Superheat and subcooling

[jakb21599]

can any one tell me what the superheat and the subcooling should be at the compressor for a reach in supermarket freezer 404A *****? which is the best method to go by?

[tonyhavcr]

Go to refrigeration 101 ]

[taz24]

can any one tell me what the superheat and the subcooling should be at the compressor for a reach in supermarket freezer 404A *****? which is the best method to go by?


Any air cooled condenser will only achieve about 4, 5 maybe 6 degs of subcooling.
Most TEV (TXV) are set to deliver about 6 degs of superheat.

Now subcooling is the temp of the liquid coming from the condenser towards the TEV
the closer to the TEV the greater the cooling. If you measured the temp at the outlet
of the condenser it could be between 1 or 2 degs to 4 or 5 degs below the saturation
temp (condensing temp). The closer to the TEV you are, the cooler the liquid will be,
but it can never cool below ambient temps.

Superheat from the evap is going to increase in temp as it moves away from the evap.
At the outlet of the evap it should be about 6 to 8 degs (could be as low as 5 or as
high as 10) and the closer to the comp you get the warmer the temp will get.

Remember subcooling does not mean cold, it just means the liquid is cooled below the
condensing temp. Superheat does not mean hot, it just means higher than evaporating temps.

The only correct way to check is with your gauges reading discarge and suction pressure
and a good touch probe thermomiter.

All of that information works as long as your system is working correctly and is ok, If there is a
fault on the system such as short of gas, blocked condenser or iced up evap the figures will not
relate to what I have just said.


taz

.

[matt p]

Hi Taz,

Should you try and aim to get equal sub-cooling and superheat temps?.

Ie on a R407c recip chiller both should be around the 6-7 degree mark?.

Regards

Matt

[taz24]

Hi Taz,

Should you try and aim to get equal sub-cooling and superheat temps?.

Ie on a R407c recip chiller both should be around the 6-7 degree mark?.

Regards

Matt


It is not important realy.

But what goes in at one side must come out at the other because fridge systems
absorb and reject, that being the case they must reject what they absorb or they fail :rolleyes:

The things that affect subcooling are state of cond and ambient temp, If all was
perfect then the subcooling would still only get to about 4 or 6 degs because the
way the systems should be designed and runs.

As for superheat, it is a nessesary evil. Superheat is added only to protect the comp,
it is an ineffiencey that is designed in to stop liquid returning to the comp. In a perfect
system you would have 1 deg of superheat at the comp and little loss of effienecey.

So the superheat and subcooling are mainly guides.
They should be between Blah Blah Blah but they will change as the system changes
under load, duty and ambient conditions.

Most fridge guys work to a rule of 10.

10 degs over the evap coil
liquid boiling 10 degs lower than air off
superheat 10 degs higher than evap temp.

All crude but they work and they will get you going for most aplications at most
times.

The challenge is to know when it is ok and when it is not and only experience
can teach you that..

Cheers taz

.

[matt p]

Thanks for that taz :)

Does sound like my old toutors rule of thumb that he used!

Just booked marked this so I can show one of the apprentices that is on site today. Mainly because I'm pants at trying to explain things!

Regards

Matt



It is not important realy.

But what goes in at one side must come out at the other because fridge systems
absorb and reject, that being the case they must reject what they absorb or they fail :rolleyes:

The things that affect subcooling are state of cond and ambient temp, If all was
perfect then the subcooling would still only get to about 4 or 6 degs because the
way the systems should be designed and runs.

As for superheat, it is a nessesary evil. Superheat is added only to protect the comp,
it is an ineffiencey that is designed in to stop liquid returning to the comp. In a perfect
system you would have 1 deg of superheat at the comp and little loss of effienecey.

So the superheat and subcooling are mainly guides.
They should be between Blah Blah Blah but they will change as the system changes
under load, duty and ambient conditions.

Most fridge guys work to a rule of 10.

10 degs over the evap coil
liquid boiling 10 degs lower than air off
superheat 10 degs higher than evap temp.

All crude but they work and they will get you going for most aplications at most
times.

The challenge is to know when it is ok and when it is not and only experience
can teach you that..

Cheers taz

.

[jakb21599]

From what i have read you can not charge an A/C by superheat if it has a TXV. you have to charge by subcooling. You can only charge by superheat if it is a fixed or capilary sytem. ( is this correct?)
Also does the same apply to a cooler or freezer with a TXV. Do you charge by superheat or subcooling for this.
( sorry if these seem like stupid questions) I am trying to gain knowledge as to the best methods of doing things.

[nike123]

From what i have read you can not charge an A/C by superheat if it has a TXV. you have to charge by subcooling. You can only charge by superheat if it is a fixed or capilary sytem. ( is this correct?)

Yes!


Also does the same apply to a cooler or freezer with a TXV. Do you charge by superheat or subcooling for this.


Same applies.

Since TXV is designed to maintain same superheat it will do that all the time when it is possible. So you could somewhat undercharge or grossly overcharge but superheat will remain same if TXV is working properly. Therefore you cannot charge TXV system by superheat. Only by subcooling.
http://www.refrigeration-engineer.com/forums/showthread.php?t=19701

[jakb21599]

on units with TXV. where do you position the bulb? I have been putting them at the 4 or 8 O`clock position. It seems like i have read somewhere that they were being put in the 2 or 10 O`clock postion. Which is right? or does one way apply to coolers,freezers and the other apply to A/C. if so which is for what?

[taz24]

on units with TXV. where do you position the bulb? I have been putting them at the 4 or 8 O`clock position. It seems like i have read somewhere that they were being put in the 2 or 10 O`clock postion. Which is right? or does one way apply to coolers,freezers and the other apply to A/C. if so which is for what?


The size of the pipe dictates where.
The smaller the pipe the higher up and the larger the pipe the lower down the bulb (phial) goes.

On new installs there is a guide on the box that the TEV comes in but if the pipe is
smaller than 7/8" then it does need to be around the 10 to 2 position.

taz

.

[jakb21599]

thanks for the info taz. regarding the position of the bulb. if it is not positioned right. what are some of the effects it will have on the system if not in the right place on the pipe?