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).