chapter 5 refrigeration & heat pumps

SKMM 2423 Applied Thermodynamics

1

Md. Mizanur Rahman

PhD, Chartered Energy Engineer, CEng, MEIFaculty of Mechanical Engineering

Universiti Teknologi Malaysia UTM

Office: C23-228

Email: [email protected]

Chapter 5Refrigeration & Heat Pumps

mailto:[email protected]

Refrigeration & Heat Pumps

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Contents

• Concepts of refrigerators and heat pumps, measure their performance.

• Analyze the ideal & actual vapor-compression refrigeration cycle.

• Selecting the right refrigerant for an application.

• Pressure-Enthalpy diagram

• Flash chamber application

• Innovative vapor-compression refrigeration systems.

• Vapour-absorption refrigeration systems.

Introduction

• Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it to an environment.

• The primary purpose of refrigeration is lowering the temperature of the enclosed space or substance and then maintaining that lower temperature.

• The term cooling refers generally to any natural or artificial process by which heat is dissipated.

• The process of artificially producing extreme cold temperatures is referred to as cryogenics, below -150C

REFRIGERATORS AND HEAT PUMPS

• The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.

• Another device that transfers heat from a low-temperature medium to a high-temperature one is the heat pump.

• Refrigerators and heat pumps are essentially the SAME DEVICES; they differ in their objectives only.

• The objective of a refrigerator is to remove heat (QL) from the cold medium

• The objective of a heat pump is to supply heat (QH) to a warm medium.

Coefficient of Performance

The performance of refrigerators and heat pumps is expressed in terms of the coefficient of performance (COP), defined as,

Both COPR and COPHP can be greater than 1.For fixed values of QL and QH

COPHP = COPR + 1

𝐶𝑂𝑃𝑅 =𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑂𝑢𝑡𝑝𝑢𝑡

𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐼𝑛𝑝𝑢𝑡=𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝐸𝑓𝑓𝑒𝑐𝑡

𝑊𝑜𝑟𝑘 𝐼𝑛𝑝𝑢𝑡=

𝑄𝐿𝑊𝑛𝑒𝑡,𝑖𝑛

𝐶𝑂𝑃𝐻𝑃 =𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑂𝑢𝑡𝑝𝑢𝑡

𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐼𝑛𝑝𝑢𝑡=𝐻𝑒𝑎𝑡𝑖𝑛𝑔 𝐸𝑓𝑓𝑒𝑐𝑡

𝑊𝑜𝑟𝑘 𝐼𝑛𝑝𝑢𝑡=

𝑄𝐻𝑊𝑛𝑒𝑡,𝑖𝑛

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THE REVERSED CARNOT CYCLEThe reversed Carnot cycle is the most efficient refrig. cycle operating between TL

and TH.

It is not a suitable model for refrigeration cycles since processes 1-2 and 3-4 are not practical because Process 1-2 involves the compression of a liquid–vapor mixture, which requires a compressor that will handle two phases, and process 3-4 involves the expansion of high-moisture-content refrigerant in a turbine.

1 – 2 Isentropic compression in an ideal compressor, W12 work input.2 – 3 Isothermal heat rejection, Q23 to the surroundings at T2.3 – 4 Isentropic expansion in an ideal turbine, W34 work output.4 – 1 Isothermal heat absorption from cold chamber at T1.

Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.

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THE REVERSED CARNOT CYCLE

*COP increase as the difference between the two temperatures decreases, that is, as TL rises or TH falls.

*Use as maximum COP or reference COP

Example 1

A refrigerator has working temperatures in the

evaporator and condenser coils of –30 and 32oC

respectively. What is the maximum COP possible? If the

actual refrigerator has a COP of 0.75 of the maximum,

calculate the required power input for a refrigeration

load of 5 kW.

Methods of refrigeration• Can be classified as non-cyclic, cyclic and thermoelectric.

• Non-cyclic refrigeration - cooling is accomplished by melting ice or by subliming dry ice (frozen carbon dioxide). Are used for small-scale refrigeration i.e. laboratories and workshops, or in portable coolers.

• Cyclic refrigeration - Consists of a refrigeration cycle, heat is removed from a low-temperature space/source and rejected to a high-temperature sink with the help of external work

• Cyclic refrigeration can be classified as Vapor cycle and Gas cycle

• Vapor cycle refrigeration can further be classified as:

➢Vapor-compression refrigeration (most common system)

➢Vapor-absorption refrigeration

VAPOR COMPRESSION REFRIGERATION SYSTEM (VCRS)

• Food Processing and storage -Refrigerator

• Building air conditioning system

• Car air conditioning system

• Water cooler

• Ice cube maker

• Low temperature drying process

Operation of Vapour Compression Refrigeration System (VCRS)

1

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4

Vapour Compression Refrigeration Cycle

To be practical, we must modify the ideal reversed Carnot cycle. The modifications are as follows.

1. Replacement of turbine by a throttle valve

Replace turbine with throttle valve. The throttle valve reduces pressure

as required, but the isenthalpic process is highly irreversible. The

penalty is that the refrigerating effect, Q41 is reduced.


2. Condition at compressor inlet

To fully utilize the enthalpy of vaporization of the refrigerant, the

evaporation process in the evaporator is continued until the vapor is dry

saturated. In practice, the process is continued such that the vapor is

slightly superheated. This prevents carry-over of liquid refrigerant

into the compressor, where it interferes with the lubrication.

The compression is now in the superheat region, and the heat rejection

is longer at constant temperature.


3. Undercooling of condensed vapour

Undercooling occurs when the condensed vapour is cooled to a

temperature below the saturation temp. at the condenser pressure.

Undercooling increases the refrigerating effect, Q41. T3 cannot go lower

than the temperature of the surroundings (e.g. cooling water for water-

cooled condeser or ambient air temperature for air-cooled condenser).

Note that line 4 –1 is moved to the left.

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Steady-flow energy balance

Analysis of Vapour Compression Refrigeration

Each component is treated separately as open system with steady flow

Analysis of theoretical cycle:Q41 = h1 – h4 refrigerating effectQ23 = h2 – h3 heat rejection at condenserW = W12 = h2 – h1 work suppliedh1 = h4 isenthalpic throttling process

The P-h diagram of vapor-compression refrigeration cycle.An ordinary household refrigerator.

Example 2

The pressure in the evaporator of a Refrigerant 134a refrigerator is 180 kPa and the pressure in the condenser is 1200 kPa. Calculate the refrigerating effect and the COPR for the following cycles:1) ideal reversed Carnot cycle2) dry saturated vapour delivered to the condenser after

isentropic compression, and no undercooling of the condensed liquid

3) dry saturated vapour delivered to the compressor where it is compressed isentropically, and no undercooling of the condensed liquid

4) dry saturated vapour delivered to the compressor, and the liquid after condensation undercooled by 10 K.

Refrigeration with compressor with isentropic efficiency

Refrigerating Load/CapacityRefrigeration Capacity = heat transfer rate from the cold chamber to

the evaporator.

Units: 1 ton = 200 Btu/min = 3.156 kW.

The pressure-enthalpy diagram▪ More convenient for refrigeration cycles since the enthalpies required

for calculation

▪ Mostly used in refrigeration cycles

Constant line in p-h diagram

Refrigeration cycle in p-h diagram

Cengel 11.112

Cengel 11.34

Cengel 11.20

Compressor type

1) Reciprocating compressors: power input up to 600 kW. Speeds from 200 to 600 rpm. Multi cylinders are used to obtain increased capacity.

2) Centrifugal compressors: power input 300 kW to 15 MW. High volume flow rates; speeds from 3000 to 20,000 rpm. For high pressure ratios, compressors are staged.

3) Screw-type compressors: power input 300 kW to 3 MW.

Compressor typeConsider a reciprocating compressor. The volume flow rate of refrigerant at suction is

vmV••

=

The swept volume for a single-acting compressor is (m3/cycle)

v

snN

VV

•

=where

n = number of cylinders

N = rotational speed (rev/s)

v = volumetric efficiency (usually 65 to 85%)

*From these equations, the larger the specific volume v, the larger is the swept volume Vs for a given mass flow rate.

For double acting compressor, v

snN

VV

2

•

=

Example 4

For the plant in example 3, the compressor is double-acting

reciprocating compressor with a bore of 250mm, a stroke of

300mm, running at 200 rpm, with a volumetric efficiency of

85%. Calculate:

(i) mass flow rate of refrigerant;

(ii) refrigeration capacity;

(iii) required power input to the electric motor when the

mechanical efficiency is 90%.

Flash Chamber▪ Also known as vapour-liquid separator

▪ Flash chamber is used to increase refrigerating effect by moving the inlet-to-evaporator (point 4) to the left.

Why ?

Flash Chamber▪ Also known as vapour-liquid separator

▪ Flash chamber is used to increase refrigerating effect by moving the inlet-to-evaporator (point 4) to the left.

Why ?

31.31C

-10.09C

800 kPa

200 kPa

Flash Chamber▪ Flash chamber is used at some intermediate pressure

▪ The vapour at this pressure can be bled off and fed back to the compression process.

▪ Thus, throttling and compression are done in two stages

4’

Flash Chamber

▪ Amount of dry saturated vapour bled off, x

▪ Total work input, Wcom

▪ Refrigerating effect, qL

▪ Heat rejected in condenser, qH

Eastop

Absorption Refrigeration Cycle

▪ Absorption refrigeration is economical when there is a source of inexpensive thermal energy at a temperature of 100 to 200°C.

▪ Some examples include geothermal energy, solar energy, and waste heat from cogeneration or process steam plants.

cool

hot

Absorption Refrigeration

Vapour Compression Refrigeration VS.

cool

hot



Low pressure vapour transform to High pressure vapour

- Using compressor

-Using 3 processes:1 – Absorb vapour in liquid while removing heat2 – Elevate pressure of liquid with pump3 – Release vapour by applying heat

➢Known as a work-operated cycle

➢Known as a heat-operated cycle (pump used small/ negligible work)



Advantage:- A liquid is compressed instead of vapour, hence work input is small, often neglected.

Disadvantage:- More expensive, complex, more space and required external heat source.

Application:- Used in large commercial and industrial area (when cost of thermal energy is low)

Advantage:-Considerably cheap and less complex.- Small area utilization, hence system location is optimized.

Disadvantage:- Bigger work input.

Application:- portable, domestic and large commercial.

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Determining the maximum COP (assuming totally reversible) of an absorption refrigeration system.

The COP of actual absorption refrigeration systems is usually less than 1.

Absorption Refrigeration Cycle

actual

ideal

Cengel

Cengel 11–48 Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.2 MPa and 200 kPa with refrigerant-134a as the working fluid. The refrigerant leaves the condenser as a saturated liquid and is throttled to aflash chamber operating at 0.45 MPa. Part of the refrigerant evaporates during this 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 flow rate 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 percent for both compressors, determine(a) the mass flow rate of the refrigerant throughthe high-pressure compressor,

(b) the rate of heat removal from the refrigerated space,and (c) the COP of this refrigerator. Also, determine(d) the rate of heat removal and

the COP if this refrigerator operated on asingle-stage cycle between the same pressure limits with the same compressor efficiencyand the same flow rate as in part (a).

chapter 5 refrigeration & heat pumps

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