Question

What are the other components that can improve the COP. Draw the P-h diagram to explain the process.

Answer 1

1) EFFECT OF SUCTION ( EVAPORATOR ) PRESSURE

Effect of increase in evaporator pressure from P'₁ to P₁ causes the states 4' and 1' to shift to 4 and 1.Hence the work donne by compressor decreases and specific volume of suction vapor decreases from v'₁ to v_1.

The refrigerating effect increases from h'1-h'4 to h1-h4. The compressor work input decreases from flatter isentropic process h'1-h'2 to steeper isentropic process h1-h2.

_{2) EFFECT OF CONDENSER PRESSURE}

As the compressor pressure is decreased, the work input decreases and hence the COP increases. When the pressure shifts from 2' to 2 the states 2' and 3' changes to 2 and 3.

Hence refrigeration effect increases from h'1-h'4 to h1-h4.

3) EFFECT OF SUCTION VAPOR SUPERHEAT

Wet compression is undesirable in compressor as they wash off the lubricants and create several mechanical difficulties. Hence in order to avoid damage compressor and increase volumetric efficiency superheating of refrigerant is prefered.In this cycle , the refrigeration effect is increased from h1-h4 to h'1-h'4 and work inout to compressor increases from h2-h1 to h2-h'1 due to diverging nature of isentropic lines.

The specific volume is also increased due to which mass flow rate decrease for same displacement of compressor. An increase in specific volume may decrease refrigeration capacity but increases refrigeration effect.The net effect is reduction in COP. But COP may increase or decrease depending on refrigerant.

In R-12 refrigerant , superheating may increase the COP while in R-22 and NH₃ Superheating will result in decrease in COP. However in general , the effect of slight superheat results in improvement of volumetric effciency and hence COP.

4) EFFECT OF LIQUID SUB-COOLING

If the liquid refrigerant after condensation is cooled below the saturation temperature before throtttling is called sub-cooling or under cooling.

The process of under-cooling is generally done by circulating more quantity of cooling water in condenser. Due to subcooling , the refrigerating effect increases,hence COP increases.