Superheat and Subcooling help !!PLZ!!

Hey guys i know this is posted all over this website, but it seems everyone has a different idea. i finished my apprenticeship about 3 years ago now, when i finished i went away and gave the fridgie game a break. i honestly forgot a few basics such as SC and SH. basically im setting up a mine site atm, i got 3 big freezer rooms and 3 big coolrooms, semi hermatics running on R404, there are two evaps in each room, each evap has its own condensor. they have been vaccedd and bombcharge by another fridgie and now im commissioning. just my luck the first one i turned on went *BANG* blew the valve plate on the comp of the freezer, i had sump heaters on for about 4 hours before i turned it on (just for the record). anywayz wat im gettin at is for the SH should i take the pressure from the access valve on the evap and work the the TD between that and the bulb temp? next for the SC i must honestly say i have never done sub-cooling ever, so really im stumped... is it TD between liquid leaving condensor and the high side service valve? please explain SC

please guys a little help would be appreciated just a push in the right direction

Hi KF,

Superheat is just them temperature above its boiling point that a refrigerant is.

So, if you measure the pressure at the evaporator out let and the temperature there also and look it up on a chart or slide rule for the refrigerant thats in the system, that will give you the superheat.
So say for example your freezer room is at -18C your evaporating temp is -30C (which you have worked out by converting 1.1bar/15psi into temp and your evap outlet temp is -23C then your evap superheat is 7K (K is for kelvin, its the same scale as C but its a temp difference and not an actual temp)

Other superheats that are important are compressor inlet superheat measured near the compressor, and discharge superheat measured near the compressor on the discharge pipe.

Make sure to sheild you temp probe from draughts.

Subcooling is the temperature below boiling point of the refrigerant, so if your system is condensing at 45C (use slide rule or gauges again to convert pressure into temp) and your condenser outlet temp is 40C you have 5K subcooling, subcooling is important to ensure a solid feed of liquid to the expansion valve, if you have little or none you will get bubbles in the liquid line which will slow down the flow through the expansion valve and lower the duty the system does.

Have a look on youtube, there are many vids posted by engineer showing how this is done on actual system.

Re your blown compressor, sounds like the install guy just dumped liquid into the compressor :eek:

Jon :)

thanks jon

yer abt the comp, thats what i first thought, but i never seen one crack the whole plate like this one did.. quite spectacular it was.

SH : so basically its the S.E.T and evap outlet temps that i compare difference.. i read something about the second superheat you mentioned, i had no idea about that lol i still dont.

SC: so is it S.C.T TD between liquid line? ( this one really confuses me, its been too long) where exactly do i test for subcooling, i realise its the liquid leaving condensor, but what should i comparing that to, you say condensing temp (this might sound silly, but) where should i be testing for that

wait i think i get it.. SC: liquid leaving condensor is colder than what it would normally be at that given pressure, Change of temp without change of pressure? am i on the right track here.
im jus guna make numbers up here but hear me out
condensor outlet temp 40C; liquid line pressure (converted to temp) tells me that liquid should be 45C so therefor i have 5K subcooling? is that right

Quote:

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Originally Posted by karara_Fridgie

thanks jon

yer abt the comp, thats what i first thought, but i never seen one crack the whole plate like this one did.. quite spectacular it was.

SH : so basically its the S.E.T and evap outlet temps that i compare difference.. i read something about the second superheat you mentioned, i had no idea about that lol i still dont.

SC: so is it S.C.T TD between liquid line? ( this one really confuses me, its been too long) where exactly do i test for subcooling, i realise its the liquid leaving condensor, but what should i comparing that to, you say condensing temp (this might sound silly, but) where should i be testing for that

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Did the comp run for a bit then go bang or was it bang straight away? If it run for a bit it sounds like a liquid slug came up the suction line, may be worth winding in the suction valve to throttle the other comps on initial start up, either that or shut the reciever valve, and run the comp for a five or ten seconds (turn off quick if it sounds noisey all of a sudden) then leave it off for a few minutes for the refrigerant to vapourize in the pipe work, once you feel there is no more liquid coming back to the comp you can open the reciever valve and run the system up, keep an eye on it though as if the installer has done something stupid with the expansion vavle then you could get liquid back again!

SH: yeah its the difference between temp refrigerant is evaporationg at and the temp the refrigerant is exiting the evap at.

SC: Yeah you got it, put your gauges on the system and see what pressure the high side is at, say 20bar/300psi the means that the refrigerant is condensing at 45C, so if your condenser outlet temp is 40C you have 5K subcooling.
I would just measure it on the liquid line near the reciever out let, but i think it technically sould be measured on the pipe between the condenser and the reciever, but on the size of equipment i work on its difficult to get to and is also in the airstream from the condenser fans which will lead to an inacurate reading.
High subcooling could point to the system being over charged. Low subcooling and it could be undercharged or poor airflow.

Jon :)

Quote:

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Originally Posted by karara_Fridgie

please guys a little help would be appreciated just a push in the right direction

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You reposted this +/- 2 hours after your first post, not everyone is sitting behind his computer to answer your question. Have a little patience, some need to work first a little bit to gain some money so that they can pay their webaccess to help finally others.

Quote:

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Originally Posted by Peter_1

You reposted this +/- 2 hours after your first post, not everyone is sitting behind his computer to answer your question. Have a little patience, some need to work first a little bit to gain some money so that they can pay their webaccess to help finally others:off topic:.

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:off topic:

ok mate, chillout.. i didnt repost anything.. simply asking another question.. if this bothers you that aint my problem... i never demanded that some should answer me straight away, simply stating my disparity. and your coment wasnt even helping me, instead you trying to talk down to me... i didnt offend anybody except mabye you...get off my case mate, if you cant help me then leave it at that, dont cry poor to me... your a fridgie so u should be good for cash...

and by the way sory if i did offend anyone, im just asking for help, and replies have been great so thanks for the help so far

:D

Quote:

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Originally Posted by monkey spanners

Did the comp run for a bit then go bang or was it bang straight away? If it run for a bit it sounds like a liquid slug came up the suction line, may be worth winding in the suction valve to throttle the other comps on initial start up, either that or shut the reciever valve, and run the comp for a five or ten seconds (turn off quick if it sounds noisey all of a sudden) then leave it off for a few minutes for the refrigerant to vapourize in the pipe work, once you feel there is no more liquid coming back to the comp you can open the reciever valve and run the system up, keep an eye on it though as if the installer has done something stupid with the expansion vavle then you could get liquid back again!

SH: yeah its the difference between temp refrigerant is evaporationg at and the temp the refrigerant is exiting the evap at.

SC: Yeah you got it, put your gauges on the system and see what pressure the high side is at, say 20bar/300psi the means that the refrigerant is condensing at 45C, so if your condenser outlet temp is 40C you have 5K subcooling.
I would just measure it on the liquid line near the reciever out let, but i think it technically sould be measured on the pipe between the condenser and the reciever, but on the size of equipment i work on its difficult to get to and is also in the airstream from the condenser fans which will lead to an inacurate reading.
High subcooling could point to the system being over charged. Low subcooling and it could be undercharged or poor airflow.

Jon :)

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:D:D:D
thats awsome...really has helped refresh my brain ... at the moment i got one running just for testing so i can get all my control settings right, i dont have a working touch probe the one the boss suplied is screwed.. so im guessing the SH.. on one evap coil only the first half is frosting up., and the other has no frost at all.. im thinking thats because there is not enough liquid flowing through the tx valve.. does that sound right

and with that compressor im pretty certain the installer dumped the charge in the suction line, the service valve is ontop of the compressor, so i think it was full of liquid,,? wat do u rekon..

So long as no one has been adjusting the expansion valves, and they have been selected correctly they shouldn't need adjusting for superheat, checking is a good idea, i think when the rooms are warm the superheat will be higher anyway and will gradually get lower as the room temperature drops so you wouldn't want to be adjusting anyway till the room had reached design temperature.

With the two evaps, the one with no frost may be working ok, it depends on the room temperature, if you have recently started the system and the room is above 10c it may not be evaporating cold enough to have frost on the coil, check your gauges and sliderule or sclae to see what temperature the system is evaporating at.

The other one may be short of refrigerant, how does the sightglass look? Did the install guys weight in the same amount to each system?
You may well have a few bubbles in the sightglass with R404A, its just they way it runs, especially if the system if working outside of its design parameters such as it may be when pulling the room down to temp.

Its difficult to explain what to do as when you are on site there are many little details you notice that point you in the right direction, how things sound, various temperatures, etc etc that you don't even realise you have noticed, with experience they all point you in the right direction.

Best advice i can give is don't make any major changes at once, if you think it is short of refrigerant just weigh a little in and see if things improve.

Have a search on google for 'sporlan expansion valve' and on the Parker Sporlan site, there is much to learn there on setting up systems.

Jon :)

;)your a champ jon... very much appreciated... im trying to get a good run out these babies but the damn sparkies ... curse them... keep turning the genarator off at the end of the day... ill keep this page posted on how im going with this.... thanks again