Back pressure valve in refrigeration - Explained !

What's the function of a Back pressure valve?

Back pressure valve prevents the evaporator pressure from falling below a set value.

Even if the compressor suction pressure is lower. 

Back pressure valve simply is to ensure that the minimum temperature is always maintained.

When the refrigeration system uses more than one evaporator which operates at different temperatures.

ie: if there are multiple refrigerating rooms with different temperatures, then there is a requirement of Back pressure valve.

 

If you have been around with the refrigeration for a while, then you know the 4 components of refrigeration by heart: Evaporator, condenser, compressor, Expansion valve.

Back pressure valve is an essential component in the refrigeration system. and this cannot be Omited.

Read on for a clear understanding.

If the above sentences are unclear, consider scanning this: 3 Essential refrigeration basics that one should know

What's a Back pressure valve in refrigeration?

Back pressure valve is a special non-return valve that is connected to the outlet of the evaporator. It's a spring-loaded valve. More like a relief valve.

As you might know, that the pressure inside an evaporator coil decides the room temperature.

as the boiling point changes with the pressure. 

(Check out this 3 Essential refrigeration basics that one should know for more info on this)

To make things  more clear,

Consider a simple real-life example: where you have 2 evaporator coils in your ship.

Back pressure valve is fitted at the end of the evaporator coil of higher room temperature.
Example refrigerant: (R134a)

Remember - Maximum heat is absorbed when there is a phase change ie, Boiling.

Meat and fish room Evaporator coil: is at -15°C (1.6 bar pressure)

Veg room evaporator coil: is at 5°C ( 3.5 bar pressure)

Since veg room refrigerant is at 3.5 bar, veg-room refrigerant starts to boil off at a higher temperature 

( 5°C )

simply put, 

If you maintain higher pressure in the veg-room evaporator coil, it starts to boil off at higher temperature and only a less amount of heat is absorbed.

and cannot maintain -15°C.

This is achieved by keeping a back pressure valve at the end of veg room evaporator coil. 

Or else, the compressor keeps on sucking refrigerant from the veg room evaporator coil, and finally reduces the pressure to 1.6 bar which is the same pressure as that of the meat room evaporator coil.

And starts to boil at -15°C.

where ever there is a pressure drop across the system, this won't get affected

This dries up all the water content in Tomatoes/vegetables. and now this can be only used for making dried tomato chutney.

How a Refrigeration back pressure valve Works?

It’s this simple, From the left side, refrigerant comes from the evaporator and goes into the chamber

when pressure of the refrigerant gas exceeded the spring bellow pressure, it lifts up.

And lifts the needle up. This needle/valve remains open until the refrigerant gas falls below the set spring pressure.

Its works like a relief valve, and a certain pressure is always maintained.

Back pressure valve always maintain a constant pressure

Main application of a back pressure valve in refrigeration.

A few may say " back pressure valve is fitted so that high-pressure refrigerant in the veg room evaporator coil won't go into low-pressure meat room evaporator". 

If there is no back-pressure vale, its not possible to have a pressure difference

or the temperature difference in the first place. 

And even though there are a few application for this back pressure valve, This is the most important one,

Permits two or more evaporators, working at different temperatures, which works with the same compressor.

I highly recommend you to know the difference between temperature and heat in order to take full advantage of any refrigeration topic available on the internet.

Same goes for the Term Enthalpy. 

for simplicity, let us say,

Temperature: The degree of heat.

Heat: Heat is the total energy.

Enthalpy: Enthalpy tells how much heat and work was added or removed from the substance.

Refrigeration: The process of removing heat.

Air conditioning: temperature, humidity, ventilation, and air cleanliness are maintained to our desired conditions.

You could find more specific molecular level definitions on the wiki.

But for the understanding of simple refrigeration onboard, plugging these to your vocabulary will eliminate most of your doubts. Puff.!

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comment down if you got any burning doubts on refrigeration topics.