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Refrigeration Capacity (RC) & THROTTLING

Rahul Kumar
18/04/2017 0 0

#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.


#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.

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.


#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)

 

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