Whats Up!!! with the Refrigerator and

Compressors -5

Anand Plappally IIT Jodhpur

Department of Mechanical Engineering

1/8/2015

1/8/2015

Pressure and Boiling Boiling of a liquid depends on the pressure acting on its surface Boiling point increases as pressure increases Boiling point decreases as pressure decreases (At sea level water boils at 212F but at places above sea level (where pressure is low) water boils at lower temperature - May be you have to design a new heater or refrigerator for high altitudes

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Mechanical Refrigeration All present day refrigerators are mechanical, that is a. Motor supplies mechanical energy . It converts electrical to

mechanical energy b. We require to operate the pump using this mechanical

energy

In refrigerator boiling takes place inside the evaporator coils----and --- boiling depends on the pressure inside these coils Point of Information In air conditioners, the boiling point is 44F In refrigerators, the boiling point is 5 F

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First, Why we go about refrigerating in the way we are doing today.

cooling

coil

Refrigeration can be produced by allowing a liquid refrigerant Say R-12 from a cylinder has to pass and boil inside a coil or evaporator

The latent heat needed for boiling is taken from the surrounding space (vegetables, fruits and air inside) in the refrigerator control volume thus cooling the volume.

If The refrigerant vapor thus can be made to escape to the atmosphere, then the pressure inside the cooling coil or evaporator will be equal to ATMOSPHERIC PRESSURE

R-12 boils at -29.4C and will boil at a pressure of 1 atm

To atm.

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SECONDLet consider a refrigeration job Room Temperature is around 32 C and we need temperature around 4.5oC for food preservation How to go about it

What do you see??? Large difference between boiling temperature of R12 (-29.4 oC ) and room temperature 32.2oC.

Then Oh! You need to boil R 12 at 4.5 oC.

How to make this happen

OK, we can provide a hand valve at the exit, such that all the vapor goes out in a controlled way (controlled with Valve opening /closing). This will help maintain specific pressure inside the cooling coil or evaporator.

cooling

coil

Valve

To Atm.

Finally, the refrigerant after performing its task leaves to the atmosphere OH Great Loss

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Third, the thought in line to more development. Thoughts are 1. Reclaim 2. Reuse

To conserve the refrigerant material R 12, high cost, ozone depletion (Montreal Protocol)

To do this.. The refrigerant must be delivered as a liquid because it can absorb maximum heat only by vaporization ------- Simplest way to do make this

happen, CONDENSE the vaporized refrigerant that leaves the evaporator..

To condense the refrigerant vapor it

LATENT HEAT has to be removed by some other medium. FOR THIS Purpose AIR OR WATER is used.

This would mean that, water or air must be at a temperature lower than the condensing temperature of refrigerant. OR ..at any given PRESSURE the condensing temperature of the refrigerant and Boiling Temperature of water or Air must be SAME

Then IF this water or air at this specification is available Then there is no need for mechanical exertion or refrigeration

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FORTH, HOW TO REFRIGERATE The air or water at 4.4 oC is not available all the time

The temperature of available air or water is always much higher than The temperature of the refrigerant vapor coming out of evaporator.. That is The refrigerant vapor cannot be condensed unless the condensing temperature is raised by some means.. CLUE:::::: Condensing point (temperature) of refrigerant vapor is raised by raising the pressure of vapor

EXAMPLE ::::::: In the above example the vapor pressure is tried to be increased to be 2.6kg/cm2G. ** If the pressure of vapor is raised to 11.25kg/cm2G------- corresponding to condensation temperature of refrigerant 49.44 oC (121oF)*****

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That is, This high vapor can them be condensed into liquid by cooling it with atmospheric Air Available.. Say about 32.3oC or 90F.

THEREFORE, we use a COMPRESSOR in Mechanical Vapor compression For the purpose Of raising the low-pressure refrigerant vapor, coming from the evaporator To a high pressure (As discussed) and then sending it to condenser

cooling

coil

Compressor

Condensor

receiver

Throttling device

OH WHAO. Ok this is the thoughts behind the development of the vapor compression system

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Condenser------ Air cooled or water cooled condenser Air or water is used for cooling the high vapor pressure in the condenser

The refrigerant vapor gives out SENSIBLE HEAT (Why fridge is hot at the back side near the coils) And LATENT HEAT And thus condenses into a liquid.. Now the liquid in the condenser at high pressure is at high pressure ------ Therefore it can be fed to the refrigerator if its pressures are lowered or throttle (to 2.6 kg/cm2 G or 37 psig) so that boiling can take place in the evaporate at 4.4oC.