Performance Enhancement of a simple VCR Cycle

Unit-2: Simple Vapour
Compression Refrigeration Cycle
Prepared By:
Ankur Sachdeva
Assistant Professor, ME

Introduction to VCRS
• As mentioned in the previous chapter, in a typical gas
cycle, the working fluid (a gas) does not undergo phase
change, consequently the operating cycle will be away
from the vapour dome.
• In gas cycles, heat rejection and refrigeration take place
as the gas undergoes sensible cooling and heating.
• In a vapour cycle the working fluid undergoes phase
change and refrigeration effect is due to the
vaporization of refrigerant liquid.
• If the refrigerant is a pure substance then its
temperature remains constant during the phase
change processes
Ankur Sachdeva, Assistant Professor, ME

Introduction to VCRS
• Vapour compression refrigeration systems are the most
commonly used among all refrigeration systems.
• As the name implies, these systems belong to the general
class of vapour cycles, wherein the working fluid
(refrigerant) undergoes phase change at least during one
process.
• In a vapour compression refrigeration system, refrigeration
is obtained as the refrigerant evaporates at low
temperatures.
• The input to the system is in the form of mechanical
energy required to run the compressor.
• Hence these systems are also called as mechanical
refrigeration systems.

Carnot Refrigeration Cycle

Carnot Refrigeration Cycle on
T-s Diagram
Component Process
Compressor Isentropic
Compression (1-2)
Heat sink (Condenser) Heat Rejection at
Constant Temperature
(2-3)
Turbine Isentropic Expansion
(3-4)
Heat Source
(Evaporator)
Heat Extraction at
Constant Temperature
(2-3)

Components of a VCR System
Component Process
Compressor Isentropic
Compression (1-2)
Heat sink
(Condenser)
Heat Rejection at
Constant Pressure
(2-3)
Expansion Device Isenthalpic
Expansion (3-4)
Heat Source
(Evaporator)
Heat Extraction at
Constant Pressure
(4-1)

Low and High Pressure Sides
(VCR System)

Components of a Simple VCRS

Suction and Discharge line in
VCRS

Working of a Simple VCRS

Pressure-Enthalpy (P-h) chart

Simple VCRS on
Pressure-Enthalpy Chart

Types of VCR cycle
Name of VCR
cycle
Condition at Inlet of
Compressor
Condition at Outlet of
Compressor
Wet Compression Wet (Dryness fraction , xr <1)
(L+V)
Wet (Dryness fraction , xr <1)
(L+V)
Wet Compression Wet (Dryness fraction , xr <1)
(L+V)
Dry, saturated Vapour
(Dryness fraction , xr =1)
Dry saturated Dry Saturated Vapour
(Dryness fraction , xr =1)
Superheated Vapour
(Dryness fraction , xr >1)
Superheated Superheated Vapour
Superheated Vapour

Wet VCR cycle
(Case-A: Refrigerant’s condition is “Wet” After Compression)
Point No. State of refrigerant
1 LT, LP, (Liquid + Vapour)
(x<1)
2 HT, HP,
(Liquid + Vapour) (x<1)
3 HT, HP, Saturated Liquid
(x=0)
4 LT, LP (Liquid +Vapour)
(x<1)

Wet VCR cycle
(Case-B: Refrigerant’s condition is “Dry & Saturated”
After Compression)
1 LT, LP, (Liquid + Vapour)
(x<1)
2 HT, HP,
(Saturated Vapour) (x=1)
(x=0)
(x<1)

Dry-Saturated VCR Cycle
(Refrigerant’s condition is “Superheated”
After Compression)
1 LT, LP, Dry & Saturated
Vapour (x=1)
2 HT, HP, Superheated Vapour
(x>1)
(x=0)
4 LT, LP (Liquid +Vapour) (x<1)
Note: It is called dry saturated cycle because
refrigerant is in dry and saturated condition
at inlet to compressor

Superheated VCR Cycle
1 LT, LP, SH Vapour (x>1)
2 HT, HP, Superheated
Vapour (x>1)
(x=0)
(x<1)
Note: It is called superheated cycle because
refrigerant is in superheated condition at inlet
to compressor

Analysis of Simple VCRS
SFEE for a control volume

• Compressor
– Neglecting the changes in K.E and P.E., Q = 0
• Work done by compressor (kW)
• Condenser
• Neglecting the changes in K.E and P.E., W = 0
– Heat Rejected in condenser

• Expansion Device,
– Neglecting the changes in K.E and P.E., Q = 0, W =
0
• Evaporator
– Neglecting the changes in K.E and P.E., W = 0
– Heat Extracted by refrigerant in evaporator (kJ/s)
or refrigeration effect

• Coefficient of Performance, 𝐶𝑂𝑃 =
𝑄𝑒
𝑊𝑐

Methods to improve Performance
of VCRS
(a) Sub-cooling of refrigerant leaving the
condenser

Methods to improve Performance
of VCRS
(b) Superheating of refrigerant entering the
compressor

Liquid-Suction Heat Exchanger
(LSHX)

Liquid-Suction Heat
Exchanger (LSHX)

Effect of Decrease in Evaporator
Pressure on VCR Cycle

Effect of Increase in Condenser
Pressure on VCR Cycle
As the condenser pressure decreases,
• Refrigerating effect decreases
• Compressor Work increases
• COP decreases

Pressure-Enthalpy Chart of R-134a

Actual VCR Cycle

Performance Enhancement of a simple VCR Cycle

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