3 Refrigeration basics that you should know | Onboard Refrigeration

Essential words for any Refrigeration system:

Getting Chills when someone says Saturation Temperature?  Probably you are not the only one. And I am here to help you.

You can find the most useful information you need to know in our quick refrigeration article, so that refrigeration and air conditioning becomes a piece of cake for you.

People may say, there are no free biryanis in this world. But this one is for you. Enjoy.

What is this Saturation Temperature?

Here is a snippet from Wiki

Saturation temperature means boiling point. Saturation temperature is the temperature for a corresponding saturation pressure at which liquid boils into its vapor. 

Yes. its just boiling temperature!

Saturated steam and Superheated Steam

Saturated Steam is the steam

that is in direct contact with water from which it was produced. 

Once water is heated to its boiling point, it gets vapourised and turns into Saturated steam.

And if all the moisture is removed, then we get dry steam.

At the Boiling point, it can Exist at both Liquid And Vapour.!

And in other words, If you have to produce superheated steam, heat the Liquid above the saturation temperature.

Always keep in mind that Superheated steam means no water content!

What's the Temperature of Normal Steam?

Say we take a bowl of water and we heat it, and the temperature of the water starts rising. Now the basic question is if you continue heating, what could happen? 

Yes. It boils at 100°C at 1 atm. 

Now steam is being formed. And the temperature of this steam cannot go above 100°C.

If you continue heating until all the water turns in to vapor, it will be still at 100°C.

Or as long as the boiling takes place, Temperature remains at 100°C this is an important basis for your understanding.

Subcooling and superheating in refrigeration

Simply put, subcooling means, Cooled below its boiling point.

Suppose if the boiling point of the water is 100°C and if the water is at 94°C,

Then its called "6 degrees of subcooling"

If the temperature of the steam is 105°C then it is called "5 degrees of superheating"

4 Components in any refrigeration :

As you might be knowing, the purpose of refrigeration is to remove heat from an Enclosed region. and that's done with the help of these 4 parts.

High-pressure side is on the left and Low-pressure is on the right

1. Evaporator

Purpose of the evaporator is to cool the enclosed space (cabin/diary room). by absorbing heat from the room and the refrigerant starts to boil. It might be fitted with a fan behind the evaporator for increasing the circulation.

2. Condensor

This is to liquify the gaseous refrigerant and then to subcool it.

(subcooling means cooling below its boiling point)

3. Compressor

If you read any first line of a compressor explanation, it will be written like this "To raise the saturation temperature of the refrigerant".

 

That simply means the compressor raises the boiling point of our refrigerant so that when it gets into the condenser, the heat is given off to the cooling water instead of absorbing heat from the cooling water.

And it also circulates the refrigerant. Probably this is the most asked question by seniors.

They may ask in this way "Ok why do you want to increase the temperature of the refrigerant again? 

A Dedicated video on compressor

Note: The compressor slightly increases heat and it mostly increases the temperature

(For increasing the boiling point of ref)

4. Useful information on TEV that you should know.

It regulates the flow of refrigerant from the high-pressure side of the system to the Low-pressure side

and it creates pressure drop which reduces the boiling point (saturation temperature) of the refrigerant so that it can be boiled at a lower temperature.

The amount of the liquid refrigerant flow is determined by the spring-loaded valve which is determined by the pressures on the diaphragm. 

The sensing bulb is filled with the same refrigerant and it is connected to chamber C and the vapor pressure in the bulb (yellow) is determined by the evaporator outlet.

Higher temperature in the evaporator outlet means- it opens more and more liquid flows to the evaporator coil

Lower the temperature in the evaporator outlet means- it opens little and lesser liquid flows to the evaporator coil

Inlet pressure together with the spring tries to close the orifice and the pressure in the refrigerant chamber B is determined by the refrigerant pressure in the evaporator

Bonus info: The screw connected here is called the superheated screw and this is to make sure that the temperature leaving the evaporator is 2 to 9 degrees above the boiling point. So that no liquid goes in tho the compressor. And if you tighten the superheat screw, Compression of the spring increases orifices closes a bit and now only lesser refrigerant flows into the evaporator.

 And "degree of superheat" increases (more superheated gas comes out from the evaporator) -

Usually, nobody goes near this screw. this is factory set for a particular length of the evaporator coil 

 TEV is to control the flow of refrigerant when there is a load change.

For an easy understanding and memorizing, consider this everyday example:

Suppose there are 8 people sitting in a room and say refrigerant of 4L/hr is circulating through the system.

and for boiling this 4 L/hr refrigerant, some amount of heat is being absorbed 

such that refrigerant in the outlet of the evaporator is slightly superheated

Now, suddenly 10 people enter into the same room, and heat in the room goes up. 

If still the same quantity of refrigerant is passing through it, then more superheat is produced.

Superheat production starts early. so the problem with more superheat production is that now in the evaporator coil only lesser liquid is available for heat absorption (Boiling).

So the Expansion valve opens up more and brings back the superheat to the original place (see diagram below)

Now we have more liquid refrigerant available inside the evaporator for boiling.!

TEV is to control the amount of refrigerant going to the evaporator so that the evaporator is always filled with saturated mixture of vapor and liquid so that the entire surface area of the Evaporator is utilized for absorbing heat.

Tev valve is pushed down and the orifice is opened more - letting more refrigerant to flow into the evaporator

TEV makes sure that the liquid completely boils off at the end of your evaporator. This superheat is also provided for safety purposes. in case if all of them suddenly leave the room and there is no heat generation and there is no boiling of the liquid refrigerant. And this goes into compressor.!

Apart from this, having an Expansion valve that the rate of drop of temperature in the room is faster,

And if instead if we are using just a capillary tube for expansion, the room temperature gets cooler, but it takes a lot of time and that adds more compressor running hours. 

Is there a TEV in our Domestic Refrigerator? A Side Quest for young Readers

 

How does your Refrigerant Flow?

Now for the purpose of oral exams or for living onboard, One must definitely now the 5 thermodynamic states.

1. High-temperature high-pressure Superheated Vapour [ HT HP SV ]

After the adiabatic compression (no heat transfer) by the compressor, Temperature of the Refrigerant rises up to 70°C (approx)

and then it passes through Condensor for cooling either by seawater or by air. and then it becomes 

High-temperature high-pressure Saturated Mixture [ HT HP SM ]

and this gets cooled further and becomes High-temperature high-pressure Subcooled Liquid 

[ HT HP SCL ] Even though the subcooling is really less. And on some ships, the liquid refrigerant leaves the condenser just above that of the seawater temperature.

If this is seawater cooled then this will be close to seawater temperature. The pressure on the low-pressure side is dependent upon the seawater temperature.

Say, seawater temperature is 28°C and the refrigerant cools to 26-25°C

and further, it is subcooled to 24-25°C.

Extra info on condenser: in some of the ships, the outlet of the evaporator is made to exchange heat by evaporator outlet. so that no liquid goes to the compressor

Now, when it passes through the expansion valve, it forms Low-pressure low-temperature saturated mixture [ LP LT SM  ]

And as there is a little bit of flashing off and thus saturated mixture is formed

finally, when it passes through the evaporator and boils off by absorbing heat from your Mutton, it changes to Low-temperature Low-pressure Superheated vapor [ LP LT SHV ]

Additional info: The low-pressure side is decided by the setting in the  LP cutoff switch and lower the pressure that you set, the liquid boils at even lower temperatures by taking more heat away from your food.

Real-life example: Imagine, you are in your cabin and you want to set your room temperature at 16°C for a great sleep. So adjust the pressure corresponding to the saturation pressure at 16°C 

At 5 bar,  refrigerant R134a  boils at 16°C 

Now, the compressor keeps on running and pressure lowers till 5 bar and your refrigerant can 

Boil-Off even at 16°C.! Isn't that nice? 

Whenever boiling takes place, maximum heat is absorbed and maximum heat rejection takes place while condensing. [ie Change of State].

Or in other ways, 

If you need to set the room temperature at 16°C, You need to boil off your refrigerant at 16°C, So, in order to do that set the pressure accordingly. (LP setting)

Pssst. Not to use this idea for freezing higher rank officers. That's Isn't Cool.

What's Flashing-Off in refrigeration?

Temperature of the liquid refrigerant going into the low-pressure side is higher than the boiling point. and it flashes off

The refrigerant has 20°C in the high pressure side (left side) and when it moves to the low pressure side (right side), it boils off. As at 4 Bar, the refrigerant can boil at 12°C.

Low-pressure is on the right side and the high-pressure is on the left.

What happens when refrigerant flashes off?

As it flashes off, the temperature is absorbed from the "refrigerant-vapor-mixture".

and some of the liquid boils off taking its heat (latent heat) from the remaining liquid refrigerant.

Why does the Seawater temperature in the condenser decides the discharge pressure of the refrigeration plant?

 An evergreen question that makes you feels like a pro (professional) when you have the answer. 

For beginners:

The condensing temperature is the temperature at which a cooling medium changes phases from a gas to a liquid

The condensing pressure is the pressure at which the refrigerant is phase changing from a gas to a liquid.

Easier Explanation: The quantity of heat can be removed depends on the seawater temperature.higher the inlet temperature of the seawater into the condenser, then less heat can be removed from the refrigerant.

The condensation rate varies as seawater temp varies. Hence the refrigerant pressure at the condenser inlet also varies. 

The same pressure acts as back-pressure to the compressor and hence compressor discharge pressure varies. Thereby increasing the compressor delivery pressure.

Compressor will keep on pumping till the Vapor refrigerant reaches its condensing pressure and if the seawater temperature is more, then the condensing pressure of the vapor refrigerant is also high.

If you go to colder regions the discharge pressure reduces to 10-12°C

and if your ship is near the equator region, it goes up to 20°C [changes with refrigerant]

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Thank You and have a nice day. Comment down if you still have doubts.