teng - ch06 mec 551 refrigeration

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MEC 551THERMAL ENGINEERING

CHAPTER 6REFRIGERATION


KJM 381Overview - Energy Consumption

Malaysia (overall)

Educational building


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Carnot (or reversed Carnot) - operates between two constant temperature reservoirs i.e. heat transfer processes take place at constant temperature.

Review – Reversed Carnot Cycle

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Review – Thermodynamics 1st Law

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Review – Control Volume

Compressors are devices used to increase pressure of a fluid. Work is supplied to these devices from an external source through a rotating shaft. Therefore, compressors involve work inputs.


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Energy balance: for compressor

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Review – Isentropic Efficiencies


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Introduction

Refrigeration is defined as a process of making something cold, and ‘cold’ can be defined as an absence of heat.

The earliest and most common of the cold substances used for taking away heat was ice or snow. The Chinese, Greeks, Romans etc all use ice for food preservation.


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Refrigeration Machine in 1834

(B) Evaporator (C) Compressor

(D) Condenser(H) Valve


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Refrigeration Machine in 1950’s


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Refrigeration Machine at Home


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Process Process Description1-2 Isentropic compression2-3 Constant pressure heat rejection3-4 Throttling process4-1 Constant pressure heat absorption

Vapor Compression Refrigeration Cycle

The processes in the vapor compression refrigeration cycle are as per following:


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Refrigeration cycle is based on the modification of reversed Carnot cycle


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The P-h diagram is another convenient diagram often used to illustrate the vapor-compression refrigeration cycle.


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COMPRESSOR: Provide the driving force for the entire system by drawing low pressure refrigerant in and adding pressure such that it exits at a higher temperature.

CONDENSER: Exhaust heat from the system by virtue of heat transfer across a temperature gradient. The refrigerant in the condenser is at a higher temperature than the ambient temperature.

Components Description


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EXPANSION VALVE: Allow the refrigerant to expand dramatically in a controlled process such that it exits the valve at a low quality liquid-vapor mixture.

EVAPORATOR: Absorb heat from the cold space by virtue of a temperature gradient, similar to the condenser.


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Component First Law ResultCompressor

Condenser

Expansion valve

Evaporator

Results of First Law


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Results of Second Law

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Refrigerator

Heat pump


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Refrigeration Effect, Load and Capacity

Refrigeration effect qL (kJ/ kg) is the heat extracted by a unit

mass of refrigerant during the evaporating process in the evaporator.

Refrigeration load QL (kW) is the required rate of heat

extraction by the refrigerant in the evaporator.

Refrigeration capacity, or cooling capacity, QRC (kW) is the

actual rate of heat extracted by the refrigerant in the evaporator.


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Example #1

A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor compression refrigeration cycle between 0.14 and 0.8 MPa. If the mass flow rate of the refrigerant is 0.05 kg/s, determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor (b) the rate of heat rejection to the environment (c) the COP of the refrigerator.


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Example #2

A refrigerator uses refrigerant-134a as the working fluid. It enters the compressor as superheated vapor at 0.14 MPa, -10°C and is compressed to 0.8 Mpa and 50 °C. The refrigerant is cooled in the condenser to 0.72 MPa, 26°C and then is throttled to 0.15 MPa. The mass flow rate of the refrigerant is 0.05 kg/s. Determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor (b) isentropic efficiency of the compressor (c) the COP of the refrigerator.

Type of refrigerants:


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Refrigerants


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Refrigerant Selection

R-134a (replaced R-12, which damages ozone layer) is used in domestic refrigerators and freezers, as well as automotive air conditioners.

R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.

Two important parameters: the temperatures of the refrigerated space and the environment with which the refrigerant exchanges heat.


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COP comparison between different refrigerants under the same operating conditions.


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Cascade Refrigeration Systems

Very low temperatures can be achieved by operating two or more vapor-compression systems in series, called cascading.


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Applying the first law to the heat exchanger,

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When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber).

Multistage Compression Refrigeration System

Flash chamber has a better heat transfer characteristics.

Such system is called multistage compression refrigeration system.


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Example 3:Consider a two stage cascade refrigeration system operating between the pressure limits of 0.8 and 0.14 Mpa. Each stage operates on an ideal vapor compression refrigeration cycle with R-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where both streams enter at about 0.32 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.05 kg/s, determine (a) the mass flow rate of the refrigerant through the lower cycle. (b) the rate of heat removal from the refrigerated space and the power input to the compressor (c) the COP of the cascade refrigerator.


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Example #4

A vapor compression plant uses R-134a and has a suction pressure of 0.14 MPa and a condenser pressure of 0.8 MPa. The vapor is dry saturated on entering the compressor and there is no undercooling of the condensate. Part of the refrigerant evaporates during flashing process and this vapor is mixed with the refrigerant leaving the low pressure compressor. The mixture is then compressed to the condenser pressure by the high pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in the evaporator. Assuming the refrigerant leaves the evaporator as a saturated vapor and both compressors are isentropic. The compression is carried out isentropically in two stages and a flash chamber is employed at a pressure of 0.32 MPa. Calculate (a) the amount of vapor bled off at the flash chamber, (b) the amount of heat removed from the refrigerated space, (c) the compressor work, and (d) the coefficient of performance.


The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the transport medium.

Absorption Refrigeration Systems

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Cooling effectWork input ,

The work input to the pump is usually very small, and the COP of absorption refrigeration systems is defined as:


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Application of Refrigeration

Food processing, preservation and distribution.


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Application of Refrigeration

Chemical and process industries.

– Separation of gases– Condensation of gases– Solidification of solute– Storage of liquid at low pressure– Removal of heat of reaction (exothermic)– Recovery of solvents


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Domestic Refrigerator & Air-Conditioner


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Chiller & Cold Room


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3 compressors (in standby mode)


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Evaporator

Condenser

CompressorExpansion valve

1

23

4

Refrigerant R-22 flows

Ethylene glycol flows

From heat exchanger

Cooling water flows

To heat exchanger

Make-up water tank

Make-up water flows

Cooling tower


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Case Study

Required process temperature in the reactor: -80°C


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Evaporator, LPC

Evaporator, HPC

Condenser, LPC

Condenser, LPC

Compressor, LPC

Compressor, HPC

Expansion valve

Expansion valve

1

23

4

5

67

8

From application

To application

Circulation pump

Refrigerant R23 flows


Main heater Fine heater

Cap tube

Cooling water supply (≈ 15°C)

Cooling water return (≈ 35°C)

Evaporator, LPC

Evaporator, HPC

Condenser, LPC

Condenser, LPC

Compressor, LPC

Compressor, HPC

Expansion valve

Expansion valve

1

23

4

5

67

8

From application

To application

Circulation pump



Main heater Fine heater

Cap tube

Cooling water supply (≈ 15°C)

Cooling water return (≈ 35°C)


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Example 5

Consider a two stage cascade refrigeration system operating between the pressure limits of 1.2 Mpa and 200 kPa with refrigerant R-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where the pressure at the upper and lower cycles are 0.4 and 0.5 Mpa, respectively. In both cycles, the refrigerant is saturated liquid at the condenser exit and saturated vapor at the compressor inlet, and the isentropic efficiency of the compressor is 80%. If the mass flow rate of refrigerant through the lower cycle is 0.15 kg/s, determine (a) the mass flow rate of the refrigerant through the upper cycle, (b) the rate of heat removal from the refrigerated space, and (c) the COP.


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Example 6

Consider a two stage cascade refrigeration system operating between the pressure limits of 1.2 MPa and 200 kPa with refrigerant R-134a as the working fluid. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.45 MPa. Part of the refrigerant evaporates during the flashing process and this vapor is mixed with the refrigerant leaving the low pressure compressor. The mixture is then compressed to the condenser pressure by the high pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in the evaporator. The mass flowrate of the refrigerant through the low pressure compressor is 0.15 kg/s. Assuming the refrigerant leaves the evaporator as a saturated vapor and the isentropic efficiency is 80% for both compressor, determine(a) The mass flowrate of the refrigerant through the high pressure compressor, (b) the rate of heat removal from the refrigerated space, (c) COP, (d) The rate of heat removal and the COP if this refrigerator operated on a single stage cycle between the same pressure limits with the same compressor efficiency and the same flow rate as in part (a).


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teng - ch06 mec 551 refrigeration

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