it provides required notes for Gate

1. 1
Refrigeration effect (RE)
The amount of heat that is to be removed from the storage space in order to maintain lower temperatures is
called Refrigeration effect.
Significance of COP
COP represents running cost of the system, greater the COP lesser is the running cost. Therefore, systems with
higher COP are desired.
Note
COP can be greater than 1, equal to 1 or less than 1.
→ For Window air conditioning, COP ≃ 3.
→ For Domestic refrigeration, COP ≃ 1.
→ Vapour refrigeration system, COP <1.
Unit of Refrigeration (TR)
1 Ton of refrigeration (TR) means the amount of heat that is to be removed from 1 American ton (907 kg) of
water at 0⁰C in order to convert into ice at 0⁰C in 1 day.
Therefore, ton of refrigeration represents heat transfer rate not mass.
𝑇𝑅 =
𝑚̇ × 𝐿𝐻
1 𝑑𝑎𝑦
=
907 × 334
24 × 3600
= 3.5
𝑘𝐽
𝑠𝑒𝑐⁄
1 𝑇𝑅 = 3.5
𝑘𝐽
𝑠𝑒𝑐⁄
Refrigeration Capacity (RC)
𝑅𝐶 = 𝑚̇ × 𝑅𝐸
RE is expressed in kJ/kg and RC is expressed in kJ/sec or kW.
Power input to compressor
𝑃𝑖𝑛 = 𝑚̇ × 𝑤𝑖𝑛
𝑚 → 𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑛𝑡 𝑖𝑛 𝑠𝑦𝑠𝑡𝑒𝑚
𝐶𝑂𝑃 =
𝑅𝐶
𝑊𝑖𝑛
=
𝑚̇ × 𝑅𝐸
𝑚̇ × 𝑤𝑖𝑛
=
𝑅𝐸
𝑃𝑖𝑛
Energy Efficient ratio (EER)
While calculating COP, work input to compressor is taken into account.
𝐶𝑂𝑃 =
𝑅𝐸
𝑤𝑖𝑛
EER is the ratio if refrigeration effect to work input to motor.
𝐸𝐸𝑅 =
𝑅𝐸
𝑤𝑖𝑛(𝑚𝑜𝑡𝑜𝑟)
𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (𝜂 𝑐𝑜𝑚𝑝) =
𝑃𝑐𝑜𝑚𝑝
𝑃 𝑚𝑜𝑡𝑜𝑟
=
𝑤𝑖𝑛,𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟
𝑤𝑖𝑛,𝑚𝑜𝑡𝑜𝑟

2. 2
Refrigeration cycles
Refrigeration is cooling of system below surroundings.
Melting of ice is the earliest method of refrigeration, here ice absorbs latent heat of fusion from surroundings.
Solid carbon dioxide (CO2) can also be used for refrigeration. Solid CO2 when exposed to atmosphere, it sublimates
(solid to vapor) by absorbing the latent heat of fusion.
Refrigeration by both ice and CO2 are non-cyclic processes.
Reversed Heat engine cycle
Heat is received from lower temperature and discharged to high temperature by receiving
inflow of work. The objective of a refrigerator is to maintain the refrigerated space at a low
temperature by removing heat from it. The objective of a heat pump, however, is to maintain a
heated space at a high temperature.
𝐶𝑂𝑃 =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑒𝑓𝑓𝑒𝑐𝑡
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑖𝑛𝑝𝑢𝑡
𝐶𝑂𝑃 𝐻𝑒𝑎𝑡 𝑝𝑢𝑚𝑝 =
𝑄1
𝑊
=
𝑄1
𝑄1 − 𝑄2
𝐶𝑂𝑃𝑅𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑜𝑟 =
𝑄2
𝑊
=
𝑄2
𝑄1 − 𝑄2
𝐶𝑂𝑃 𝐻𝑒𝑎𝑡 𝑝𝑢𝑚𝑝 = 𝐶𝑂𝑃𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑜𝑟 + 1
Reversed Carnot cycle
Carnot cycle is a totally reversible cycle that consists of two reversible isothermal and two isentropic processes.
Reversing the cycle does also reverse the directions of any heat and work interactions. The result is a cycle that
operates in the counterclockwise direction on a T-s diagram, which is called the reversed Carnot cycle. A
refrigerator or heat pump that operates on the reversed Carnot cycle is called a Carnot refrigerator or a Carnot
heat pump.
𝐶𝑂𝑃 𝐻𝑒𝑎𝑡 𝑝𝑢𝑚𝑝 =
𝑄 𝐻
𝑊𝑁𝑒𝑡
=
𝑄 𝐻
𝑄 𝐻 − 𝑄 𝐿
=
𝑇 𝐻
𝑇 𝐻 − 𝑇𝐿
=
1
1 −
𝑇𝐿
𝑇 𝐻
⁄
𝐶𝑂𝑃𝑅𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑜𝑟 =
𝑄 𝐿
𝑊𝑁𝑒𝑡
=
𝑄 𝐿
𝑄 𝐻 − 𝑄 𝐿
=
𝑇𝐿
𝑇 𝐻 − 𝑇𝐿
=
1
𝑇 𝐻
𝑇𝐿
⁄ − 1

3. 3
Vapour Compression refrigeration cycle
In an actual refrigeration cycle, turbine or expansion engine is not used as power output is very less and not cost
efficient. So, throttling valve is used for expansion. In Carnot cycle, the expansion and compression of liquid
vapour mixture in turbine and compressor are very difficult. So, these difficulties are eliminated by completely
evaporating the refrigerant in evaporator and by replacing the turbine with a throttling device.
1 → 2 ⟹ 𝑅𝑒𝑣𝑒𝑟𝑠𝑖𝑏𝑙𝑒 𝐴𝑑𝑖𝑎𝑏𝑎𝑡𝑖𝑐 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛
2 → 3 ⟹ 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 ℎ𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛
3 → 4 ⟹ 𝑇ℎ𝑟𝑜𝑡𝑡𝑙𝑖𝑛𝑔 (𝐼𝑠𝑒𝑛𝑡ℎ𝑎𝑙𝑝ℎ𝑖𝑐 𝑝𝑟𝑜𝑐𝑒𝑠𝑠)
4 → 1 ⟹ 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 ℎ𝑒𝑎𝑡 𝑎𝑑𝑑𝑖𝑡𝑖𝑜𝑛
→ Dry compression is preferred to wet compression
because the liquid refrigerant may damage the compressor.
→ In expansion valve, i.e., throttling, enthalpy remains
constant. Even though the process is adiabatic entropy is not
constant as process is irreversible and entropy increases in
irreversible process.
→ By throttling, the pressure of refrigerant drops at which
saturation temperature of refrigerant is below the
temperature of surroundings.
→ In evaporator, cooling effect is produced by absorbing
the heat at constant pressure from refrigerator space.
Analysis of cycle
1. 𝑊𝑐 = 𝑊𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 = ℎ2 − ℎ1
2. 𝑄 𝐻 = 𝑄 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 = ℎ2 − ℎ3
3. ℎ3 = ℎ4
4. 𝑄 𝐿 = 𝑄 𝑎𝑑𝑑𝑒𝑑 = ℎ1 − ℎ4 = ℎ1 − ℎ3
Refrigeration effect is the amount of heat absorbed in evaporator.
𝐶𝑂𝑃 =
𝑄 𝐿
𝑊𝑐
=
ℎ1 − ℎ3
ℎ2 − ℎ1

4. 4
Compressors are of 3 types. They are reciprocating, rotating and centrifugal.
When the volume flow rate is high, Centrifugal compressors are used. For small capacity rotary compressors are
used. Reciprocating compressors are used in plants up to 100 tonnes capacity, more than that centrifugal
compressors are used.
Volumetric efficiency of reciprocating compressor
𝐶𝑙𝑒𝑎𝑟𝑒𝑛𝑐𝑒 𝑟𝑎𝑡𝑖𝑜 (𝐶) =
𝑉𝑐
𝑉𝑠
=
𝑉3
𝑉1 − 𝑉3
𝜂 𝑣𝑜𝑙 =
𝑉𝑎𝑐𝑡𝑢𝑎𝑙
𝑉𝑠𝑤𝑒𝑝𝑡
=
𝑉1 − 𝑉4
𝑉1 − 𝑉3
𝜂 𝑣𝑜𝑙 =
𝑉1 − 𝑉3 + 𝑉3 − 𝑉4
𝑉1 − 𝑉3
= 1 −
𝑉4 − 𝑉3
𝑉1 − 𝑉3
𝜂 𝑣𝑜𝑙 = 1 +
𝑉3
𝑉1 − 𝑉3
(
𝑉4
𝑉3
− 1) = 1 + 𝐶 (
𝑉4
𝑉3
− 1) = 1 + 𝐶
𝑉4
𝑉3
− 𝐶
(
𝑉4
𝑉3
= (
𝑃2
𝑃1
)
1
𝑛 𝑒
) 𝑛 𝑒 → 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑒𝑥𝑝𝑎𝑛𝑠𝑖𝑜𝑛
𝜂 𝑣𝑜𝑙 = 1 − 𝐶 + 𝐶 (
𝑃2
𝑃1
)
1
𝑛 𝑒
𝜂 =
𝑉𝑎𝑐𝑡𝑢𝑎𝑙
𝑉𝑠𝑤𝑒𝑝𝑡
=
𝑚̇ · 𝑣1
𝜋
4
𝐷2 · 𝐿 · 𝑁 · 𝐾 ·
1
60
𝑊ℎ𝑒𝑟𝑒, 𝒎̇ = 𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑛𝑡, 𝒗 𝟏 = 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑣𝑜𝑙𝑢𝑚𝑒 𝑎𝑡 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑖𝑛𝑙𝑒𝑡
𝑫 & 𝑳 → 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 & 𝐿𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑏𝑜𝑟𝑒, 𝑵 → 𝑅𝑃𝑀, 𝑲 → 𝑛𝑜. 𝑜𝑓 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟𝑠
Use of Flash chamber in Vapour compression cycle
The state of refrigerant when entering evaporator is in wet region. It is the liquid refrigerant that absorbs heat
from storage space. Therefore, flash chamber is used before evaporator, it separates liquid from vapour
refrigerant and allows only liquid refrigerant to pass through evaporator decreasing the size of evaporator.
There is no change in COP, RE and Winput when flash chamber is used.

5. 5
Actual Vapour compression cycle
An actual vapor-compression refrigeration cycle differs from the ideal one mostly to the irreversibilities that
occur in various components. Two common sources of irreversibilities are fluid friction (causes pressure drops)
and heat transfer to or from the surroundings.
→ The refrigerant is slightly superheated at the
compressor inlet. This ensures that the refrigerant is
completely vaporized when it enters the compressor.
→ In compressor the pressure drop caused by fluid friction
and heat transfer from the surroundings to the
refrigerant can be very significant. Superheating the
refrigerant in turn increases the specific volume of
refrigerant increasing the power input.
→ Entropy of refrigerant can increase or decrease in
compression process depending upon if frictional loss or
heat loss dominates.
→ Entropy loss is desirable as less work input is enough
which can be done by cooling.
→ It is undesirable to route the refrigerant to the throttling
valve before the refrigerant is completely condensed.
Therefore, the refrigerant is subcooled before it enters
the throttling valve.
→ Throttling valve and the evaporator are usually located
very close to each other, so the pressure drops in the
connecting line is small.
Cascade Refrigeration System
For large industrial applications efficiency, not simplicity, is the major concern. Some industrial applications
require moderately low temperatures, and the temperature range they involve may be too large for a single vapor
compression refrigeration cycle to be practical. Solution is performing refrigeration in stages. Such refrigeration
cycles are called Cascade refrigeration cycles.
The two cycles are connected through the heat exchanger in the middle. The heat transfer from the fluid in the
bottom cycle should be equal to the heat transfer to the fluid in the top cycle.

6. 6
Use of Heat exchangers in VC cycle
The primary purpose of using heat exchanger is to sub-cool the refrigerant which results in the increase of
refrigerant and refrigerant gets superheated results in dry compression.
𝐶𝑂𝑃 =
ℎ6 − ℎ5
ℎ2 − ℎ1
Applying steady flow equation, heat lost is equal to heat gain.
ℎ1 − ℎ6 = ℎ3 − ℎ4
𝑐 𝑣𝑎𝑝𝑜𝑢𝑟(𝑇1 − 𝑇6) = 𝑐𝑙𝑖𝑞𝑢𝑖𝑑(𝑇3 − 𝑇4)
Vapour Absorption Refrigeration system

7. 7
Vapour absorption refrigeration system is used where there is a
source of inexpensive thermal energy at a temperature.
The most widely used absorption refrigeration system is the
ammonia–water system, where ammonia (NH3) serves as the
refrigerant and water (H2O) as the transport medium.
System looks very much like the vapor-compression system, except
that the compressor has been replaced by a complex absorption
mechanism consisting of an absorber, a pump, a generator, a
regenerator, a valve, and a rectifier.
Compared with vapor-compression systems, a liquid is compressed
instead of a vapor and thus the work input is very small.
The absorption refrigeration systems are much more expensive than
the vapor-compression refrigeration systems. They are more
complex and occupy more space, they are much less efficient.
𝐶𝑂𝑃𝑚𝑎𝑥 = (
𝑇𝐺 − 𝑇𝑜
𝑇𝐺
) · (
𝑇𝑅
𝑇𝑜 − 𝑇𝑅
)
𝐶𝑂𝑃𝑚𝑎𝑥 = 𝜂 𝑐𝑎𝑟𝑛𝑜𝑡 × 𝐶𝑂𝑃𝑐𝑎𝑟𝑛𝑜𝑡
Gas Refrigeration system
Gas refrigeration cycle operates on reverse Brayton cycles or reverse Joule cycle or Bell-Coleman cycle.
In gas refrigeration cycles, air is used as a refrigerant and the system is used in air-craft refrigeration system.
Though the COP of gas refrigeration system is low, it is still used in air-craft refrigeration systems because of its
low weight per ton of refrigeration.
The COP of gas refrigeration is lower because the refrigerant doesn’t undergo phase change, unlike vapour
compression refrigeration system it absorbs sensible heat whereas vapour refrigeration system absorbs latent
heat.
1 → 2 ⟾ 𝑅𝑒𝑣𝑒𝑟𝑠𝑖𝑏𝑙𝑒 𝑎𝑑𝑖𝑎𝑏𝑡𝑖𝑐 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛
2 → 3 ⟾ 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 ℎ𝑒𝑎𝑡 𝑟𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛
3 → 4 ⟾ 𝑅𝑒𝑣𝑒𝑟𝑠𝑖𝑏𝑙𝑒 𝑎𝑑𝑖𝑎𝑏𝑡𝑖𝑐 𝑒𝑥𝑝𝑎𝑛𝑠𝑖𝑜𝑛
4 → 1 ⟾ 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 ℎ𝑒𝑎𝑡 𝑎𝑑𝑑𝑖𝑡𝑖𝑜𝑛
Analysis of the cycle
Assumptions
1. Each device can be treated as steady flow device.
2. Kinetic and potential changes are neglected.
3. The working fluid is treated as ideal gas.
4. Compression and expansion are assumed to be reversible
adiabatic.
5. Specific heats (cp and cv) are constant.

8. 8
→ 𝑊𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 = ℎ2 − ℎ1 = 𝑐 𝑝(𝑇2 − 𝑇1)
→ 𝑄 𝑟𝑒𝑗𝑒𝑐𝑡𝑒𝑑 = ℎ2 − ℎ3 = 𝑐 𝑝(𝑇2 − 𝑇3)
→ 𝑊𝑡𝑢𝑟𝑏𝑖𝑛𝑒 = ℎ3 − ℎ4 = 𝑐 𝑝(𝑇3 − 𝑇4)
→ 𝑄 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 = 𝑅. 𝐸 = ℎ1 − ℎ4 = 𝑐 𝑝(𝑇1 − 𝑇4)
→ 𝐶𝑂𝑃 =
𝑅. 𝐸
𝑊𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 − 𝑊𝑡𝑢𝑟𝑏𝑖𝑛𝑒
→ 𝐶𝑂𝑃 =
𝑐 𝑝(𝑇1 − 𝑇4)
𝑐 𝑝(𝑇1 − 𝑇2) − 𝑐 𝑝(𝑇3 − 𝑇4)
=
(𝑇1 − 𝑇4)
(𝑇1 − 𝑇2) − (𝑇3 − 𝑇4)
→ 𝐶𝑂𝑃 =
1
(
𝑇2 − 𝑇3
𝑇1 − 𝑇4
) − 1
=
1
𝑇3
𝑇4
− 1
=
1
𝑟𝑝
𝛾−1
𝛾
− 1
{
𝑇3
𝑇4
= (
𝑃3
𝑃4
)
𝛾−1
𝛾
= (
𝑃2
𝑃1
)
𝛾−1
𝛾
= 𝑟𝑝
𝛾−1
𝛾
}
COP of gas refrigeration cycle depends on compression ratio or pressure ratio (rp).
With increase in pressure ratio, the work input increases and COP decreases.
If throttling is used in gas refrigeration cycle instead of isentropic expansion as the working fluid is an ideal gas
and during throttling enthalpy remains constant, temperature after throttling remains same and can’t absorb
heat from storage space.
Actual cycle
1 − 2𝑠 − 3 − 4𝑠 ⟹ 𝐼𝑑𝑒𝑎𝑙 𝑐𝑦𝑐𝑙𝑒
1 − 2 − 3 − 4 ⟹ 𝐴𝑐𝑡𝑢𝑎𝑙 𝑐𝑦𝑐𝑙𝑒
→ 𝜂 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 =
ℎ2𝑠 − ℎ1
ℎ2 − ℎ1
=
𝑇2𝑠 − 𝑇1
𝑇2 − 𝑇1
→ 𝜂 𝑡𝑢𝑟𝑏𝑖𝑛𝑒 =
ℎ3 − ℎ4
ℎ3 − ℎ4𝑠
=
𝑇3 − 𝑇4
𝑇3 − 𝑇4𝑠

9. 9
Refrigerants
Desirable properties of a refrigerants or selection of a refrigerant
The properties of refrigerant are broadly classified into
1. Thermodynamic
2. Chemical
3. Physical
Thermodynamic properties
Critical temperature
The critical temperature of a refrigerant must be high as possible. The critical temperature of CO2 and ethylene are
undesirable because their critical temperature is less than ambient summer temperature.
Refrigerant H2O NH3 R-12 R-22 R-134a CO2 Ethylene
Critical
temperature (⁰C)
374.1 132 111.5 96.5 101.2 31 10.6
Desirable Undesirable
Specific Heat
We know that 𝑐 = 𝑇 (
𝑑𝑆
𝑑𝑇
), as the liquid refrigerant undergoes throttling and for lesser irreversibilities dS must be small.
Therefore, specific heat of liquid refrigerant must be small.
As the vapour refrigerant is undergoing compression, the compression work must be small. For smaller compression work,
the degree of superheat (dT) must be small. Therefore, the specific heat of vapour refrigerant must be high.
Enthalpy of vaporization
Refrigerant with large enthalpy of vaporization (RE) is preferred because the larger the enthalpy of vaporization, lesser is
the mass flow rate for a given refrigeration capacity.
Refrigerant H2O NH3 R-22 R-12 R-134 a
Enthalpy of vaporization
(kJ/kg)
2261 1369 234.7 16.7 197.3
Larger the enthalpy of vaporization, lesser the mass flow rate.
Conductivity
Thermal conductivity of refrigerant must be high for better heat transfer.
Condenser & Evaporator pressure
Evaporator pressure must be close to atmospheric pressure as possible because if evaporator pressure is very low
atmosphere air can leak into system.
The condenser pressure must be moderate i.e., if the condenser pressure is high the compressor must do more work.
Pressure ratio
Low pressure ratios are desirable because higher the pressure ratio larger is the compressor work and less volumetric
efficiency.
Freezing point
The freezing point of refrigerant must be as low as possible, so that very low temperature can be achieved.
Freezing point of water is 0⁰C, which is very high. Hence, it can’t be used as refrigerant for producing low temperatures.
Specific volume of refrigerant at the inlet of compressor
The specific volume at the inlet of compressor should be small, because if the specific volume is large, the compressor size
will be large and hence reciprocating compressors are not used if specific volumes of refrigerant are large and, in that case,
centrifugal compressors are used. Ex- R-113.
COP
High COP’s are desired because larger is the COP smaller is the running cost. Though the latent heat of vaporization of NH3
is very large. It doesn’t help in anyway in the improvement of COP because the work input to the compressor is large
(because of high γ) and hence COP of NH3 refrigeration system is almost same as other common refrigerants.

10. 10
Compressor Discharge temperature
The compressor discharge temperature (T2) should not be very high. If the compressor discharge temperature is very high it
will damage the compressor. The compressor discharge of NH3 refrigeration system is very high (120⁰C) compared to other
common refrigerants. Therefore, NH3 compressors are generally water cooled/water jacketed.
For common refrigerants, the compressor discharge temperature is less than 75⁰C.
Boiling point
The boiling point if refrigerant must be low, if the Boiling point is low for a given evaporator pressure, low temperatures can
be achieved.
Chemical Properties
Flammability
Refrigerants must be inflammable.
Toxicity
Refrigerants must be non-toxic.
Ammonia is toxic and hence not used as refrigerant in domestic refrigerators, though it is cheap.
Action with oil
In compressors some oil is carried by high temperature refrigerant to condenser and finally to evaporator. In evaporator
refrigerant vaporizes and oil separates from refrigerant. This accumulation of oil in evaporator results in reduction of heat
transfer.
Refrigerants are immiscible with oil (ex- CO2, NH3) don’t present any problems because the oil is brought back to compressor
from evaporator.
Refrigerant that are partially miscible with oil (ex- R-22) create problems. So, synthetic oils are used instead of mineral oil.
Action with material of construction
Ammonia attacks copper and hence when ammonia is used as refrigerant, wrought iron or steel is used as material of
construction. Similarly CFC’s attack aluminium and hence CFC’s, copper is used as material of construction.
Physical Properties
Viscosity
For easy flow of refrigerant, viscosity should be low.
Leak detection
Refrigerant must not leak from the system and if at all it leaks it must be detectable.
Ammonia leaks are detectable by sulphur stick method. In presence of NH3, white fumes of ammonium sulphide are formed.
Freon leaks are detected by allied torch method. In presence of Freon’s colour of light changes from blue to bluish green.
Refrigerant Application
R-134a Domestic refrigerator
R-22 A/C
R-717 (NH3) Industrial application
Dry ice (Solid CO2) Transportation
Psychrometry
At temperatures below the critical temperature, the gas phase of a substance is frequently referred to as a
vapor. The term vapor implies a gaseous state that is close to the saturation region of the substance, raising the
possibility of condensation during a process.
Air in the atmosphere normally contains some water vapor (or moisture) and is referred to as atmospheric
air. By contrast, air that contains no water vapor is called dry air. Composition of dry air remains relatively
constant, but the amount of water vapor changes as a result of condensation and evaporation. Although the
amount of water vapor in the air is small, it plays a major role in human comfort. Therefore, it is an important
consideration in air-conditioning applications.
Dry air can be treated as an ideal gas with a constant cp value of 1.005 kJ/kg·K.

11. 11
ℎ 𝑑𝑟𝑦 𝑎𝑖𝑟 = 𝑐 𝑝 · 𝑇 ℎ 𝑎 = 𝑐 𝑝 · 𝑇 = 1.005
𝑘𝐽
𝑘𝑔 · 𝐾
· 𝑻 𝐾 = 1.005 · 𝑻
𝑘𝐽
𝑘𝑔⁄
𝛥ℎ 𝑑𝑟𝑦 𝑎𝑖𝑟 = 𝑐 𝑝 · 𝛥𝑇
Water vapor in air behaves as ideal gas and obeys the ideal-gas relation Pv = RT.
Then the atmospheric air can be treated as an ideal-gas mixture whose pressure is the sum of the partial
pressure of dry air Pa and that of water vapor Pv
𝑃 = 𝑃𝑎 + 𝑃𝑣
The partial pressure of water vapor is usually referred to as the vapor pressure. It is the pressure water
vapor would exert if it existed alone at the temperature and volume of atmospheric air.
Since pv is very small, the saturation temperature of water vapor at that pressure is less than atmospheric
temperature. So, it exists in a superheated state.
The enthalpy of water vapor in air can be taken to be equal to the enthalpy of saturated vapor at the same
temperature.
Taking 0⁰C as the reference temperature, the enthalpy and enthalpy change of dry air can be determined.
The enthalpy of water vapor at 0⁰C is 2500.9 kJ/kg. The average cp value of water vapor in the temperature
range -10⁰C to 50⁰C can be taken to be 1.88 kJ/kg·⁰C.
ℎ 𝑣 = ℎ00 𝐶 + 𝑐 𝑝 · 𝑇 = 2500.9 + 1.88 · 𝑇
Specific Humidity (ω)
It is the mass of water vapor present in a unit mass of dry air.
𝜔 =
𝑚 𝑣
𝑚 𝑎
=
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟 𝑝𝑟𝑒𝑠𝑒𝑛𝑡 𝑖𝑛 𝑎𝑖𝑟
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟
𝜔 =
𝑃𝑣 · 𝑉
𝑅 𝑣 · 𝑇⁄
𝑃𝑎 · 𝑉
𝑅 𝑎 · 𝑇⁄
=
𝑃𝑣
𝑃𝑎
×
𝑅 𝑎
𝑅 𝑣
=
𝑃𝑣
𝑃𝑎
×
𝑀 𝐻2 𝑂
𝑀 𝑎𝑖𝑟
=
𝑃𝑣
𝑃𝑎
×
18
29
= 0.622 ×
𝑃𝑣
𝑃𝑎
𝜔 = 0.622 ×
𝑃𝑣
𝑃𝑎
= 0.622 ×
𝑃𝑣
𝑃 − 𝑃𝑣
Dry air contains no water vapor, and thus its specific humidity is zero.
Relative Humidity (ϕ)
Amount of moisture the air holds (mv) relative to the maximum amount of moisture (mvs) the air can
hold at the same temperature is relative humidity.
𝜙 =
𝑚 𝑣
𝑚 𝑣𝑠
=
𝑃𝑣
𝑃𝑣𝑠
𝜙 =
𝑚 𝑣
𝑚 𝑣𝑠
=
𝑃𝑣 · 𝑉
𝑅 𝑣 · 𝑇⁄
𝑃𝑣𝑠 · 𝑉
𝑅 𝑣 · 𝑇⁄
=
𝑃𝑣
𝑃𝑣𝑠
⟹ 𝜔 = 0.622 ×
𝑃𝑣𝑠 · 𝜙
𝑃 − 𝑃𝑣𝑠 · 𝜙
⟹ 𝜙 =
𝜔 · 𝑃
(0.622 + 𝜔) · 𝑃𝑣𝑠
The relative humidity ranges from 0 for dry air to 1 for saturated air.
Relative humidity is generally expressed in percentage.
The amount of moisture air can hold depends on its temperature. Therefore, the relative humidity of air
changes with temperature even when its specific humidity remains constant.

12. 12
The total enthalpy (an extensive property) of atmospheric air is the sum of the enthalpies of dry air and the water
vapor,
𝐻 = 𝐻 𝑎 + 𝐻𝑣
𝐻 = 𝑚 𝑎 · ℎ 𝑎 + 𝑚 𝑣 · ℎ 𝑣
Dividing it by ma gives,
𝐻
𝑚 𝑎
= ℎ 𝑎 +
𝑚 𝑣
𝑚 𝑎
· ℎ 𝑣 ⟹ 𝒉 = 𝒉 𝒂 + 𝝎 · 𝒉 𝒗 → ℎ = ℎ 𝑎 + 𝜔 · ℎ 𝑣𝑠 (ℎ 𝑣 = ℎ 𝑣𝑠)
𝒉 →
𝒆𝒏𝒕𝒉𝒂𝒍𝒑𝒉𝒚 𝒐𝒇 𝒂𝒊𝒓
𝒌𝒈 𝒐𝒇 𝒅𝒓𝒚 𝒂𝒊𝒓
Ordinary temperature of atmospheric air is frequently referred to as the dry-bulb temperature.
Dew point temperature (Tdp)
The dew-point temperature Tdp is defined as the temperature at which condensation begins when
the air is cooled at constant pressure.
Tdp is the saturation temperature of water corresponding to the vapor
pressure.
As the air cools at constant pressure, the vapor pressure Pv remains
constant, the vapor in the air (state 1) undergoes a constant-pressure cooling
process until it strikes the saturated vapor line (state 2). The temperature at this
point is Tdp, if the temperature drops any further, some vapor condenses out. As a
result, the amount of vapor in the air decreases, which results in a decrease in Pv.
The air remains saturated during the condensation process and thus follows
a path of 100 % Relative humidity (the saturated vapor line). The ordinary
temperature and the dew-point temperature of saturated air are identical.
Adiabatic Saturation and wet-bulb temperature
One way of determining the relative humidity is to determine the dew-point
temperature of air. Knowing the dew-point temperature, we can determine the
vapor pressure Pv and thus the relative humidity. This approach is simple, but not
quite practical.
Another way of determining the absolute or relative humidity is related to
an adiabatic saturation process, shown schematically and on a T-s diagram.
Unsaturated air which has specific humidity ω1 and T1 passes over liquid
water, as the air flows over the water, some water evaporates and mixes with the
airstream. Water vapor content increases and temperature decreases during the
process. If air flows long enough air becomes fully saturated at T2 and that
temperature is called Adiabatic saturation temperature.
If makeup water is supplied to the channel at the rate of evaporation at
temperature T2. The adiabatic saturation process described above can be
analyzed as a steady-flow process.
→ 𝑚̇ 𝑎1
= 𝑚̇ 𝑎2
= 𝑚̇ 𝑎 (𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟 𝑟𝑒𝑚𝑎𝑖𝑛𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡)
→ 𝑚̇ 𝑣1
+ 𝑚̇ 𝑓 = 𝑚̇ 𝑣2
(𝑚̇ 𝑓 → 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛)
→ 𝜔1 · 𝑚̇ 𝑎 + 𝑚̇ 𝑓 = 𝜔2 · 𝑚̇ 𝑎 ⟹ 𝑚̇ 𝑓 = 𝑚̇ 𝑎 × (𝜔2 − 𝜔1)

13. 13
Energy balance,
𝐸𝑖𝑛 = 𝐸 𝑜𝑢𝑡
→ 𝐻1 + 𝐻𝑓 = 𝐻2
→ 𝑚̇ 𝑎 · ℎ1 + 𝑚̇ 𝑓 · ℎ𝑓2
= 𝑚̇ 𝑎 · ℎ2 ⟹ 𝑚̇ 𝑎 · ℎ1 + 𝑚̇ 𝑎 × (𝜔2 − 𝜔1) · ℎ 𝑓2
= 𝑚̇ 𝑎 · ℎ2
𝐷𝑖𝑣𝑖𝑑𝑖𝑛𝑔 𝑏𝑦 𝑚̇ ,
→ ℎ1 + (𝜔2 − 𝜔1) · ℎ 𝑓2
= ℎ2
→ (𝑐 𝑝 · 𝑇1 + 𝜔1 · ℎ 𝑣1
) + (𝜔2 − 𝜔1) · ℎ 𝑓2
= (𝑐 𝑝 · 𝑇2 + 𝜔1 · ℎ 𝑣2
)
𝜔1 =
𝑐 𝑝 · (𝑇2 − 𝑇1) + 𝜔2 · ℎ 𝑣𝑓2
ℎ 𝑣1
− ℎ𝑓2
→ 𝜔2 =
𝜔1 · (ℎ 𝑣1
− ℎ𝑓2
) − 𝑐 𝑝 · (𝑇2 − 𝑇1)
ℎ 𝑣𝑓2
𝜔2 =
0.622 · 𝑃𝑣2
𝑃2 − 𝑃𝑣2
(𝜙2 = 100%)
The adiabatic saturation process discussed above provides a means of determining the absolute or relative
humidity of air.
A more practical approach is to use a thermometer whose bulb is covered with a cotton wick saturated with
water and to blow air over the wick, as shown in. Temperature measured in this manner is called the wet-bulb
temperature Twb.
Degree of saturation (μ)
It’s the ratio of actual specific humidity to specific humidity under saturation conditions.
𝜇 =
𝜔
𝜔𝑠
𝜇 =
𝜔
𝜔𝑠
=
0.622 ⋅ (
𝑃𝑣
𝑃𝑡 − 𝑃𝑣
)
0.622 ⋅ (
𝑃𝑣𝑠
𝑃𝑡 − 𝑃𝑣𝑠
)
→ 𝜇 =
𝑃𝑣
𝑃𝑣𝑠
×
𝑃𝑡 − 𝑃𝑣𝑠
𝑃𝑡 − 𝑃𝑣
𝜇 = 𝜙 ⋅ (
𝑃𝑡 − 𝑃𝑣𝑠
𝑃𝑡 − 𝑃𝑣
)

14. 14
Developing of Psychometric chart
𝑇 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒𝑠 → 𝑃𝑣 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒𝑠 𝜔 = 𝑓(𝑃𝑣)
We know that from T-S diagram, as Pv increases corresponding saturation temperature also increases, if these values are
plotted on Pv vs T diagram, we get the diagram.
We know that water vapour in air exists in super-heated state. Therefore, the region towards the right of saturation curve
shows superheated shows superheated region and hence this is the region of interest in psychrometry, we also now that ω is
a function of Pv, therefore, in psychrometry chart is replaced with ω.
Psychrometric chart
Various lines on Psychrometric chart

15. 15
Sensitive Heating or Cooling
Only the dry bulb temperature of air changes.
→ 𝑚̇ 𝑎1
= 𝑚̇ 𝑎2
= 𝑚̇ 𝑎 (𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟 𝑟𝑒𝑚𝑎𝑖𝑛𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡)
→ 𝑚 𝑣1
= 𝑚 𝑣2
= 𝑚̇ 𝑣 (𝑚𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒 𝑟𝑒𝑚𝑎𝑖𝑛𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡)
→ 𝜔1 · 𝑚 𝑎 = 𝜔2 · 𝑚 𝑎 ⟹ 𝜔1 = 𝜔2 = 𝜔
→ 𝑚̇ 𝑎1
· ℎ1 + 𝑄1−2 = 𝑚̇ 𝑎2
· ℎ2 (𝐸𝑛𝑒𝑟𝑔𝑦 𝐶𝑜𝑛𝑠𝑒𝑟𝑣𝑎𝑡𝑖𝑜𝑛)
→ 𝑄1−2 = 𝑚 𝑎 · (ℎ2 − ℎ1)
→ 𝑄1−2 = 𝑚 𝑎 · [(𝑐 𝑝 · 𝑇2 + 𝜔 · (2500 + 𝑐 𝑝 𝑣
· 𝑇2)) − (𝑐 𝑝 · 𝑇1 + 𝜔 · (2500 + 𝑐 𝑝 𝑣
· 𝑇1))]
→ 𝑄1−2 = 𝑚 𝑎 · [(𝑐 𝑝 · (𝑇2 − 𝑇1)) + 𝜔 (𝑐 𝑝 𝑣
· (𝑇2 − 𝑇1))]
→ 𝑄1−2 = 𝑚 𝑎 · [(1.005 · (𝑇2 − 𝑇1)) + 𝜔(1.88 · (𝑇2 − 𝑇1))]
→ 𝑄1−2 = 𝑚 𝑎 · (𝑇2 − 𝑇1) · (1.005 + 1.88 · 𝜔)
Cooling and Dehumidification
When the humidity ratio of air decreases, air is said to be dehumidified, when humidity increases, then air is
said to be humidified.
Cooling and dehumidification process is obtained when the air at
the given dry bulb temperature (DBT) and dew point (DPT)
temperature is cooled below the dew point temperature.
When the air comes in contact with the cooling coil that is
maintained at the temperature below its dew point temperature, its
DBT starts reducing and relative humidity (ϕ) starts increasing. The
process of cooling continues and at some point, it reaches the value of
dew point temperature of the air. At this point the water vapor within
the air starts getting converted into the dew particles due to which the
dew is formed on the surface of the cooling and the moisture content
of the air reduces thereby reducing its humidity level. Thus, when the
air is cooled below its dew point temperature, there is cooling as well
as dehumidification of air.
Air may be cooled and dehumidified by
a) Placing the evaporator coil across the air flow
b) Circulating chilled water in a tube placed across the air flow
c) By spraying chilled water to air in the form of fine mist to expose a large surface

16. 16
→ 𝑚̇ 𝑎1
= 𝑚̇ 𝑎2
= 𝑚̇ 𝑎
→ 𝑚̇ 𝑣1
= 𝑚̇ 𝑣2
+ 𝑚̇ 𝑓 ⟹ 𝜔1 · 𝑚̇ 𝑎 = 𝜔2 · 𝑚̇ 𝑎 + 𝑚̇ 𝑓
→ 𝑚̇ 𝑓 = 𝑚̇ 𝑎 · (𝜔1 − 𝜔2)
→ 𝑚̇ 𝑎 · ℎ1 = 𝑚̇ 𝑎 · ℎ2 + 𝑚̇ 𝑓 · ℎ𝑓 + 𝑄1−2
→ 𝑄1−2 = 𝑚̇ 𝑎 · ℎ1 − 𝑚̇ 𝑎 · ℎ2 − 𝑚̇ 𝑓 · ℎ𝑓
→ 𝑄1−2 = 𝑚̇ 𝑎 · [(ℎ1 − ℎ2) − (𝜔1 − 𝜔2) · ℎ𝑓]
Here there is both sensible cooling and latent heat of vaporization.
→ ℎ1 − ℎ2 = (ℎ1 − ℎ 𝑥) + (ℎ 𝑥 − ℎ2) = 𝑆𝑒𝑛𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡(𝑆𝐻) + 𝐿𝑎𝑡𝑒𝑛𝑡 ℎ𝑒𝑎𝑡 (𝐿𝐻)
Heating and Humidification
Problems associated with the low relative humidity for winter air-
conditioning resulting from simple heating can be eliminated by
humidifying the heated air. This is accomplished by passing the air first
through a heating section (process 1-2) and then through a humidifying
section (process 2-3).
→ 𝑚̇ 𝑎1
= 𝑚̇ 𝑎2
= 𝑚̇ 𝑎
→ 𝑚̇ 𝑣1
+ 𝑚̇ 𝑓 = 𝑚̇ 𝑣2
⟹ 𝜔1 · 𝑚̇ 𝑎 + 𝑚̇ 𝑓 = 𝜔2 · 𝑚̇ 𝑎
→ 𝑚̇ 𝑓 = 𝑚̇ 𝑎 · (𝜔2 − 𝜔1)
→ 𝑚̇ 𝑎 · ℎ1 + 𝑚̇ 𝑓 · ℎ𝑓 + 𝑄1−2 = 𝑚̇ 𝑎 · ℎ2
→ 𝑄1−2 = 𝑚̇ 𝑎 · [(ℎ2 − ℎ1) − (𝜔2 − 𝜔1) · ℎ𝑓]
Chemical Dehumidification
Some substances like silica gel and activated alumina have great
affinity for water vapour. They are called adsorbents.
When air passes through a bed of silica gel, water vapour molecules
get adsorbed on its surface. Latent heat of condensation is released. So,
the temperature of air is increased.

17. 17
Adiabatic evaporative cooling
When liquid water is circulated / sprayed through air (dry air +
water vapour), water is evaporated by absorbing heat from air thereby
increasing specific humidity and in process air is cooled as it gave
latent heat of evaporation to water. No heat transfer takes place
between the chamber and the surroundings. therefore, the energy
required for evaporation is suppled by the air and consequently DBT is
decreased. After the process has been in operation for a sufficient
length of time, circulating water approaches Wet bulb temperature
(WBT) of air.
→ 𝑚̇ 𝑎1
= 𝑚̇ 𝑎2
= 𝑚̇ 𝑎
→ 𝑚̇ 𝑣1
+ 𝑚̇ 𝑓 = 𝑚̇ 𝑣2
→ 𝑚̇ 𝑓 = 𝑚̇ 𝑎 × (𝜔2 − 𝜔1)
→ 𝑚̇ 𝑎 · ℎ1 + 𝑚̇ 𝑓 · ℎ 𝑓2
= 𝑚̇ 𝑎 · ℎ2
→ ℎ1 + (𝜔2 − 𝜔1) · ℎ 𝑓2
= ℎ2
Adiabatic mixing of 2 streams
→ 𝑚 𝑎1
+ 𝑚 𝑎2
= 𝑚 𝑎3
→ 𝑚 𝑣1
+ 𝑚 𝑣2
= 𝑚 𝑣3
→ 𝜔1 · 𝑚 𝑎1
+ 𝜔2 · 𝑚 𝑎2
= 𝜔3 · 𝑚 𝑎3
⟹ 𝜔1 · 𝑚 𝑎1
+ 𝜔2 · 𝑚 𝑎2
= (𝑚 𝑎1
+ 𝑚 𝑎2
) · 𝑚 𝑎3
→ ℎ1 · 𝑚 𝑎1
+ ℎ2 · 𝑚 𝑎2
= 𝑚 𝑎3
· ℎ3 ⟹ ℎ1 · 𝑚 𝑎1
+ ℎ2 · 𝑚 𝑎2
= (𝑚 𝑎1
+ 𝑚 𝑎2
) · ℎ3
𝑚 𝑎1
𝑚 𝑎2
=
ℎ3 − ℎ2
ℎ1 − ℎ3
=
𝜔3 − 𝜔2
𝜔1 − 𝜔3