#1
Refrigeration is the process of MAINTAINING a lower temperature as
compared to the surroundings.
Be it continuous generation of work from heat or maintaining a lower
temp, we have to conduct these operations in cycles. Every continuous
operation runs on cycle. The cycles on which refrigeration processes
function are known as refrigeration cycles.
General direction of heat flow in from a higher temperature to a lower
one. Obviously to make the heat flow in opposite direction, we would
have to do some external work.
#2
Cycles discussed in Power Cycles (work producing cycles) were running
on different working fluids. It was sometimes water, sometimes vapour,
sometimes air. In refrigeration cycles too, the cycles need some fluid
which is to be circulated. That fluid is known as refrigerant.
There are many refrigerants being used, for example, ammonia, water,
air, R-134, etc
#4
Refrigeration effect:
It is the amount of heat which is to be extracted from storage space in
order to maintain a lower temperature. Its unit is kJ/kg.
CAUTION: In some questions, you might be asked to calculate
refrigeration effect but unit might be in kW, so be flexible in dealing with
this term!!!
#5
Unit of Refrigeration: Tonne of Refrigeration (TR)
It is the amount of heat that is to be removed from one tone water at 0°C
in order to covert it into ice at 0°C in one day (=24 hours).
The amount of heat require to change water into ice is nothing but latent
heat.
Calculating the latent heat to be removed from 1 ton (=1000 kg) in 24
hours, we get:
1 TR = 3.5 kW = 211 kJ/min
Basically 1 TR would mean the amt of heat removed = 211KJ/min from
storage space.
Latent heat for solid to liquid does not vary significantly with change in
pressure.
#6
On P-v and T-s diagrams, all work producing cycles are clockwise and all
work consuming (refrigeration cycles) are counter-clockwise.
#7
Ideal Refrigeration Cycle:
As I stated earlier that reversing a power cycle will give refrigeration
cycle, reversing a work producing Carnot cycle will give a Carnot
Refrigeration Cycle.
#8
Refrigeration Capacity (RC):
• RC = mass flow rate of refrigerant * Refrigeration Effect (RE)
• After multiplying RE (kJ/kg) with Mass Flow Rate (kg/s), units will be in
kW (=kJ/s), so units of RC will be in kW or W
• COP = RE/W in
multiplying mass flow rate to numerator and denominator, it will become,
COP = RC/Power in
Once again go through the CAUTION in #4.
#9
A carnot refrigerator requires 1.5 kW per tonne of refrigeration to
maintain a region at -30°C, then the COP is?
#10
Like there were different work producing cycles(Otto, Brayton, Rankine),
there are different work consuming cycles too. We will start with Ideal
Vapour Compression cycles.
First one is Ideal Vapour Compression cycle:
Just like we tried to conduct Rankine inside saturation dome (to take
advantage of isothermal phase change) and modified it into Rankine, we
would do the same for refrigeration in this. Just the cycle will be
reversed.
There are just few modifications in reverse Rankine:
1. The refrigerant is completely vapourized before compressing
2. Use of throttling valve (will be discussed in further posts)
Is it necessary to completely vaporize before compressing refrigerant?
yes. We do not have any device which will compress both liquid and gas
from same initial state to same final state SIMULTANEOUSLY.
DOUBT- in reverse rankine cycle we would always land up in
superheated zone
ANS - Yes , due to the reason I stated above
ANS -Pump can act as compressor
Compressor can not act as pump
DOUBT-sir heat same pressure pr remove ni kr sakte using condenser..
high pressure pr hi kyon?.. #no compressor
ANS -T should be higher than T atm to reject heat
warna difficult hoga compression main right?
Yes. Ek sath dono nai ho skte.
#11
Why throttling is used?
1. It is not feasible to use a turbine for reducing the pressure of
refrigerant in all refrigerators (Can you imagine a turbine running beside
your kitchen refrigerator :P )
2. Even if we used a turbine between state 3 and 4, work produced will
be negligible since it is in liquid state for which specific volume is very
less hence v.dP work will be very less
DOUBT- sir lekin ideal cycle me throttling ko laga kar hamne cycle ko
irrev bana dia and iski efficiency ghata di. yadi throttling ke jagah turbine
lagate to ideal(though its not possible) carnot cycle ki hi tarah iski bhi
maximum COP milti na..
ANS - The cost of manufacturing and installing a turbine + Added
complexity will not justify the money saved due to somewhat higher
efficiency
Ismey throttling ko ideal ki tarah liye hai ?
No. throttling is not ideal.
#12
We know that area under T-s dia give heat transfer, so
• Area under 4-1: heat absorbed by the refrigerant
• Area under 2-3: heat rejected in condenser
• Higher the area under 4-1, more will be the refrigeration effect.
#13
There is one more very useful diagram used for this cycle: P-h diagram.
Reason?
Out of the 4 process, 2 are const pressure and one is isenthalpic, hence
on P-h dia, we will have three straight lines and thus process will be
easier to comprehend.
DOUBT- Sir 3-4 irrev process ... dotted line se represent hoga na 3-4?
ANS-
In this case we know that it is isenthalpic hence it will along line of
constant h, so no dotted
#14
Unlike other ideal cycles we have discussed, the ideal Vapour
Compression Ref. cycle is not an internally reversible cycle. Reason is
clear because throttling is an irreversible process.
Note that since process 3-4 is isenthalpic, so h3 = h4.
COP (R) = (h1 – h4) / (h2 – h1)
COP (HP) = (h2 – h3) / (h2 – h1)