Refrigeration & Air-Conditioning

Lesson 5. Effect of change of operation conditions on the working of vapour compression refrigeration plant.

Module 1. Fundamentals of refrigeration

Lesson 5
EFFECT OF CHANGE OF OPERATION CONDITION ON THE WORKING OF VAPOUR COMPRESSION REFRIGERATION PLANT

5.1 Introduction

The operating conditions of the plant vary depending on the evaporating and condensing temperature. Operating condition changes due to variation in the condensing pressure, as it is governed by the temperature of the cooling medium available at the condenser. It is very essential to understand the effect of variations of operation conditions in order to operate the refrigeration plant under optimum operating conditions. The effect of change of operating conditions is discussed bellow.

5.2 Effect of Evaporating Pressure (Fig. 5.1)

Enthalpy (H)
Here as shown in figure 1-2-3-4 is cycle operated at P_e evaporating pressure and cycle 1’-2’-3-4’ is operated at evaporating pressure P_e’ which is lower than the first cycle.

(a) Effect on RE (Refrigerating Effect):

RE₁ = (h₁ – h₃) kJ/kg (1^{st Cycle})

RE₂ = (h₁’– h₄’) kJ/kg (2^{ed Cycle})

So from P-H chart it can be said that RE₁ > RE₂

Thus it is clear that lower evaporating/suction pressure/temperature decreases RE per kg of refrigerant circulated in the system.

(b) Effect on work done (WD):

WD₁ = (h₂ – h₁) kJ/kg (1^{st Cycle})

WD₂ = (h₂’– h₁’) kJ/kg (2^{ed Cycle})

So from P-H chart it can be said that WD₂ > WD₁

Thus, it is clear that lower evaporating/suction pressure/temperature increases the work of compression per kg of refrigerant circulated in the system.

(c) Heat removed at condenser (Q):

Q₁ = (h₂ – h₃) kJ/kg (1^{st Cycle})

Q₂ = (h₂’– h₃) kJ/kg (2^{ed Cycle})

It is clear from the P-H chart that Q₂ > Q_1.

Thus, lower evaporating/suction pressure/temperature increases the heat to be removed at the condenser.

(d) Co-efficient of Performance (COP):

It is obvious that if the work done is higher, then the COP is bound to decrease as it is numerator value in the calculation of COP. Lower evaporating temperature/pressure reduces COP of the system.

Therefore, it can be concluded that lower suction pressure/temperature is not desirable for the refrigeration system. It is recommended to operate vapour compression refrigeration system at highest possible evaporating pressure/temperature. However the evaporating pressure maintained is dependent on the requirement of temperature at the cold store.

5.3 Effect of Condensing Pressure

The effect of condensing pressure is shown on P-H diagram in

As shown in P-H diagram (Fig. 5.2), a vapour compression refrigeration system operates as 1-2-3-4 having evaporating pressure P_e while cycle 1-2’-3’-4’ operates at higher condensing pressure p_e. In practical situation, such variations are taking place depending on the temperature of the cooling medium employed at the condenser.

(a) Effect on RE (Refrigerating Effect)

RE₁ = (h₁ – h₄) kJ/kg (1^{st Cycle})

RE₂ = (h₁– h₄’) kJ/kg (2^{ed Cycle})

From P-H chart it can be said that RE₁ > RE₂

Thus, it is clear that higher condensing temperature/pressure reduces refrigerating effect per unit weight of refrigerant circulated in the system.

(b) Effect on work done (WD)

WD₁ = (h₂ – h₁) kJ/kg (1^{st Cycle})

WD₂ = (h₂’– h₁) kJ/kg (2^{ed Cycle})

From P-H chart it clear that WD₂ > WD₁

Thus, higher condensing temperature/pressure increases the work of compression of the system.

(c) Heat removed at condenser (Q)

Q₁ = (h₂ – h₃) kJ/kg (1^{st Cycle})

Q₂ = (h₂^’– h₃^’) kJ/kg (2^{ed Cycle})

Higher condensing pressure/temperature slightly increases the total heat to be removed at the condenser.

(d) Co-efficient of Performance (COP)

It is clear from the above analysis that higher condensing temperature/pressure adversely affects the COP of the system as refrigerating effect decreases and work of compression increases. Therefore, operation of the refrigeration plant at higher condensing temperature/pressure is not desirable. The power consumption of the plant which is operating at higher condensing temperature consumes more electrical power. The temperature of cooling medium used at the condenser as well as heat transfer rate in the condenser are very important factors affecting the condensing temperature/pressure of the system.

(1) Effect of sub-cooling

The cooling of liquid refrigerant after condensation is called sub-cooling of refrigerant. Little sub-cooling always takes place in the condenser but for achieving higher degree of sub-cooing low temperature water or refrigerant is used. The effect of sub-cooling of liquid refrigerant is shown on P-H diagram in

The sub-cooling of the refrigerant increases the refrigerating effect per kg of refrigerant circulated in the system without any change in the work of compression. Therefore, sub-cooling is always desirable to reduce the operating cost of the system.

(1) Effect of super heating

The increase in temperature of refrigerant after formation of saturated vapour at given suction pressure is known super heating of suction vapour. The super heating may take place with in the useful area (cold store/ice-bank tank) or out side the useful area. The cycle 1-2-3-4 is a saturated cycle while cycle 1^’-2^’-3-4 operates with suction vapour super heated.

It appears from the P-H diagram (Fig. 5.4) that super heating useful cooling improves the COP of the system. However, excessive superheating is not desirable as it increases the volume of refrigerant to be pumped by the compressor and increases the temperature of gas leaving the compressor. Higher temperature of gas leaving the compressor creates problem of head cooling of compressor. This problem is more serious in ammonia plant as compared to R-22 refrigerant. Effective insulation of suction pipeline is very essential to reduce the level of superheating of suction gas. Little superheating is recommended for smooth running of compressor without liquid pumping but excessive superheating should be avoided in vapour compression refrigeration system.