discharge superheat

[philjd26]

hi,

i was wondering about where discharge superheat comes from, i imagine it gets its temp from suction superheat,running temp of compressor, heat of compression,and ambient temparture....can anyone ryhme of good explaination of this and explain the effects of certain instances like liquid floodback/high p[ressur/lowpressure? also is there a normal temp discharge superheat for all refrigerants @ the same ambient... eg. 5c suction temp, goes through compression comes out @ 45c..ie 40c dis superheat....does it vary with compressor types?

rgrds phil

[taz24]

hi,

i was wondering about where discharge superheat comes from, i imagine it gets its temp from suction superheat,running temp of compressor, heat of compression,and ambient temparture....can anyone ryhme of good explaination of this and explain the effects of certain instances like liquid floodback/high p[ressur/lowpressure? also is there a normal temp discharge superheat for all refrigerants @ the same ambient... eg. 5c suction temp, goes through compression comes out @ 45c..ie 40c dis superheat....does it vary with compressor types?

rgrds phil

Phil you have answered your own question.

You are right the superheat comes from friction in the comp, compresion and so such, but there is no one rule of thumb for working out what it will be.

The effiency of the comp, the type of refrigerant and the ambient temp will all effect the final superheat reading.
I would say that the actual temp out of the comp is somwhere near the 20deg c above the condenser saturation temp.

Cheers taz.

[philjd26]

hi taz

i know what your saying,maybe i did more or less answer it ,i was wondering is there anything else i missing because i have seen discharge reach 80c with a suction temp of 5c which is 75c in total,this is on an inverter mind you, with ambient temps of 23c...any thoughts on that?

rgrds phil

[Josip]

Hi, philjd26

hi,

i was wondering about where discharge superheat comes from, i imagine it gets its temp from suction superheat,running temp of compressor, heat of compression,and ambient temparture....can anyone ryhme of good explaination of this and explain the effects of certain instances like liquid floodback/high p[ressur/lowpressure? also is there a normal temp discharge superheat for all refrigerants @ the same ambient... eg. 5c suction temp, goes through compression comes out @ 45c..ie 40c dis superheat....does it vary with compressor types?

rgrds phil

Maybe a small contribution to all what you said...

…..superheating of suction to compressor can occur in any one of or in combination of the following:


-in the end of evaporator – useful cooling

-in the end pipe inside the refrigerated space – useful cooling

-in the suction pipe outside of refrigerated space – non useful cooling

-in the liquid suction heat exchanger – indirect useful cooling

…. we need superheated gas to be sure to have only a gas in front of suction flange of compressor

….calculation and choose of equipment we made according to some experience and rules trying to obtain some suction superheat….regarding to used refrigerant suction superheat is higher for ***** and lower for ammonia and to have it under control all must be done before suction flange of compressor….

....now we compress our gas within compressor thus suddenly rising pressure and temperature transforming mechanical energy to heat energy...and we call that the Thermal equivalent of compression work....

....yes, we have a small differences within discharge temperatures using different compressors ...it is slightly higher with piston compressors then with screws, but that is due to a large amount of cooling oil passing through screw compressor used also for cooling of discharge gases..

...but, it is not all so bad, we can use this heat in discharge desuperheater for heating...i.e. sanitary water, glycol for freezer rooms floors...etc

....my English is not the best (still learning) and maybe something is not expressed in clear way, but I hope you can understand it all

Best regards, Josip

[Thermatech]

Phil

You tend to mainly ask questions about splits & vrf.

So every time you put your gauges on a system watch the discharge pressure & convert it to deg C.Use the gauge temperature reading or comparitor. ( make sure you are connected to the compressor discharge service port).
Also measure the actual temperature of the compressor discharge pipe.( you need a half decent digital temperature meter & probes).

You will notice a difference between the two temperature readings.
This is the discharge super heat.

If you get into the habit of always monitoring the DSH you will gradually get a feel for what DSH is good for different systems & refrigerant & also any difference between cooling & heating mode.

For instance R410a VRF.
If the DSH less than 10deg C system stops on overcharge fault.
More than 120 deg C discharge temp systems stops on under charge fault & you will find DSH of 80 deg c or more.

So you might find looking at systems which are working ok & have known correct charge that the DSH will be between 25 to 45 deg C on flat out cooling mode.

DSH is very usefull for working out short of gas problems.
If the system has low suction pressure & low discharge pressure then you will naturally suspect SOG. But look at the DSH & compare with what you know it normally is with correct charge.

I recommend always look at the DSH when you commission every system so that you see what it should be with a correct refrigerant charge & always log that in your memory along side suction & discharge pressures.

Its all about the cooling of the compressor motor windings.
If the refrigerant volume & velocity is small the DSH will be high.
As the refrigerant volume & velocity increase the DSH reduces.

High DSH = suspect SOG
low DSH = suspect over charge

[philjd26]

hi l

thanks for replies lads...jossip you english is better that mine pal...

thermatech just one more question yousay that making a note of discharge temp on commisioning is a good idea for future referance, in the case of the ambient temp being differant i take it, it doesnt matter as the fans speeds are increased accoreding to the head pressure...

rgrds phil

[Thermatech]

Phil

discharge superheat

Foats like a butterfly

watch carefully before it stings

[US Iceman]

Several factors control the effect of discharge superheat.

One is the suction superheat present in the gas entering the compressor.

Another is the type of compressor being used. A screw compressor is oil injected (and the oil is cooled). This tends to limit the actual discharge temperature for any condensing temperature and hence the discharge superheat at that condition. When a recip. compressor is used the discharge temperatures are much higher than a screw compressor being used.

One of the main factors is the specific heat ratio of the refrigerant. This value is quite high for ammonia and air, but much lower for other refrigerants. Therefore, as the pressure ratio increases the actual discharge temperature also increases with the same suction superheat entering the compressor.

[hendry]

superheat at discharge is due to the increase molecular activities of the refrigerant vapour.

such activities increase the overall temp of the vapour.

yes, in other parts of the system, superheating exists & it usefulness varies.