a few qick questions....

[sid]

hi everyone,
just a few quick questions..

how do i work out superheat...and what is it..
what is the basic refrigeration cycle..

thanks sid

[taz24]

hi everyone,
just a few quick questions..

how do i work out superheat...and what is it..
what is the basic refrigeration cycle..

thanks sid

Sid how long have you been into fridge?
Have you done any formal fridge schooling?

I ask because in my oppinion someone who does not know how to measure superheat and needs to know what the basic cycle is, should not be let loose on their own.

taz.

[monkey spanners]

Superheat

Right, water boils at 100c at 1 bar (the normal air pressure that we breath), so you can have water at 100c and steam at 100c. If you heat the steam to 110c at 1 bar you have 10c (or 10k for kelvin as its not a temperature but a difference in temperature) of superheat, that is to say its 10k above its boiling point.
So say you have a suction pressure of 2 bar (measured at the evaporator outlet as the pressure drop in the suction line wil effect acuracy) with a system on R134a, this equates to a evaporating temerature of -10c. Now measure the temperature at the evaorator outlet, it can not be any lower than -10c. If it is -3c you have 7k of superheat. That is all it is. You need superheat to be sure that all the liquid has boiled off in the evaporator, if it is too low or you have none, you will have liquid refrigerant entering your compressor and a trip to the wholesalers in the near future.

Subcooling, water at 90c at 1 bar has 10k of subcooling, that is to say its 10k below its boiling point.

In vapour compression refrigeration systems we make things a bit more complicated by varying the temperature and pressure throughout the system. You'll need a thermometer some gauges and a sliderule to know what is going on where and when.

As to your second question the clue is in the name of the parts.
Compressor, the compressor compresses the superheated suction vapour and superheats it a bit more.
Condenser, this cools the superheated vapour from the compressor below its boiling point so it turns back into a liquid. (the temperature it cools the liquid below boiling point is subcooling)
Expansion device, this is fed with subcooled liquid still at high pressure. the liquid passes through the device and undergoes a drop in pressure. The liquid now at a lower pressure begins to boil and gets the heat to do this from the evaporator surface and what ever the system is cooling. The refrigerant exits the evaporator as a superheated vapour to go around again and again.

Get yourself a book on the subject, modern refrigeration and air conditioning has pleanty of colour pictures and cut away diagrams to show the workings of various parts. It would be money well spent.

Cheers Jon

[monkey spanners]

[pendlesteve]

God help us

[Brian_UK]

Yep, in his biography he is an "Air-con Engineer".

A slight overstatement I believe.

[Peter_1]

"Thank you Monkey Spanners" said Sid or something similar.

[expat]

Very concise Monkey Spanners.

[Paulajayne]

Kelvin Scale

The Kelvin temperature scale (K) was developed by Lord Kelvin in the mid 1800s. The zero point of this scale is equivalent to -273.16 °C on the Celsius scale. This zero point is considered the lowest possible temperature of anything in the universe. Therefore, the Kelvin scale is also known as the "absolute temperature scale". At the freezing point of water, the temperature of the Kelvin scale reads 273 K. At the boiling point of water, it reads 373 K.

Whereas the Kelvin scale is widely used by scientists, the Celsius or Fahrenheit scales are used in daily life. These two scales are easier to understand than the large numbers of the Kelvin scale. Could you imagine waking up to your radio and hearing the DJ give a weather report like this: "It's going to be a beautiful day today with sunny skies and a balmy temperature of 297 K!" That's 24 °C or 75 °F.

Paula

[TXiceman]

I hope that Sid was just pulling our leg a bit.

Ken