5/19/2016

Sana’a University Mechanical Engineering department

Vapor Compression Refrigeration System

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

• The coefficient of performance is an index of performance of a

thermodynamic cycle or a thermal system. Because the COP can
be greater than 1, COP is used instead of thermal efficiency. The
coefficient of performance can be used for the analysis of the
following:
• A refrigerator that is used to produce a refrigeration effect only,
that is, COPref
• A heat pump in which the heating effect is produced by rejected
heat COPhp
• A heat recovery system in which both the refrigeration effect
and the heating effect are used at thesame time, COPhr

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

►Most common refrigeration cycle in use today

►There are four principal
control volumes involving
these components:
►Evaporator
►Compressor
►Condenser
►Expansion valve Two-phase
liquid-vapor mixture

All energy transfers by work and heat are taken as positive in

the directions of the arrows on the schematic and energy
balances are written accordingly.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

• Evaporation. In this process, the refrigerant evaporates at a lower
temperature than that of its surroundings, absorbing its latent heat of
vaporization.

• Superheating. Saturated refrigerant vapor is usually superheated to

ensure that liquid refrigerant does not flow into the compressor.

• Compression. Refrigerant is compressed to a higher pressure and

temperature for condensation.

• Condensation. Gaseous refrigerant is condensed to liquid form by

being desuperheated, then condensed, and finally subcooled,
transferring its latent heat of condensation to a coolant.

• Throttling and expansion. The higher-pressure liquid refrigerant is

throttled to the lower evaporating pressure and is ready for
evaporation.

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Sana’a University Mechanical Engineering department

►The processes of this cycle are

Process 4-1: two-phase liquid-vapor
mixture of refrigerant is evaporated
through heat transfer from the
refrigerated space.
Process 1-2: vapor refrigerant is
compressed to a relatively high
temperature and pressure requiring
work input. Two-phase
liquid-vapor mixture
Process 2-3: vapor refrigerant
condenses to liquid through heat
transfer to the cooler surroundings.
Process 3-4: liquid refrigerant
expands to the evaporator pressure.
Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

►Engineering model:
►Each component is analyzed as a control
volume at steady state.
►Dry compression is presumed: the
refrigerant is a vapor.
►The compressor operates adiabatically.
►The refrigerant expanding through the valve
undergoes a throttling process.
►Kinetic and potential energy changes are
ignored.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

►Applying mass and energy rate balances

Evaporator Q in
 h h 1 4
m
►The term Qisin referred to as the
refrigeration capacity, expressed in kW in
the SI unit system or Btu/h in the English
unit system.
►A common alternate unit is the ton of
refrigeration which equals 200 Btu/min or
about 211 kJ/min.

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

►Applying mass and energy rate balances

Compressor Wc
 h2  h1
Assuming adiabatic 
m
compression
Condenser Q out
 h2  h3
m
Expansion valve
Assuming a throttling h4  h3
process

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

►Performance parameters
Coefficient of Performance (COP)
ℎ −ℎ
= =
ℎ −ℎ

Carnot Coefficient of Performance

=
−

This equation represents the maximum theoretical

coefficient of performance of any refrigeration cycle
operating between cold and hot regions at TC and TH,
respectively.
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 5/19/2016

Sana’a University Mechanical Engineering department

Effect of evaporator temperature

Effect of evaporator temperature on Effect of evaporator temperature on cycle

cycle performance (P-h diagram) performance (T-s diagram)
For example, Fig. shows the effect of decreasing evaporator temperatures on T s and P h
diagrams. It can be seen from the T s diagrams that for a given condenser temperature, as
evaporator temperature decreases: (1) Throttling losses increase , (2) Superheat losses increase
(3) Compressor discharge temperature increases , (4) Quality of the vapour at the inlet to the
evaporator increases , (5) Specific volume at the inlet to the compressor increases
As a result of this, the refrigeration effect decreases and work of compression increases as shown
in the P h diagram. Coefficient of performance is decreased
Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Example 4.1
Refrigerant-134a enters the compressor of a vapor-compression refrigeration cycle at
120 kPa as a saturated vapor and leaves at 900 kPa and 75 ◦C (Figure 4.2a) . The
refrigerant leaves the condenser as a saturated liquid. The rate of cooling provided by
the system is 18,000 Btu/h. Determine (a) the mass flow rate of R-134a and (b) the
COP of the cycle. (c) Also, determine the COP of the cycle if the expansion valve is
replaced by an isentropic turbine. Do you recommend such a replacement
for refrigeration systems? (d) Determine the COP if the evaporator pressure is 160 kPa
and other values remain the same. (e) Determine the COP if the condenser pressure is
800 kPa and other values remain the same

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Sana’a University Mechanical Engineering department

Condenser and evaporator temperature selection

For condenser ( 10 – 15 ): For practical air cooled condenser calculations use

for DT = 15 K. Most manufacturers are determining in catalogues this number.
For tropical countries it is recommended to reduce this number to DT = 10 K.
Than larger condensers, produce less condenser pressure pc.
For Evaporator (5 – 10 )
The most important factor guiding the humidity in the refrigerated space is the
evaporator DT. The smaller the temperature difference between evaporator to and
room tR the higher the relative humidity in the space. Likewise, the greater the
evaporator DT the lower is the relative humidity in the space.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

Subcooling

Condensed liquid refrigerant is usually

subcooled to a temperature lower than
the saturated temperature
corresponding to the condensing
pressure of the refrigerant, shown in
Fig. as point 6. This is done to
increase the refrigerating effect, The
degree of subcooling depends mainly
on the temperature of the coolant (e.g.,
atmospheric air, surface water, or well
water) during condensation, and the
construction and capacity of the
condenser.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Superheating
As mentioned before, the purpose of superheating is to avoid compressor slugging
damage. Superheating is shown in Fig. The degree of superheat depends mainly on
the type of refrigerant feed and compressor as well as the construction of the
evaporator

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Sana’a University Mechanical Engineering department

Using Heat Exchanger

(a) A vapor-compression refrigeration system with a heat exchanger for superheating

and subcooling, (b) its T–s diagram, and (c) its log P–h diagram.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Practical Vapor-Compression Refrigeration Cycle

An actual vapor-compression refrigeration system and its T–s diagram.

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

Using for flash chamber

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Flash gas removal using flash tank
It is mentioned above that one of the problems with high temperature lift applications is the
high quality of vapour at the inlet to the evaporator. This vapour called as flash gas develops
during the throttling process. The flash gas has to be compressed to condenser pressure, it does
not contribute to the refrigeration effect as it is already in the form of vapour, and it increases
the pressure drop in the evaporator .

Working principle of a flash tank Expansion process using a flash tank on P-h diagram
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Sana’a University Mechanical Engineering department

Fraction of Evaporated Refrigerant in Flash Cooler

The fraction x also indicates the quality, or

dryness fraction, of the vapor and liquid
mixture in the flash cooler at the interstage
pressure

Heat balance of entering and leaving

refrigerants in a flash cooler and at the mixing
point: (a) in the flash cooler; (b) at the mixing
point 3 before entering the second-stage
impeller.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

Using for Accumulator

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

MULTISTAGE VAPOR COMPRESSION SYSTEMS

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

MULTISTAGE VAPOR COMPRESSION SYSTEMS

When a refrigeration system uses more than single-stage compression process, it is
called a multistage system (as shown in Fig. 9.7), and may include the following:
1. A high-stage compressor and a low-stage compressor
2. Several compressors connected in series
3. Two or more impellers connected internally in series and driven by the same motor
or prime mover, as shown in Fig. 9.7
4. A combination of two separate refrigeration systems
The discharge pressure of the low-stage compressor, which is equal to the suction
pressure of the
high-stage compressor, is called the interstage pressure. The reasons for using a
multistage vapor compression system instead of a single-stage system
are as follows:
1. The compression ratio Rcom of each stage in a multistage system is smaller than that
in a single stage unit, so compressor efficiency is increased. Compression ratio Rcom is
defined as the ratio of the compressor’s discharge pressure pdis, psia (kPa abs.), to
the suction pressure at the compressor’s inlet psuc, psia (kPa abs.), or

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

2. Liquid refrigerant enters the evaporator at a lower enthalpy and increases the
refrigeration effect.
3. Discharge gas from the low-stage compressor can be desuperheated at the
interstage pressure.
This results in a lower discharge temperature from the high-stage compressor
than would be
produced by a single-stage system at the same pressure differential between
condensing and
evaporating pressures.
4. Two or three compressors in a multistage system provide much greater
flexibility to accommodate
the variation of refrigeration loads at various evaporating temperatures during
part-load operation.
The drawbacks of the multistage system are higher initial cost and a more
complicated system
than that for a single-stage system.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Interstage Pressure
Interstage pressure is usually set so that the compression ratio at each stage is
nearly the same for higher COPs. For a two-stage compound system, Interstage
pressure pi, psia (kPa abs.), can be calculated

Flash Cooler and Intercooler

In compound systems, flash coolers are used to subcool liquid refrigerant to the
saturated temperature corresponding to the interstage pressure by vaporizing part of
the liquid refrigerant. Intercoolers are used to desuperheat the discharge gas from the
low-stage compressor and, more often, to subcool also the liquid refrigerant before it
enters the evaporator.

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

Compound Systems
Multistage vapor compression systems are classified as compound systems or cascade
systems. Cascade systems are discussed in a later section.
A compound system consists of two or more compression stages connected in series.
For reciprocating, scroll, or screw compressors, each compression stage usually
requires a separate

Two-stage compound system with a flash cooler: (a) schematic diagram; (b) refrigeration cycle.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Fraction of Evaporated Refrigerant in Flash Cooler

The fraction x also indicates the quality, or

dryness fraction, of the vapor and liquid
mixture in the flash cooler at the interstage
pressure

Heat balance of entering and leaving

refrigerants in a flash cooler and at the mixing
point: (a) in the flash cooler; (b) at the mixing
point 3 before entering the second-stage
impeller.

Dr. Abduljalil Al-Abidi HVAC

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Sana’a University Mechanical Engineering department

A two-stage compression system with flash tank for flash gas removal only (a) System
schematic; (b) Cycle on P-h diagram

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Example 9.1. A 500-ton (1760-kW) single-stage centrifugal vapor compression system
uses HCFC-22 as refrigerant. The vapor refrigerant enters the compressor at dry
saturated state. The compression process is assumed to be isentropic. Hot gas is
discharged to the condenser and condensed at a temperature of 95°F (35°C). The
saturated liquid refrigerant then flows through a throttling device and evaporates at a
temperature of 35°F (1.7°C). Calculate:
1. The refrigeration effect
2. The work input to the compressor
3. The coefficient of performance of this refrigeration cycle
4. The mass flow rate of the refrigerant

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Sana’a University Mechanical Engineering department

Intercooling in multi-stage compression

The specific work input, w in reversible, polytropic compression of refrigerant vapour

is given by:

where P1 and P2 are the inlet and exit pressures of the compressor, v1 is the specific
volume of the refrigerant vapour at the inlet to the compressor and n is the polytropic
exponent. From the above expression, it can be seen that specific work input reduces
as specific volume, v1 is reduced. At a given pressure, the specific volume can be
reduced by reducing the temperature. This is the principle behind intercooling in
multi-stage compression. Figures (a) and (b) show the process of intercooling in two-
stage compression on Pressure-specific volume (P-v) and P-h diagrams.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Fig. (a) & (b): Intercooling in two-stage compression

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Intercooling using external water cooled heat Intercooling using liquid refrigerant in flash
exchanger tank

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

Using Flash tank and Intercooling in multi-stage compression

Two —stage vapour compression refrigeration Two —stage vapour compression refrigeration
system with flash gas removal using a flash system with flash gas removal using a flash
tank and intercooling tank and intercooling — P—h diagram

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Use of flash tank for intercooling only

A two-stage compression system with the flash tank used for intercooling only (a) System
schematic (b) Cycle on P-h diagram

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

(a) A two-stage vapor-compression refrigeration system, (b) its T−s diagram, and (c)
its log P−h diagram.

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Sana’a University Mechanical Engineering department

Two-stage compound system with a vertical coil intercooler: (a) schematic diagram; (b)
refrigeration cycle.
Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

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Sana’a University Mechanical Engineering department

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

5.3 Cascade Refrigeration Systems

(a) Schematic of a two-stage (binary) cascade refrigeration system, (b) its T–s
diagram, and (c) its log P–h diagram.

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Example:
Consider a two-stage cascade refrigeration system operating between the pressure
limits of 1.6MPa and 180 kPa with refrigerant-134a as the working fluid (Figure 5.5).
Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic
counter-flow heat exchanger where the pressure in the upper and lower cycles are
0.4 and 0.5MPa, respectively. In both cycles, the refrigerant is a saturated liquid at
the condenser exit and a saturated vapor at the compressor inlet, and the
isentropic efficiency of the compressor is 85%. If the mass flow rate of the refrigerant
through the lower cycle is 0.07 kg/s, (a) draw the temperature–entropy diagram of the
cycle indicating pressures; determine (b) the mass flow rate of the refrigerant through
the upper cycle, (c) the rate of heat removal from the refrigerated space, and (d) the
COP of this refrigerator; and (e) determine 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. Also, take the mass flow rate of R-134a
through the cycle to be 0.07 kg/s.

Dr. Abduljalil Al-Abidi HVAC

Sana’a University Mechanical Engineering department

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