MISR UNIVERSITY FOR SCIENCE &TECHNOLOGY
Mechanical department
First semester
Faculty of engineering
Academic year 2020/2021
Refrigeration and air conditioning
Sheet No 1: Introduction
1- A machine follows the reversed Carnot cycle between the temperature limits of 0 oC and 30 oC.
a) Determine the coefficient of performance when it works as a refrigerator and as a heat pump.
What would be the coefficient of performance if the heat-sink temperature was increased to 40
o
C
b) If the rate of heat extraction from the low-temperature source is 58.15 kW. Calculate the power
input and rate of heat rejection in kW for both cases (refrigerator and heat pump). Will they be
different?
2) A new refrigerator is said to require 1.103 kW for a duty of 2.791 kW when working between the
temperatures limits of -15 oC and 30 oC. Can this be true?
3) A heat pump system is used to serve a heating load of 116.3 kW at a temperature level of 40 oC. If the
heat-source temperature is 0 oC. Determine the power-input requirement in kW assuming that the relative
efficiency is 70 %
4) A system works on the reversed Carnot cycle between a high temperature of 40 oC and a low temperature
of:
 -50 oC
 -25 oC
 0 oC
 20 oC
Determine the C.O.P. of the system as a refrigerator and as a heat pump. Represent the results on a suitable
diagram. Repeat the results if the high temperature is lowered to 30 oC. Comment on the results.
5) A refrigeration system follows the reversed Carnot cycle between a heat source and sink temperatures of
T1 and T2 respectively. It is possible either to raise T1 or reduce T2 by 2 K. Determine which of the two
alternatives is more effective for increasing the C.O.P.
MISR UNIVERSITY FOR SCIENCE &TECHNOLOGY
Mechanical department
First semester
Faculty of engineering
Academic year 2020/2021
Refrigeration and air conditioning
Sheet No 2: single stage Refrigeration systems
1. Catalogue data for a refrigerant R-12 compressor with a piston displacement of 1.7 m³/min shows the
capacity to be 13.7 tons and brake power 14 kW when the suction conditions are 18 °C and 3 bar and the
condensing temperature is 35 °C. The refrigerant leaves the condenser as saturated liquid. Calculate the
actual volumetric efficiency and the adiabatic compression efficiency of the compressor at these conditions.
(Take the mechanical efficiency to be 80 %).
nd a condensing temperature of 32 °C. The expansion valve is supplied
with the liquid at 24 °C and the vapor leaves the evaporator with 9 °C superheat. If the compressor used
has 4 cylinders with a strokeine:
a. Weight of refrigerant to be circulated per minute.
b. Power of the compressor, assuming a compression efficiency of 84 % and a mechanical efficiency of 78
%.
c. Bore and stroke of the compressor if volumetric efficiency is 82 %.
d. Cooling water required for the condenser in m³/min allowing for 8 °C temperature rise in the cooling
water.
e. The C.O.P. and the refrigeration efficiency.
r
temperature of – 8 °C and a condenser temperature of 30 °C. The refrigerant, ammonia, is sub cooled 5 °C
before entering the expansion valve, and vapor leaves the evaporator with 6 °C superheat. The compressor
has compression efficiency of 80 %, volumetric efficiency of 85 %, and mechanical efficiency of 76 % and
runs at 900 rpm. Strokea. The weight of refrigerant to be circulated per minute.
b. The power of the compressor.
c. The bore and stroke of the compressor.
d. The C.O.P. of the cycle and its refrigerating efficiency.
ar, ammonia liquid leaves condenser
– 12 °C. The vapor
A compr
single stage, water-jacketed, with a maximum rpm of 600. Cylinder diameter is 10 cm and stroke is 12 cm.
The following supplemental assumptions to al
compressor mechanical efficiency 80 %.
a. Determine if the available compressor can be used. What rpm should it operate at?
b. Estimate the power required to drive the compressor.
.3 bar,
of – 6.5 °C and a condenser temperature of 30 °C. The refrigerant, ammonia, is sub cooled 5 °C before
entering the expansion valve, and the vapor is superheated 5 °C before leaving the evaporator coil. A twocylinder, vertical, single-acting, water-valve pressure drop of 0.3 bar and a dischargevalve pressure drop of 0.15 bar. Compression follows a polytropic path with
n=1.2. The compressor
mechanical efficiency is 80 %. Clearance volume of the compressor is 2 %. Determine:
a. Refrigerating effect.
b. Weight of refrigerant to be circulated per minute.
c. Volumetric efficiency.
d. Piston displacement per minute.
e. Bore and stroke of the compressor. f. Power required to drive the compressor.
g. Coefficient of performance.
h. Heat removed through condenser.
i. Heat rejected to compressor cooling water.
6. It is proposed to use a heat pump working on the ideal vapor-compression cycle for the purpose of
heating the air supply to a building. The supply of heat is taken from a river at 7 oC. Air is required to be
delivered into the building at 1 bar and 30 oC at a rate of 0.5 m3/s. The air is heated at constant pressure
from 10 oC as it passes over the condenser coils of the heat pump. The refrigerant is R12 which is dry
saturated when leaving the evaporator; there is no subcooling in the condenser. A temperature difference
of 17 oC is necessary for the transfer of heat from the river to the refrigerant in the evaporator. The
o
C. Calculate:
a) The mass flow rate of the refrigerant.
b) The motor power required to drive the compressor if the mechanical efficiency is 87 %.
c) The COP of the heat pump.
d) The stroke volume of the compressor which is running at 240 rpm with a volumetric efficiency of 85 %.
MISR UNIVERSITY FOR SCIENCE &TECHNOLOGY
Mechanical department
First semester
Faculty of engineering
Academic year 2020/2021
Refrigeration and air conditioning
Sheet No 3: Malty stage Refrigeration systems
1- The following conditions apply to 100 ton, ammonia compound vapour-compression system with water
intercooler. Condenser pressure is 1.5 MPa, evaporator pressure is 0.15 MPa, intercooler pressure is 0.5
MPa and volumetric efficiency is 85% for low-pressure stage and 78% for high-pressure stage. Assume
pressure drop through compressor valve as follows: low-pressure suction= 0.015MPa, low-pressure
discharge=0.035MPa, high-pressure suction= 0.03 MPa, high-pressure discharge=0.07 MPa. Ammonia may
be cooled to 32 oC in intercooler ; and subcooled liquid from condenser is at 30 oC and suction temperature
is -18 oC. Temperature leaving the brine cooler= -21 oC. Low-pressure compression is isentropic. Highpressure compression has an index (n) of 1.27 Both cylinders are double acting. Calculate:
(a) Ammonia to be circulated per second.
(b) Indicated power of high pressure cylinder.
(c) Indicated power of low pressure cylinder.
(d) Heat rejected to intercooler (kW).
(e) Piston displacement of low pressure cylinder and high pressure cylinder m3 / s.
(f) Coefficient of performance.
2-a- A cold store for frozen fish is to be maintained at a temperature of -7oC. The estimated refrigeration
load for the store is 174.45 kW, and is to be carried by an ammonia plant working at a condenser pressure of
1 MPa, and an evaporator pressure of 0.25 MPa. If a 4-cylinder compressor running at 600 R.P.M. is used
and having a compression efficiency of 80%, volumetric efficiency of 75% and mechanical efficiency 80%
calculate:
 The brake power in kW of the compressor.
 The bore and stroke of the compressor.
 The C.O.P of the plant and its relative efficiency.
b- If the above-mentioned ammonia compressor is to be two stages with a water intercooler, make a suitable
design of the plant and determine :
 The intermediate pressure.
 The %age saving in the broke power of the compressor.
3- An ammonia plant is to be designed for an ice factory having a daily output of 65 tons of ice under the
following design conditions:
· Average temperature of available water is 22oC.
· Temperature of produced ice is – 6 oC.
· Freezing time is 19 hours and the estimated heat gain from surroundings into ice tank amounts to
10% of the freezing load.
a) Select suitable operating conditions for a simple ammonia system and then calculate the brake power of
the compressor; assuming a compression efficiency of 84% and a mechanical efficiency of 90%.
b) If using a two-stage compressor with a water intercooler, determine the percentage saving in the power of
the compressor assuming a compression efficiency of 84% for low-pressure stage and 86% for high-pressure
stage. Calculate also the percentage improvement in the C.O.P of the system due to compounding.
4- Determine:
(a) The C.O.P.
(b) The maximum cycle temperature.
(c) The total piston displacement per sec. per ton for the
theoretical single-stage cycle and for the cycle shown in
figure. For each cycle assume an evaporating temperature of
– 40oC condensing temperature of 38 oC and a compressor
clearance of 5%. The refrigerant is ammonia. For the twostage cycle, assume that the intermediate saturation
temperature is -11oC, the vapour leaves the water intercooler
at 38°C and the compression is isentropic.
5- An industrial plant uses a R- 12 system with one compressor to serve both an air conditioning evaporator
and a low-temperature evaporator for process refrigeration. The air conditioning evaporator is a liquid
chiller having a capacity of 80 tons and is maintained at a temperature of 4.5oC by means of a pressure
regulating valve located at the outlet of the evaporator. The low-pressure evaporator has a capacity of 25
tons and operates at a temperature of -12oC. The compressor suction pressure is the same as the pressure in
the low temperature evaporator and the condensing temperature is 32 oC. Calculate the power required by
compressor.
6- An ammonia system having a 45 ton evaporator operating
at -1o 0C, and a 10 ton evaporator operating at -40oC utilizes
flash gas removal and intercooling. The condensing
temperature is 32 oC. The arrangement is as shown in figure.
Calculate the power required for each compressor.
Note:
Assume saturated refrigerant leaves condenser and each
evaporators
7- An ammonia vapor-compression system is arranged
as shown in figure. Assume isentropic compression in
both stages. The following temperatures are given :
t1 = 32°C
t10 = -7°C
t5 = -40°C
t3 = 4.5°C
t7 = 38°C
If the low-pressure evaporator produces twice the
capacity of the high-pressure evaporator, find the C.O.P
of the cycle.
8- A two-stage vapor-compression ammonia
refrigerator is shown in figure. Condenser pressure is
1.5 MPa. The low-temperature evaporator is kept at
a pressure of 0.08 MPa while the high-temperature
evaporator is at 0.4 MPa. Liquid leaving the
condenser and vapor leaving each evaporator are
saturated. Temperature of vapor leaving the water
intercooler is 30°C. Both compression stages have an
isentropic efficiency of 85%. The condenser is water
cooled; the cooling water temperature rise in the
condenser is 9°C and its flow rate is 20 m3/hr. The
C.O.P. of the cycle is 3.5. Draw a P-h diagram of the
cycle and calculate the refrigeration capacity of each
evaporator in tons.
9- A three-stage ammonia refrigeration machine operates between evaporating and condensing temperatures
of -70 and 40°C. Two open-type flash intercoolers are installed; one at each of the two intermediate
pressures 0.07 and 0.35 MPa. Moreover, a water intercooler is used at each of the intermediate pressures to
cool the vapor down to 25°C. Refrigerant leaves the condenser and evaporator at saturation conditions.
Compression in all stages has a compression efficiency of 85%. Draw a block diagram and plot the cycle on
the P-h plane; then determine the C.O.P of the cycle. If, for some reason, the water supply to the two water
intercoolers was cut off, find the percentage effect on the C.O.P.
10- three-stage R-12 vapor-compression refrigeration
system is arranged as shown in figure. The system serves
an air-conditioning cooling load on evaporator (A) in
addition to two process-cooling loads of 10 and
15 tons on evaporators (B) and (C) respectively.
Condensing pressure is 1 MPa and liquid is subcooled in
the condenser to 30°C. All compression processes are
assumed to be isentropic. Liquid is subcooled to -20°C in
the closed flash intercooler. The ratio of the indicated
power of the high-pressure stage to that of
the intermediate-pressure stage is 1.7. Evaporating
pressures are 0.015, 0.08 and 0.35 MPa. The refrigerant is
saturated at exit from each evaporator. Draw the P-h
diagram and determine :
(a) Refrigeration capacity of the air-conditioning
evaporator (water chiller) in tons.
(b) Total indicated power input.
(c) C.O.P. of the system (mechanical efficiency of all
compressors is 90%)
(d) Approximate piston displacement (m3/sec) for each
compression stage if the clearance ratio is 0.04 for all
compressors.
11- The refrigerants R-12 and R-13 are used in the cascade
system shown in figure. The lower-temperature cycle
follows a simple saturation cycle between evaporating and
condensing temperatures of -80 and -20°C. The indicated
power input to the R-13 compressor is 1.471 kW/ton. The
R-12 cycle has two evaporators operating at the two
pressures 0.35 and 0.08 MPa. Their refrigeration capacities
are 17 and 40 tons respectively.
Liquid leaving the condenser is saturated at 40°C. The
liquid supplied to the expansion valve of the lowtemperature evaporator is at 20°C. Vapor leaving the hightemperature evaporator is at 10°C. The low- temperature
evaporator is a flooded one ; and the vapor
leaving it is superheated in the liquid-to-suction heat
exchanger to -20°C. The F-12 fed to the cascade condenser
has a quality at inlet of 0.1 and a temperature of -20°C at
exit. Consider compression in both stages to be isentropic.
Draw the P-h diagram and determine :
(a) Refrigeration capacity of the R-13 evaporator in tons.
(b) Power input to the overall binary system (mechanical
efficiency is 0.9 for all units)
(c) C.O.P. of the system.
MISR UNIVERSITY FOR SCIENCE &TECHNOLOGY
Mechanical department
First semester
Faculty of engineering
Academic year 2020/2021
Refrigeration and air conditioning
Sheet No 4: Air refrigeration cycles
1- A closed air cycle refrigeration system produces 10 T.R. Air enters the compressor at 0.412 MPa and
-7°C and is then compressed to 1.47 MPa ; compression index is 1.35. The temperature of the air leaving the
air cooler is 28°C. Expansion index is 1.3. Assume frictionless flow; and calculate the C.O.P. of the system
and the thermodynamic relative efficiency.
2- The schematic air-cycle cooling system for a jet
plane is shown in figure. Air is bled from the jetengine compressor at 330°C and 0.69 MPa at a rate
of 0.133 kg/sec. Air enters the turbine at 101°C. For
the turbine, assume a polytropic exponent n =1.2 and
a mechanical efficiency of 0.8. The cabin pressure is
0.081 MPa and air is exhausted from the cabin at
23°C. Assume that no air is bypassed around the
turbine. Determine :
(a) The turbine power output.
(b) The T.R. produced.
3- For a bootstrap unit the following performance data is given:
Turbine efficiency 85%, secondary-compressor efficiency 77%, and secondary heat exchanger effectiveness
90%. The unit is designed for a cabin pressure of 0.098 MPa. Ambient-air temperature is 32°C and
compressed air leaves the primary heat exchanger at 65°C. Air is supplied to cabin at 5°C. Calculate:
a) Temperature of air leaving the secondary compressor.
b) Pressure at turbine entrance.
c) Temperature of air entering the turbine.
d) Pressure at entrance of secondary compressor.
4- A bootstrap air refrigeration system is used for an airplane to produce 10 tons of refrigeration. The
ambient conditions are t=5°C and P = 0.083 MPa. The air pressure increases to 0.108 MPa isentropically
before entering the compressor. Pressure of air bled from the main compressor is 0.343 MPa and this air is
further compressed in the secondary compressor to 0.441 MPa. The isentropic efficiency of each compressor
is 90% and that of the cooling turbine is 80%. Heat exchanger effectiveness of the primary heat exchanger is
60% and that of the secondary heat exchanger is 62%. The airplane cabin is maintained at 0.098 MPa and
25°C. The cooling turbine drives the secondary compressor and its surplus power is used for the fan.
Calculate :
a) The power required to cool the cabin.
b) C.O.P. of the system based on the power required by the compressor.
N.B.: air used for heat exchangers cooling is from inlet to compressor (i.e. at 0.108 MPa)
5- An air refrigeration system for a jet aero plane operates on the simple cycle. The cockpit is maintained at
22°C. The ambient-air pressure and temperature are 0.086 MPa and 10°C. The stagnation pressure of the
ram air is 0.126 MPa. The pressure ratio of the main compressor is 3. The airplane speed is 277.8 m/s.
Temperature of the air entering the turbine is 83°C. Pressure drop in the heat exchanger is 0.02 MPa.
Compressor and turbine isentropic efficiencies are both 80%. Pressure in the cockpit is kept at 0.098 MPa.
Cockpit cooling load is 1 ton. Draw a schematic T-s diagram of the cycle and determine:
a) Temperature of air entering and leaving the compressor.
b) Temperature of air supplied to the cabin.
c) Brake power output of the turbine if its mechanical efficiency is 82%.
d) Rate of heat rejection in the heat exchanger in kW.
6- A bootstrap air refrigeration system is used to cool an aircraft cabin. The ambient-air temperature is 5°C.
The aircraft speed is 222.2 m/sec. Air is extracted from the main engine compressor at a pressure of 0.226
MPa and a rate of 0.389 kg/sec. Air is cooled in the primary heat exchanger down to 50°C and then
compressed to 0.352 MPa in the secondary compressor. The isentropic efficiencies of the turbine and
secondary compressor are 0.8 and 0.84 respectively. The cabin is pressurized at a pressure of 0.098 MPa and
air is exhausted from it at 22°C. The mechanical efficiencies of the turbine and the secondary compressor
are 0.84 and 0.87 respectively. Determine:
(a) The refrigeration load in tons.
(b) The effectiveness of the secondary heat exchanger.
7- The air refrigeration system of a small turbopropelled airplane is shown in figure. Air at a pressure
of 0.588 MPa is extracted from the main gas-turbine
compressor at a rate of 0.333 kg/sec. Temperature at
inlet to the turbine is 100°C. The turbine has a
mechanical efficiency of 85% and an expansion index
of 1.2. Cabin pressure is 0.098 MPa and the maximum
allowable temperature inside it is 25°C. At a certain
time 15 % of the air flow is bypassed around the
turbine. Determine:
(a) The cooling load, in tons.
(b) The turbine power output, in kW.
(c) What is the full load, in tons, that can be handled by
the system under the same inside and outside
conditions?
(d) By-passed fraction of air stream under half- load
condition.
MISR UNIVERSITY FOR SCIENCE &TECHNOLOGY
Mechanical department
First semester
Faculty of engineering
Academic year 2020/2021
Refrigeration and air conditioning
Sheet No 5: Psychrometric
1. Moist air exists at 27 °C dry-bulb temperature; 0.015 kg water / kg dry air specific humidity, and 75 cm
Hg pressure. Determine:
a. The dew point.
b. The relative humidity.
c. The specific volume.
d. The enthalpy.
e humidity. Air at 22 °C and 75 %
exit of t
temperature and relative humidity of the air leaving the heater.
3. Saturated air at standard atmospheric pressure and 10 °C passes through a duct at a rate of m a = 60
kg/min. A heating steam coil is placed in a side of the duct, where a part of the air is heated by this coil.
The rest of the air is bytwo parts of the steam mix again and the temperature of the mixture becomes 25 °C. Determine:
a. The amount of air which passes through the coil per minute.
b. The relative humidity of the mixture at outlet.
4. Air is available at standard atmospheric pressure, 13 °C d.b.t. and 10.2 °C w.b.t. Before being supplied to
initial relative humidity and dew point of the air and describe the process, which must be carried out on it
of moist air supplied per minute and what is its volume? what must be the mass of humidity injected into
5. A stream of 800 m³/h of outdoor air at 12 °C dry-bulb temperature and 5 °C thermodynamic wet-bulb
temp
-bulb temperature and 40 %
relative humidity. The mixture is heated and then humidified with dry saturated steam at atmospheric
-bulb temperature and 0.017 kg water per kg dry air
specific humidity. All the processes are at standard atmospheric pressure. Show the processes on the
psychrometric chart and then determine:
a. Temperature and relative humidity of the moist air before heating.
b. Heat added to the air in kW.
c. Temperature and specific humidity of the air after heat addition.
d. Amount of steam required for humidification.
6. An industrial process requires 6.86 m³/s of saturated air at a temperature of 22 °C. In a winter day,
2.04 kg/s moist air having a temperature of 31 °C and a specific humidity of 20 gm water per kg dry air. The
mixture is heated and then humidified with dry saturated steam at atmospheric pressure to reach the
required state. All the processes are at standard atmospheric pressure. Show the processes on the
psychrometric chart, and then determine:
a. The required mass of atmospheric air in kg/s.
b. The heat added to the air in kW.
c. The temperature and specific humidity of air after heat addition.
d. The amount of steam required for humidification in kg/s.
7. Atmospheric air at 1 atm, 30 o
inlet flow rate of 1500 m3/min. The liquid which condenses is removed from the system at the final cooling
temperature which is 17 o
a) Sketch the processes on the psychrometric chart.
b) Sketch the processes which water vapor undergoes on the T-s diagram.
c) Calculate the heat removed in the cooling section in kJ/min.
d) Calculate the heat added in the heating section in kJ/min.
e) Estimate the amount of vapor condensed in kg/min.
MISR UNIVERSITY FOR SCIENCE &TECHNOLOGY
Mechanical department
First semester
Faculty of engineering
Academic year 2020/2021
Refrigeration and air conditioning
Sheet No 6: Air-conditioning
o
humidity. The air off the coil has 12 oC dbt and 95% RH. Find the heat removed and moisture condensed
per kg of dry air.
o
C dbt and 80 % RH. The air is heated
C dbt. Find the RH of the air off the heater and the rate of heat
o
transfer to air in kW.
o
o
C dbt and 22.5 o
C dbt and 45 %
RH. The sensible heat factor (SHF) has been determined as 0.72. The air off the coil is 90 % saturated. Find:
(a) The apparatus dew point (ADP), outside air dew point and the off-coil air.
(b) The cooling rate of the unit.
(c) The amount of moisture condensed per hour.
o
4. A mixing box
C dbt and 70 % RH with
o
1558 L/s of return air at 24 C dbt and 50 % RH. The mixture is passed over a cooling coil and comes out 90
% saturated. The room SHF has been determined as 0.67. Find:
(a) The apparatus dew point (ADP), mixed air dew point and the off-coil air temperature.
(b) The cooling rate of the unit.
5. A space is required to be air-conditioned for summer, the following data are obtained:
Outside design conditions: 35 oC dbt, 70 % RH.
Space latent heat gain:
15 kW.
o
Inside design conditions:
25 C dbt, 50 % RH.
Space sensible heat gain:
90 kW.
Required ventilation:
1000 L/s
Conditioned air RH:
90 %
Find:
i) The amount of needed supply air in L/s.
ii) The thermodynamic state at:
a. Mixing of fresh and return air.
b. Supply air to space.
iii) Apparatus dew point (ADP).
iv) The required A/C machine capacity in TR.
o
Outside design conditions: 32 C dbt, 60 % RH.
Inside design conditions:
24 oC dbt, 55 % RH.
Space sensible heat gain:
40 kW.
Space latent heat gain:
5 kW.
Bypass factor:
0.08
Calculate:
i) The supply air volume flow rate and its thermodynamic state.
ii) Machine capacity in kW.
tained:
7. A space is required to be air-conditioned for winter. The following data are obtained:
Outside design conditions: 5 oC dbt, 2 oC wbt.
Inside design conditions:
23 oC dbt, 50 % RH.
Ventilation needed:
25.34 kga./s
Supply air rate:
44.16 kga/s
Room sensible heat factor: 1
Room heat loss:
1,000 kW
Spray efficiency:
95 %
Estimate the capacity of the equipment needed and make a sketch for the air handling unit components.