Setting superheat settings

[Abe]

This topic has probably been covered upteenth times already.
I am getting liquid floodback on a unit........Ive increased superheat which seems to have cured it.

I need to know, the correct way to check superheat settings, so Im not fluffing about.

[chemi-cool]

hi abe,

assuming system charged and head pressure is around 200psi
and room temp is 2 to 3C from setpoint.

I use a two sensor thermometer. T1 sensor is connected to the TEV outlet. T2 sensor is connected to evaporator outlet.

T2-T1= about 4C. adjust the TEV untill you get that differential.

this can be done also but not as accurate with gages connected to the same places but you have to preper a schreder connections in advance. bet you didnt.

over 90% dont.

chemi

[Brian_UK]

Abe, have a look at the Profs' site www.sporlan.com and go to the Literature section. They have tech docs on tev's which are worthwhile. Well I learnt from it anyway

[chemi-cool]

hi Brian.

the sporlan site as many others, is not very clear.
I attach the relevant pdf, please read carefully and see that "not recommended method" is not a serious answer.
what would you learn if they don't tell you WHY?

I use the temp difference method for years ( since I've found two sensors digital thermometer) and it is accurate.

the gages pressure at compressor valve can help but it is not the same pressure at the evaporator outlet.

chemi

p.s. file is too big so go to http://www.sporlan.com/tevs.shtm
and download zip 10-11
I would suggest to keep this page in your favorits as it contains some useful information.

[Abe]

Thank You.

Im inclined to use the two temperature sensor method as Chem suggested.

[Brian_UK]

Makes sense to me

[TAKO]

Tx Valve Bulb Temp - Evap Temp = Operating Superheat

[Dan]

please read carefully and see that "not recommended method" is not a serious answer.
what would you learn if they don't tell you WHY?

I use the temp difference method for years ( since I've found two sensors digital thermometer) and it is accurate.

the gages pressure at compressor valve can help but it is not the same pressure at the evaporator outlet.

Let's say you have an R22 evaporator with 10 psi pressure drop. Midway through the evaporator where you measure your first temperature the pressure is 37 psig and at the end of the evaporator the pressure is 32 psig. If you assume the saturation point midway through the evaporator, you accept the temperture reading as 15 deg F and consider a temperature reading at the end of the evaporator being 20 deg F as a superheat of 5 deg F.

But an outlet temperature of 20 deg F at 32 psig is actually a superheat of 10 deg F. Thus, in this example anyway, using two temperture probes compared to using a temperature probe and a pressure reading, you will tend to set the superheat higher than you think you are setting it.

The two-temperature probe method will tend to deliver the higher superheat and fool you into thinking that you are providing a 3 to 5 deg superheat when you are more likely providing a more reasonable 7 to 10 deg superheat.

[chemi-cool]

The two-temperature probe method will tend to deliver the higher superheat and fool you into thinking that you are providing a 3 to 5 deg superheat when you are more likely providing a more reasonable 7 to 10 deg superheat

hi Dan,


please explain this bit better, why??

if I measure the temp at the evaporator inlet and get -25C
and at the outlet I measure -22C, then how do I get higher superheat?

using the pressure/temp method should be exactly the same if you have a scdraeder valve at the same points where you measure temp using the same gages for accurate reading but I've never seen schraeder valves ready for pressure check so what you say is, lets compromise.

sorry mate, compromising on superheat settings, can easily cost you a new compressor either from flood back or lack of cooling.

my approach to superheat settings is one of the most important things, we know what will happened to the system if its not.

chemi

[Dan]

If I measure the temp at the evaporator inlet and get -25C
and at the outlet I measure -22C, then how do I get higher superheat?

Hi Chemi.

The reason Sporlan recommends to use the temperature pressure method of measuring superheat is to compensate for pressure drop that may exist in the evaporator tubing. If we assume -25C to be saturated liquid at 14 psig, and there is no pressure drop in the evaporator, then a -22C outlet is 3C superheat.

On the other hand, if there is a pressure drop of 4 psig from the point you measured the -25C temperature to the point where you measured the -22C temperature, your superheat is actually 7C because the saturation temperature at 10 psig is -29C.

[chemi-cool]

hi dan,

got your point.

we are talking about pro's, if there is a pressure drop and the temp stays the same and as you said.

this is called a starved evaporator and it will not function well.

of course there are always gages on the suction valve. but its only as a safety.

the real problems start when the condensing unit is remote and there is no place to read pressure at sensing bulb, you will not run from the TEV to compressor like a poisoned rat but you will check the temp.

you will observe the suction line for condensation, watch the accumulator for frost and read the pressure.

there is no " black or white". all things together will give you the full picture of evaporator function but at the end of the day its the right superheat that you adjust makes the difference, and its down to temp check.

well,thats how I work but as an open minded person, I will try the "sporlan" way, even just to see if I was wrong.

chemi

[terrygoodrich]

The preferred location to read the pressure is at the evaporator outlet just upstream from the equlizer tube.(Just in case any liquid escapes the valve through the equalizer) If the suction line is not excessively long and sized properly the pressure drop should be negligible and predictable. You should be able to read the pressure at the compressor and estimate the pressure at the evaporator outlet. The temperature at the evaporator inlet can be misleading and is not reliable to calculate superheat. Compressor manufacturers dictate maximum and minimum superheat readings at the compressor, however these values should be achieved by setting the valve properly at the evaporator and then providing adequate insulation on the suction line.

[Gary]

Another factor to keep in mind is the evaporator air in temp. If the TXV bulb is within the refrigerated space, the bulb temp cannot possibly exceed the space temp. There just simply is no heat source to raise it above this temp. The superheat can be no higher than the difference between SST and evap air in temp (evap TD). Given an oversized evap, the TD can be very low.