The Back pressure valve fitted in the refrigeration system is a spring loaded non-return valve.
It is fitted just in a multi temperature rooms system at the point of exit of the refrigerant from the evaporator coil .
This valve is being fitted at the exit of the compartments which temperature is set higher (usually around at about 4-5 degrees centigrade).
Onboard ship, this valve is fitted outside vegetable room compartment.
Function of back pressure valve
The function of the pressure valve is to maintain the the system equilibrium as the pressure of the gas at the exit of each compartment varies.
The vegetable room is maintained at a temperature of +4 ° C to-+6 ° C, while the fish room and the meat room are kept at a temperature of-12 ° C to-14 ° C.
Thus, the flow of refrigerant to the fish room and the meat room is more than the veg room.
So, the pressure at the outlet of evaporator is more from the fish room and the meat space, so they appear to reach the vegetable room.
As a result, the back pressure valve is fitted to the veg room outlet, allowing the refrigerant to flow out only if the pressure inside the evaporator coil of the veg room is greater than the pressure outside (pressure due to evaporation).
Let us understand it in details that
What is the importance of back pressure valve in vegetable room
As we know that the temperature of vegetable room is maintained at 4 degree celcius while the room temperature of fish and meat is about -15 ℃ to -20 ℃.
Thus,we can say that the requirements of refrigerant for fish and meat room is more than that of the vegetable room and hence the quantity of refrigerant for vegetable room is less than that of other two rooms I.e. meat room and fish room.
The Quantity of the refrigerant is controlled by the thermostatic expansion valve.
The difference of flow of the refrigerant of the veg room and other two rooms and at the outlet where all outlet of all three valves meet cause flow of refrigerant from fish and meet room to veg room in reverse direction can lead to drop in temperature of veg room which is not desired.
A spring loaded back pressure valve is fitted at the outer of veg room to counter this ,which will open only when the outlet pressure of refrigerant of veg room equals to the pressure of refrigerant on the common line.