What is the function of the back pressure valve in the Refrigeration System?

Back pressure valve is a spring loaded non-return valve. It is fitted just at the exit of the refrigerant from the evaporator coil in a multi temperature rooms system. This being fitted at the exit of the compartments whose temperature is set higher (usually at about 4-5 degrees centigrade). Onboard ship, it is fitted outside vegetable room compartment.

Function of back pressure valve

The function of the valve is to maintain equilibrium of the system as the pressure of the gas at the exit of each compartment differs. The vegetable room is maintained at a temperature of  +4°C to -+6° C while fish room and meat room is at a temperature of  -12°C to -14°C. Hence, flow of refrigerant at fish room and meat room is more as compared to veg room. So, pressure at the evaporator outlet will be more from fish room and meat room and they will tend to enter veg room. So, a back pressure valve is fitted at the veg room outlet which will allow the refrigerant to flow out only if the pressure inside the evaporator coil of veg room is more than the pressure outside(pressure due to evaporated refrigerant of other two compartments).

About the author

Anand Gautam

Anand Gautam is a marine engineer, Graduated from M.E.R.I. Mumbai. He likes to sail onboard ship and is one who is passionate about his job. He loves to share his knowledge, information, and ideas to everyone in a simplified way just for his satisfaction.


  • I’m sorry, but your explanation is incorrect. Lower temperature room will have lower pressure. By that fundamental your logic and explanation stands incorrect.

  • the function of back pressure valve is to maintain saturation pressure in the vegroom… if veg room pressure in evaporator drops to 1.2bar as in meat or fish room it will cool the veg room to -18c more… so we use back pressure valve to maintain that pressure to 3.9 bar so that we maintain temp 5 to 8 C in veg room

  • I cannot see a direct relationship between the higher flow rate (by the way, mass flow rate or volumetric flow rate?) and lower evaporation temperature. At lower evaporation temperatures the pressure is also lower, so in the system you describe the pressure of the refrigerant leaving the vegatables compartment is higher that that of the fish and meat compartment.

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