superheated region. Thus ultimately there may not be any improvement in system
COP due to this arrangement. It is easy to see that this modification does not result in
significant improvement in performance due to the fact that the refrigerant vapour at
t
now for the reversible, isothermal heat transfer processes 2-3 and 4-1, we can write:
3
q c = q 2 3 = T.ds = Tc (s 2 s 3 )
(10.2)
2
1
q e = q 41 = T.ds = Te (s 1 s 4 )
(10.3)
4
where Te and Tc are the evaporator and condenser temperatures, respectively, a
6.1.1. Conservation of mass:
As the name implies, this law states that mass is a conserved parameter, i.e., it can
neither be generated nor destroyed; it can only be transferred. Mathematically, the
equation of conservation of mass for a control volume is
In multi-effect systems a series of generators operating at progressively
reducing pressures are used. Heat is supplied to the highest stage generator operating
at the highest pressure. The enthalpy of the steam generated from this generator is
used to ge
COPNH3,1st = 1.363
Power input to single stage ammonia system is given by:
Wc,NH3,1st = Qe/ COPNH3,1st = 35.17/1.363 = 25.8 kW
(Ans.)
Comments:
1) Using a cascade system the power consumption could be reduced by about 9.5 %.
2) More importantly, in actual
QB,in = 2880 kJ
(Ans.)
Neglecting the heat transferred during initial sensible cooling of vapour, the total heat
transferred at Vessel A is:
QA,out = Amount of water vapour condensed X latent heat of vapourization = 2500 kJ
(Ans.)
The difference in energy
Process 4-1: Reversible, isothermal heat absorption in a turbine
Fig. 9.1(a). Schematic of a reverse Carnot refrigeration system
Fig. 9.1(b). Reverse Carnot refrigeration system in P-v and T-s coordinates
The heat transferred during isothermal processes 2
Qc
m, Pe, Te, he, se
m, Pc, Td, hd, sd
Wc
Fig.18.3. Energy balance across a steady flow compressor
b) Ideal compressor with clearance:
In actual compressors, a small clearance is left between the cylinder head
and piston to accommodate the valves and to t
V,cl
n
0,1
rp
Fig.18.5. Effect of pressure ratio (rp) and index of compression (n) on
clearance volumetric efficiency (V cl)
Work input to the compressor with clearance:
If we assume that both compression and expansion follow the same
equation Pvn = const
3V
h
T3
h2
h3
h1
2
3
T3
0
2
1
3L
3
1
1
Fig.16.9: Adiabatic mixing of two streams on h-T- diagram
Version 1 ME, IIT Kharagpur 15
m1 + m 2 = m 3
m 1 1 + m 2 2 = m 3 3
m1 h 1 + m 2 h 2 = m 3 h 3 + Q
(16.21)
(16.22)
(16.23)
From the above equations, the entha
systems are available to suit almost all applications with the refrigeration capacities
ranging from few Watts to few megawatts. A wide variety of refrigerants can be used in
these systems to suit different applications, capacities etc. The actual vapour
2
.The specific objectives are to:
1. State principles of evaluating thermodynamic properties of pure substances using:
a) Equations of State (Section 5.2)
b) Thermodynamic charts (Section 5.2)
c) Thermodynamic tables (Section 5.2)
2. Derive expressions f
Since the mass of refrigerant in the cylinder at points C and D are same,
we can express the ratio of cylinder volumes at points D and C in terms of ratio of
specific volumes of refrigerant at D and C, i.e.,
VD v D
=
V
C vC
(18.18)
Hence, the cleara
Lesson
19
Performance Of
Reciprocating
Compressors
Version 1 ME, IIT Kharagpur
1
The specific objectives of this lecture are to:
1. Discuss the performance aspects of ideal reciprocating compressors with
clearance, specifically:
a) Effect of evaporator te
Fig.20.8: Working principle of a scroll compressor
Fig.20.9: Main parts of a scroll compressor
Version 1 ME, IIT Kharagpur 11
Fig.20.10: Different views of a scroll compressor
Version 1 ME, IIT Kharagpur 12
From the above data and using refrigerant property values for R 22 at various state
points are:
State
T
P
h
s
v
Quality
o
3
Point
( C)
(bar)
(kJ/kg) (kJ/kg.K) m /kg
1
7.2
6.254
407.6
1.741
0.03773
1.0
2
37.88
6.254
430.7
1.819
0.04385 Superheated
3
104.9
7. Which of the following statements concerning screw compressors are true?
a) The capacity of a screw compressor can be varied over a large range by using the
slide valve
b) Compared to reciprocating compressors, screw compressors are compact for
small c
Design point
Reciprocating
Reciprocating
Design point
Load
Centrifugal
Centrifugal
Condensing temperature
Evaporator temperature
Fig.21.8(a) and (b): Effects of condensing and evaporator temperatures on the
performance of reciprocating and centrifugal com
6
some property must be same for the three systems. This property is temperature. Thus this law is
the basis for temperature measurement. Equality of temperature is a necessary and sufficient
condition for thermal equilibrium, i.e. no transfer of heat.
Th
10
Therefore if ln (P) is plotted against 1/T, the saturated vapour line will be a straight line.
Also, it has been observed that for a set of similar substances the product of Mhfg/Tnb called
Trouton number is constant. Here M is the molecular weight of
10
Thermal efficiency for a heat engine, HE is defined as:
HE =
Wcycle
QH
=1
QC
QH
(4.12)
where Wcycle is the net work output, QC and QH and are the heat rejected to the low temperature
reservoir and heat added (heat input) from the high temperature rese
replacement of isothermal heat rejection process of Carnot cycle by isobaric heat
rejection in case of VCRS.
Since the heat rejection increases and refrigeration effect reduces when the Carnot cycle
is modified to standard VCRS cycle, the net work input t
vapour, since reciprocating compressors are most widely is refrigeration, traditionally dry
compression (compression of vapour only) is preferred to wet compression.
ii. The second practical difficulty with Carnot cycle is that using a turbine and extract
Q. What is meant by IAQ and what does it involve?
Ans.: IAQ stands for Indoor Air Quality and it refers to the ways and means of reducing
and maintaining the pollutants inside the occupied space within tolerable levels. IAQ
involves specifying suitable le
8. An ammonia based refrigeration system with a refrigeration capacity of 100TR
(1TR=3.5167 kW) operates at an evaporating temperature of 36oC (saturation
pressure = 0.8845 bar) and a condensing temperature of 30oC (saturation
pressure = 11.67 bar). Assum
.
.
Q c = m r (h 2 h 3 )
(10.25)
where h3 and h2 are the specific enthalpies (kJ/kg) at the exit and inlet to the condenser,
respectively.
The condenser pressure Pc is the saturation pressure corresponding to evaporator
temperature Tc, i.e.,
Pc = Psat (Tc
change of angular momentum of the system. This equation is applied for hydraulic
machines such as pumps, turbines, compressors etc.
6.1.3. Bernoullis equation:
The Bernoullis equation is one of the most useful equations that is applied in a wide
variety o
a) For fixed heat rejection temperature (T3) and fixed refrigeration temperature
(T1), the COP of reverse Brayton cycle is always lower than the COP of
reverse
Carnot
cycle
(Fig.
9.3),
that
is
T4
T1
< COPCarnot =
COPBrayton =
T3 T4
T3 T1
Fig. 9
12
1Q2
= ( U 2 U1 )+ 1W2
1W2
= P.dV
2
(5.21)
1
If the working fluid behaves as an ideal gas and there are no phase changes, then, the work done,
heat transferred and entropy change during the isothermal process are given by:
1 Q 2 = 1W2
2
( U = f (T)
v
P