ME−411 3(2, 1)

REFRIGERATION & AIR-

CONDITIONING (RAC)
Teacher In-charge
PROF. DR. ASAD NAEEM SHAH
anaeems@uet.edu.pk

DEPARTMENT OF MECHANICAL ENGINEERING

UNIVERSITY OF ENGINEERING AND TCHNOLOGY LAHORE
 RAC PART 1

REFRIGERATION
Cont.…

Arranged by Prof. Dr. Asad Naeem Shah

 PROPERTIES OF REFRIGERANTS
o The only properties of refrigerant that have been
discussed so far are the characteristic temperature-
entropy ( 𝑇 − 𝑠) relationships of saturated liquid and
vapor. Other thermodynamic properties are also
necessary for refrigeration work.
o Following are some useful charts (or diagrams) to
discuss the various properties of refrigerants:
1. 𝑇 − 𝑠 diagram
2. 𝑇 − 𝑣 diagram
3. 𝑃 − 𝑣 diagram
4. 𝑃 − ℎ diagram
5. ℎ − 𝑠 diagram
Arranged by Prof. Dr. Asad Naeem Shah
 PROPERTIES OF REFRIGERANTS Cont.
➢ PROPERTIES ON 𝑷 − 𝒉 CHART:
o In refrigeration practice, the enthalpy is one of the most
important properties sought, and the pressure can
usually be determined very easily. A skeleton pressure-
enthalpy diagram is shown in Fig. 1. The pressure is the
ordinate and the enthalpy the abscissa.
o The saturated-vapor (SV) and saturated-liquid (SL) lines
are the reference lines, while the constant-temperature
line is horizontal in the mixture region because here the
temperature must correspond with the saturation
pressure. The subcooled-liquid or compressed-liquid
region is to the left of SL line, and the constant-
temperature line is vertical in this region.
Arranged by Prof. Dr. Asad Naeem Shah
 PROPERTIES OF REFRIGERANTS Cont.
o The temperature of a compressed liquid therefore
determines the enthalpy ℎ and not the pressure. To find
the enthalpy of liquid water that is subcooled, the enthalpy
is read as the enthalpy of SL at the existing temperature,
even though the actual pressure is higher than the
saturation pressure.

Fig. 1: The 𝑃 − ℎ diagram of a refrigerant. Arranged by Prof. Dr. Asad Naeem Shah
 PROPERTIES OF REFRIGERANTS Cont.
o The superheat region is to the right of SV line. In this
region the line of constant temperature drops first slightly
to the right and then vertically. When the line of constant
temperature becomes vertical, ∆ℎ = 𝑐𝑜𝑛𝑠𝑡 ∆𝑡 , the
typical relationship of enthalpy and temperature of a
perfect gas.
o The line of constant specific volume (𝑣) slopes to the
right such that the lines of higher ‘ 𝑣 ’ are found at
progressively lower pressures (Fig. 1).
o The line of constant entropy (𝑠) runs upward to the right.
An isentropic compression shows the expected increase
in enthalpy as the pressure increases during a
compression.
Arranged by Prof. Dr. Asad Naeem Shah
 PROPERTIES OF REFRIGERANTS Cont.
o Pressure-enthalpy charts for the superheated region of
ammonia, refrigerants 11, 12, 22 (Freon 22) & 502 are
shown in the appendix Figs. A-1 to A-5, respectively.
o Tabular property data are shown for these refrigerants in
Tables A-3 to A-8. All the tables pertain to liquid and
saturated vapor except for Table A-7 which applies to
superheated refrigerant 22 vapor.

Arranged by Prof. Dr. Asad Naeem Shah

THE STANDARD VAPOR COMPRESSION
CYCLE (VCC)
o The standard VCC on the 𝑃 − ℎ diagram is shown in Fig.
1(a) , while the schematic diagram of the equipment is
given in Fig. 1 (b).

𝑏
𝑎
Fig. 1: (a) The standard VCC on the 𝑃 − ℎ diagram, (b) flow diagram
Arranged by Prof. Dr. Asad Naeem Shah
 THE STANDARD VCC Cont.
➢ PERFORMANCE OF THE STANDARD VCC:
o The following significant quantities of the standard VCC
can be determined with the help of 𝑃 − ℎ chart:
• The work of compression (w): It is the change in enthalpy
in process 1-2 along the constant-entropy line from
saturated vapor to the condenser pressure. As per SFEE:
∵ ℎ1 + 𝐾𝐸1 + 𝑃𝐸1 + 𝑞 = ℎ2 + 𝐾𝐸2 + 𝑃𝐸2 + 𝑤
𝑤 = ℎ1 − ℎ2 → 1
• The heat rejection rate (𝑞𝑟 ): It is the constant-pressure de-
superheating and condensation along a straight horizontal
line on the chart, i.e., it is the heat transferred from the
refrigerant during the process 2-3.
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 THE STANDARD VCC Cont.
Therefore,
𝑞𝑟 = ℎ3 − ℎ2 → (2)
• The throttling effect: The throttling process 3-4 is an
isenthalpic process and therefore is vertical on the chart,
ℎ3 = ℎ4 → (3)
• The refrigerating effect (𝑞𝑎 ): It is the heat transferred in
process 4-1, and is expressed as
𝑞𝑎 = ℎ1 − ℎ4 → (4)
• The coefficient of performance (COP): As it is the
ratio of refrigerating effect to the work of
compression, so:
ℎ1 − ℎ4
𝐶𝑂𝑃 = → (5)
ℎ2 − ℎ1
Arranged by Prof. Dr. Asad Naeem Shah
 THE STANDARD VCC Cont.
ሶ It is a rough indication of the
• The volume flow rate (𝑉):
physical size of the compressor, and is given in 𝑚3 Τ𝑠 as:
𝑉ሶ = 𝑚ሶ × 𝑣 → (5)
• The power per kilowatt of refrigeration: It is the
compressor power per kW of refrigeration, and is inverse
of the COP. It should be as low as possible.
• The compressor discharge temperature: It is the
temperature of superheated vapor at point 2 on the 𝑃 − ℎ
chart.

Arranged by Prof. Dr. Asad Naeem Shah

 PROBLEM
A standard vapor-compression cycle developing 50 kW of
refrigeration using refrigerant 22 operates with a condensing
temperature of 35˚C and an evaporating temperature of -
10˚C. Calculate (a) the refrigerating effect in kilojoules per
kilogram, (b) the circulation rate of refrigerant in kilograms
per second, (c) the power required by the compressor in
kilowatts, (d) the coefficient of performance, (e) the volume
flow rate measured at the compressor suction, (f) the power
per kilowatt of refrigeration, and (g) the compressor
discharge temperature.
Arranged by Prof. Dr. Asad Naeem Shah
SOLUTION

Arranged by Prof. Dr. Asad Naeem Shah

Arranged by Prof. Dr. Asad Naeem Shah
Arranged by Prof. Dr. Asad Naeem Shah
o From Table A-6 the properties at key point corresponding
to −10℃ are:
𝒉𝟏 = ℎ𝑔1 = 401.6 𝑘𝐽/𝑘𝑔
𝑠1 = 𝑠𝑔1 = 1.767 𝑘𝐽Τ𝑘𝑔 𝐾
𝑣1 = 𝑣𝑔1 = 65.3399 𝐿Τ𝑘𝑔 = 0.0654 𝑚3 Τ𝑘𝑔
o To find ℎ2 move at a constant entropy from point 1 until
reaching the saturation pressure corresponding to 35℃.
This corresponding pressure is 1354𝑘𝑃𝑎, and thus:
𝒉𝟐 = 435.2 𝑘𝐽/𝑘𝑔
o The values of ℎ3 and ℎ4 are identical and are equal to the
enthalpy of saturated liquid (ℎ𝑓 ) at 35℃ i.e.,
𝒉𝟑 = 𝒉𝟒 = 243.1 𝑘𝐽/𝑘𝑔
Arranged by Prof. Dr. Asad Naeem Shah
a) The refrigerant effect = ℎ1 − ℎ4
= 401.6 − 243.1 = 𝟏𝟓𝟖. 𝟓 𝒌𝑱/𝒌𝒈
𝑞𝑎 50
b) Mass flow rate of refrigerant = 𝑚ሶ = =
ℎ1 −ℎ4 158.5
= 𝟎. 𝟑𝟏𝟓 𝒌𝒈/𝒔
c) Compressor power = 𝑚ሶ ℎ2 − ℎ1
= 0.315 435.2 − 401.6
= 𝟏𝟎. 𝟔 𝒌𝑾
𝑅𝐸 50
d) COP = = = 𝟒. 𝟕𝟐
𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑝𝑜𝑤𝑒𝑟 10.6
ℎ1 − ℎ4
(𝐴𝑙𝑠𝑜, 𝐶𝑂𝑃 = )
ℎ2 − ℎ1
e) As from Table A-6 or Fig. A-4;
𝑣1 = 65.3399 𝐿Τ𝑘𝑔 = 0.0654 𝑚3 Τ𝑘𝑔
⇒ 𝑉ሶ = 𝑚ሶ × 𝑣1 = 0.315 × 0.0654 = 0.0206𝑚3 /𝑠 = 𝟐𝟎. 𝟔 𝑳/𝒔
Arranged by Prof. Dr. Asad Naeem Shah
f) The compressor power per kW of refrigeration (i.e.,
1
reciprocal of COP) = = 𝟎. 𝟐𝟏𝟐 𝒌𝑾/𝒌𝑾
4.72

g) The compressor discharge temperature is the

temperature of superheated vapor at point 2 which from
Fig. A-4 (corresponding to constant 𝑃 −line & constant
𝑠 −line on 𝑃 − ℎ chart) is found to be:
𝑡2 = 57℃

NOTE: The values of ℎ2 & 𝑡2 can only be determined either

from Fig.A-4 or by interpolating in Table A-7 at the pressure
and entropy applicable.

Arranged by Prof. Dr. Asad Naeem Shah

 USE OF HEAT EXCHANGERS
o Some refrigeration systems use a liquid-to-suction heat
exchanger (HE), which sub-cools the liquid from the
condenser with suction-vapor coming from the
evaporator. The arrangement is shown in Fig.1(a) and
the corresponding 𝑃 − ℎ diagram in Fig. 1(b).

𝑎 𝑏

Fig. 1: (a) Refrigeration system with HE, and (b) 𝑃 − ℎ diagram of the system
Arranged by Prof. Dr. Asad Naeem Shah
 USE OF HEAT EXCHANGERS Cont.
o Saturated liquid at point 3 coming from the condenser is
cooled to point 4 by means of vapor at point 6 being
heated to point 1.
o From the heat balance:
ℎ3 − ℎ4 = ℎ1 − ℎ6
⇒ ℎ3 − ℎ5 = ℎ1 − ℎ6
---------

⇒ ℎ6 − ℎ5 = ℎ1 − ℎ3
which is the refrigerating effect.
➢ APPARENT ADVANTAGES:
• The system with HE may seem to have obvious
advantages of increased refrigerating effect (RE) relative
to the standard VCC. Both the capacity & COP may
seem to be improved.
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 USE OF HEAT EXCHANGERS Cont.
• This is not necessarily true, however.
➢ REALITY:
• Even though the RE is increased, the compression is
pushed farther out into the superheat region, leading to
the more work input than that it is close to the SV-line.
• From the standpoint of capacity, point 1 has a higher
specific volume than point 6, so that a compressor which
is able to pump a certain volume delivers less mass flow if
the intake is at point 1.
• The potential improvements in performance are thus
counterbalanced, and the HE probably has negligible
thermodynamic advantages.
Arranged by Prof. Dr. Asad Naeem Shah
 USE OF HEAT EXCHANGERS Cont.
➢ CONCLUSIONS:
• The HE (Fig. 2) is definitely justified, however, in
situations where the vapor entering the compressor must
be superheated to ensure that no liquid enters the
compressor.
• Another practical reason for using the HE is to sub-cool
the liquid from the condenser to prevent bubbles of vapor
from impeding the flow of refrigerant through the
expansion valve (EV).

Fig. 2: A liquid-to-suction heat exchanger.

Arranged by Prof. Dr. Asad Naeem Shah
 ACTUAL VAPOR-COMPRESSION
CYCLE (VCC)
o The actual VCC suffers from inefficiencies compared
with the standard cycle. There are also other changes
from the standard VCC, which may be intentional or
unavoidable.
o Some comparisons can be drawn by superimposing the
actual cycle on the 𝑃 − ℎ diagram of the standard cycle
as shown in Fig. 1.

Fig. 1: Actual VCC compared with

standard VCC.
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 ACTUAL VCC Cont.
o The essential differences between the two cycles are
the pressure drops in the condenser & evaporator, in
the sub-cooling of the liquid leaving the condenser & in
the superheating of the vapor leaving the evaporator.
o The pressure drops in the actual cycle occur because
of friction, and thus lead to the more demand of work
input during the process 1-2 than in the standard cycle.
o Sub-cooling of the liquid in the condenser is a normal
occurrence and serves the desirable function of
ensuring that 100 percent liquid will enter the expansion
device.
Arranged by Prof. Dr. Asad Naeem Shah
 ACTUAL VCC Cont.
o Superheating of the vapor usually occurs in the
evaporator and is recommended as a precaution against
droplets of liquid being carried over into the compressor.
o The final difference in the actual cycle is that the
compression is no longer isentropic and there are
inefficiencies owing to friction and other losses.

Arranged by Prof. Dr. Asad Naeem Shah

Arranged by Prof. Dr. Asad Naeem Shah
PROBLEM # 6

SOLUTION

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SOLUTION Cont.

Arranged by Prof. Dr. Asad Naeem Shah