new to f gas and confused about heating up just to cool back down

howdy lads
just start studying f gas lately and joined this site a couple of days ago. the main thing that confuses me about the vapour compression cycle is why we need to compress and heat up the vapour when we actually want it cold.

lets say we have -10 saturated liquid on entering the evap and -4 superheated vapour on leaving. the whole point is to cool the room so we need to get that -4 vapour back to -10 liquid to enter the evap again.

the first thing we do is compress it and heat it up then we cool it down through the condensor, but its still not cold enough (and still hotter than when we started compressing it) so we reduce the pressure through the expansion valve which cools it down sufficiently. obviously this is a stupid question but if the expaansion valve is capable of cooling from lets say 33 degrees to -10, why can it not just cool from -4 to -10. i presume its something to do with having to change state but im hoping one of you genius's can help me get my little brain around it. (without making it too complicated)

Perpetual motion hasn't been invented yet, sadly.

Using your figures it required a temperature difference of (+33 to -10) 43° to remove the heat energy from the room; you now suggest that (-10 to -4) a difference of 6° will remove the same amount of heat energy.

It is the action of absorbing the heat from the cooled space that boils the refrigerant liquid and changes it to a vapour. If you start with a vapour then all you can do is superheat it.

Got me thinking for a minute there Brian.

You presume correct - it's to do with changing state. Short answer is because you want liquid to go into the expansion device, not vapour.

Heat is absorbed by boiling the refrigerant in the evaporator. It goes in as a liquid, pressure is reduced through the expansion device it boils but needs energy to do so. It gets this energy from the heat around the evaporator - that's why the air is cold coming out of the evaporator.
More heat is absorbed per unit of mass when the refrigerant is boiling/condensing (latent heat) than if it stays at the same state but just gets hotter or colder (sensible heat).

Now we need to get more LIQUID to the inlet of the expansion device. It has to be liquid because we need to change it to a vapour to get the most amount of heat energy removed from the air. If we put gas in the evaporator and gas came out the only heat you'd take out is by how much the gas was cooled. This would imply the gas was cooled before it went in for it to work. How you gonna cool it? You'd need to refrigerate it first! No way it could work.

So how to get liquid at the beginning of the expansion device? Lets get it to a pressure where its saturated temperature (the temp read from the comparator) is above ambient, then blow ambient air over it to cool it down enough to condense then we have liquid refrigerant ready for expansion.

The refrigerant will only travel from high to low pressures so now we have this (pressure difference caused by the compressor and expansion device) all systems go. The expansion device will need a pressure difference across it to work correctly. To little difference and not enough refrigerant passing through it.

Does that make sense?
Have you gone through Pressure Enthalpy charts? Once you get the hang of those it should make it clear(er!)

Cheers,
Andy.

The heat absorbed in the evaporator needs to go somewhere. That somewhere is the condensor.
Some heat is transfered to the refrigerant during compression but this is unavoidable and also rejected by the condensor.

Cheers,
Andy.

Do some goolging on latent heat and sensible heat also check this book out
http://www.amazon.co.uk/gp/aw/d/1590...dir_mdp_mobile

hey brian. the 33 degrees to -10 degrees was actually the temp drop across the expansion valve and not taking heat out of the room.

i know absorbing the heat from the cooled space boils the refrigerant but this doesnt explain to me why we then go on and heat it up even further with the compressor when we actually want the refrigerant to be cold again so we can asorb more heat from the room

Quote:

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Originally Posted by brian connolly

i know absorbing the heat from the cooled space boils the refrigerant but this doesnt explain to me why we then go on and heat it up even further with the compressor when we actually want the refrigerant to be cold again so we can asorb more heat from the room

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Read above posts.

thanks andy

that made a lot of sense, we need the refrigerant changing state in the evaporator because then it is absorbing latent heat which is far more than sensible heat.....

just one thing, why do we pressurise to where its saturated temp is above ambient and then cool it down with ambient air to condense it. we seem to be heating it up higher than we need , why not just heat it up to the slightly lower temp? im sure you'll have no bother making sense of this for me after your last great explanation....

think i may have it andy, we're losing heat in the condensor but its a change of state so its latent heat were losing and the temp/pressure stay the same so we still have the pressure we need whereas if we hadnt compressed to the higher temp originally we wouldnt have had the pressure we needed to operate the expansion valve. am i on the right track?

sorry brian was replying to posts as i read them. i should probably read them all first before responding.........

believe it or not james i actually bought that book about 8 years ago when i was going for a maintenance technician interview, i went to a friends house one night who worked as a technician in this factory to prep me for interview and i went home after paying 110euro for that book from amazon. the interview was a week later and i hadnt a clue about refrigeration, my friend said all id need to know were the four main components. im not sure what i was suppose to say if they asked me anything about them........ that book is beside me on the table, but im not sure if i have enough spare decades to read it