Noobie question.Evaporator Pressure.

[kfjoe]

A bit of a noobie question.
But can anyone explain in simple terms what governs the evaporator pressure in say a tev system.
Ie. If the load/temp is high, is it this temp that would cause the evap pressure to rise? or is it the tev opening letting in more refrigerant/pressure that would cause it to rise.
As a system on pull down seems to have a highish evap temp that comes down when the room temp is reduce.
I just cant get my head around wether it is the temp/pressure relationship that causes a higher pressure in the evaporator or the metering device letting in more refrigerant and therfore pressure.
I am a little confused as i thought it would be latent heat at evaporation and therefore no increase in temp and along constant pressure line.

Would appreciate any help.
Thanks

[Prof Sporlan]

But can anyone explain in simple terms what governs the evaporator pressure in say a tev system.

The Prof will give it a try...

If the load/temp is high, is it this temp that would cause the evap pressure to rise?

Yes

or is it the tev opening letting in more refrigerant/pressure that would cause it to rise.

Yes.

Perhaps it is better explained as follows. A high heat load on the evaporator will be sensed by the TEV due to the resulting higher superheat at the outlet of the evaporator coil, which will cause the TEV to feed more refrigerant into the coil, which places a larger load on the compressor.

But a high load will also cause evap pressure to rise on a evaporator employing a cap tube or plug restrictor. The high load raises the operating TD of the coil, giving it more capacity. But with a TEV, the evaporator is better utilized at the high load condition, and one would expect higher evaproator pressures with a TEV, ignoring for the moment, any MOP (maximum operating pressure) feature the TEV bulb charge might have.

I am a little confused as i thought it would be latent heat at evaporation and therefore no increase in temp and along constant pressure line.

You have to consider what is happening with evaporator coil capacity with changes in heat load.

[kfjoe]

Ahh
Thankyou Prof.
That has cleared it up a bit better.

[superheat]

Prof explaination was good, but I have a different focus on this situation.
Gas pressure is governed by the ideal gas law. PV/nT.
The biggest factor on evap pressure is, n, the number of molecules entering the coil and how fast it boils. As the valve feeds more ***** into the evap, the pressure will rise. (n is increasing) A high heat load will cause the ***** to boil faster and raise the pressure also.
In a condenser you have to think about the space left to the vapor by the liquid in the bottom. Increased subcool will leave more volume in the coil to hold more molecules, n.

[kfjoe]

Am i right in thinking that there should be no temp increase from the refrigerant boiling off as it is latent heat.
But is there a Pressure increase from vapourisation (i assume there is ?)
I think maybe i need to get my head in my old books again lol
I actually did my Refrigeration Technician certificates but never really ended up going down that lane so never used it.

[Prof Sporlan]

kfjoe, you're viewing evaporator pressure from a refrigerant-centric perspective. A single constituent refrigerant or azeotrope will boil at a constant temperature given a constant pressure. As pressure rises, its boiling temperature rises. As pressure falls, its boiling temperature falls. But what causes high or low pressure in an evaporator coil is primarily a function of the heat load on the coil, and the resulting balance you obtain with evaporator and compressor capacity. Knowing the refrigerant's refrigerating effect and pressure temperature characteristics is more of a sidebar to the central question of what causes evaporator pressure to rise or fall.

[superheat]

The ***** enters the expansion valve as a warm liquid under high pressure. The expansion valve meters ***** into the evap. This lowers the ***** pressure below the saturated pressure. The ***** boils. As it boils it absorbs heat. (Water on your stove has a gas flame to provide heat.) The ***** around losses heat and the temperature drops to close to saturated evap temp. SO there can be latent and sensible at the same time.
In the evap as the ***** boils, the air stream provide the warm media to be cooled. The ***** expands as it boils. This expanding ***** causes the evap pressure. There is another thing, the compressor is continuing to suck ***** out of the evap while the TEV continues to add ***** to the evap. These 2 detirmine n, the number of molecules (or pounds, moles, Kg) in the evap. The heat available from the air detirmines how fast the ***** will boils and expand. The split (DT) and coil area give you the heat transfer from air to ***** Q = Area X Split. High superheat always comes with low evap pressure too. High SH is caused by a lack of ***** in the evap. More ***** into the evap (n), causes the pressure to rise.

Getting a little long winded, still have my head in book writing mode. I try to write a few pages every morning. In a few years I might have a book ready.

[frank]

One of the best explanations I've ever heard!! - can;t wait to buy the book!!

[superheat]

Thank you for the vote of confidence. With a little luck, many people will want to buy it.