithium bromide where water serves as a refrigerant (limited applications-Why?).

Absorption Refrigeration Systems The basic principles can be discussed by the Ammonia absorption refrigeration cycle shown in Fig. ARS are: complex, occupy more space and less efficient (hence, expensive compared to vapor compression systems). In ARS liquid is compressed instead of vapor, thus the work input is very small compared to vapor compression systems. Write an expression for the COP of an ARS? Derive an expression for the maximum COP of an absorption refrigeration system and comment?

Absorption chillers Absorption chillers are air-conditioning systems based on absorption refrigeration. Absorption chillers cooling capacity decreases sharply with decrease in source temperature. The COP is affected less by decrease in source temperature. Read more about absorption chillers.