A couple of questions.

[marc5180]

I've got a few questions that i would like to ask you all, i tried ringing my college tutor but he hasn't gotten back to me yet so i thought i would post here instead.
I know these are probably very basic questions to you all:

1) I have been called to different units several times and with them all generally if the unit is short of refrigerant, then i would expect to see a low suction pressure of say 3bar which on R22 is -10deg but if i am getting -10deg after the evaporator then i would expect to see say -20deg going in to the evaporator. If this is the case then why would i be getting no cooling from the indoor coil ( i know it's short of refrigerant)

2) Do exansion devices have a limit of how much they can reduce the temperature, i.e can they only reduce temperature by 20degC or more or less?

[US Iceman]

Do exansion devices have a limit of how much they can reduce the temperature, i.e can they only reduce temperature by 20degC or more or less?

NO!

The expansion devices only reduce the liquid pressure down to the evaporating pressure maintained by the compressor capability (assuming the charge is correct).

For example, a cap tube capacity can be changed by increasing or decreasing it's bore or length. A TXV is a complex orifice that only controls superheat if properly applied.

[nswilkinson]

I maybe wrong but am i right in thinking that due to the system been short of gas there will be alot more sensible heat transfer than latent heat transfer, causing a huge reduction in the total heat absorbed at the evap. I think you would also have an increase in superheat which would reduce your c.o.p. i hope i'm right in thinking this i'm still learning but thought i'd have a go at posting.

[marc5180]

I maybe wrong but am i right in thinking that due to the system been short of gas there will be alot more sensible heat transfer than latent heat transfer, causing a huge reduction in the total heat absorbed at the evap. I think you would also have an increase in superheat which would reduce your c.o.p. i hope i'm right in thinking this i'm still learning but thought i'd have a go at posting.

Can anyone explain this better for me?

[Thermatech]

The expansion device needs pure liquid.
At 3 bar suction you might not have any liquid in the liquid line it might be high presure vapor which cannot flash off on entry to the evaporator so,,,, no cooling.
As the system becomes SOG then you start to get bubbles in liquid line & so even if the expansion device opens up to max still cooling performance starts to become reduced due to reduced refrigerant volume passing through into the evaporator due to the bubbles in liquid.
As the system looses more refrigerant the liquid volume passing through the expansion device becomes smaller & smaller untill there is no liquid at all & then no cooling.
Mean time compressor is flat out & suction pressure / temperature is very low.

I am sure there are some engineers who can explain better with technical reference to R22 characteristics.

[marc5180]

The expansion device needs pure liquid.
At 3 bar suction you might not have any liquid in the liquid line it might be high presure vapor which cannot flash off on entry to the evaporator so,,,, no cooling.
As the system becomes SOG then you start to get bubbles in liquid line & so even if the expansion device opens up to max still cooling performance starts to become reduced due to reduced refrigerant volume passing through into the evaporator due to the bubbles in liquid.
As the system looses more refrigerant the liquid volume passing through the expansion device becomes smaller & smaller untill there is no liquid at all & then no cooling.
Mean time compressor is flat out & suction pressure / temperature is very low.

I am sure there are some engineers who can explain better with technical reference to R22 characteristics.

this is what confuses me though, if it got to the point where there was no liquid present at the evaporator and it is just vapour then surely the pressures/temperatures should be higher?

[nike123]

this is what confuses me though, if it got to the point where there was no liquid present at the evaporator and it is just vapour then surely the pressures/temperatures should be higher?

Temperatures difference's (TD) are lower and pressures are lower then with correct charge.
That mean that air leaving evaporator is at higher temperature then if the refrigerant charge is OK.
Also, air leaving condenser is at lower temperature then if the refrigerant charge is OK.
Subcooling is non existent and superheat is high (suction pipe is not cold).

[powell]

1) I have been called to different units several times and with them all generally if the unit is short of refrigerant, then I would expect to see a low suction pressure of say 37 psi which on R22 is +14 F but if I am getting +14 F after the evaporator then I would expect to see say -4 F going in to the evaporator. If this is the case then why would I be getting no cooling from the indoor coil (I know it's short of refrigerant).

Marc,

I converted your questions to PSI and F to help me think through your question.

The way I read the question about the refrigerant TD between the inlet and outlet of the evaporator is that the TEV, once set for the proper superheat, will always keep this SH. This thought is incorrect as the SH fluctuates as the load changes. The TEV SH setting is fined tuned with the system fully charged and working properly and near the designed temperature. When the load increases or decreases the SH will change.

Next, concerning compressor performance and the evaporator. A typical HT R22 compressor is rated at a 0 to 45 F SST. The lower the SST, the less btuh’s it pumps. For example: A 1 HP HT R22 compressor pumps 12,000 btuh at a +45 F SST and 9600 btuh at a +20 F SST.

So a system low on refrigerant will have a lower SST and pump less btuh’s. If the evaporator is designed for a capacity rating at +45 F then the refrigeration affect is lost when the SST drops. Basically the evaporator is starved.

Here are some links that may help you. Enjoy, there’s lots to read.


Sporlan TEV theory of operation: http://www.parker.com/literature/Literature%20Files/Parker.com/Literature/Sporlan/Sporlan%20pdf%20files/Sporlan%20pdf%20010/10-9.pdf

Sporlan – Using a PT chart: http://www.parker.com/literature/Literature%20Files/Parker.com/Literature/Sporlan/Sporlan%20pdf%20files/Sporlan%20pdf%20010/10-135.pdf

Sporlan – Common TEV problems: http://www.parker.com/literature/Lit...010/10-143.pdf

System Troubleshooting: http://www.hvacrinfo.com/system_trouble.htm

Copeland AE bulletins: http://www.hvacrinfo.com/ae_index.htm

[wambat]

The greatest amount of heat transfer in the evaporator takes place when you have a change of state, like from a liquid to a gas so whan you are short of refrigerant you run out of liquid to early to take advantage of all the heat transfer avialable to do much cooling.Therefore, lowsuction and discharge pressures because the compressor can remove more vapor then is being generated in the evaporator. This by the way would result in high superheat in the suction and discharge lines.

[marc5180]

Sorry it's taken me so long to reply, i've been reading through the posts and the links which have kept me busy. I think i understand it better now.
Thanks for all the replies.

[Pooh]

NSWilkinson
you must have a good tutor?

Ian