Superheat?

[cretan]

what is the proper way to check and adjust superheat?Do I take temp reading on suction line close to compressor or suction line at the evaporator?The diagrams I have seen are too confusing.

[US Iceman]

Hi,

Review the article found at this link:
http://www.sporlan.com/10-135.pdf

It describes the of the Pressure/Temperature chart and finding superheat.

You actually have three locations where you want to measure superheat.


At the evaporator. This tells you the superheat the TXV is adjusted for. This is called evaporator superheat.
At the compressor suction service valve. This tells you how much superheat the suction has picked up, in addition to the evaporator superheat. This is called suction superheat.
At the compressor discharge service valve. This tells you how much the gas temperature has increased after flowing through the compressor. If the superheat in this location is low, you may have liquid carryover problems from the evaporator(s) and would want to find the problem causing this. This is called discharge superheat.These are three distinct types of superheat an should not be used interchangeably.;)

Hope that helps.

[mohamed khamis]

what is the proper way to check and adjust superheat?Do I take temp reading on suction line close to compressor or suction line at the evaporator?The diagrams I have seen are too confusing.

Hi Cretan

firstly you should know that the superheat is defined as the difference in temperature between the evaporator exit refrigerant temperature and saturation temperature corresponding to evaporator pressure (which is some what equals to suction pressure regardless the suction pressure drop). To check the system superheat, you should take reading on suction line close to the evaporator. The reading on suction line close to compressor gives u an indication to the magnitude of the increase to the suction temperature as the heat gain from the environment and the difference between this reading (close to compressor) and reading (close to evaporator) is called UnUseful Superheat because this superheat decreases the density of refrigerant entering compressor and thus a drop in compressor volume capacity (volumetric efficiency) and an increase in discharge temperature affects badly on oil viscosity. So the suction line should be insulated to avoid the much increase in superheat and prevents any moisture condensation on the suction line. Regarding to the reading at compressor discharge service valve this indicates as US Iceman said to liquid carryover problem but it is also named as superheated temperature and it refers to the magnitude of the temperature of vapor entering condenser not the superheat which is regularly cheeked to know the system performance, it just indicates if there is a liquid carry over. In short, should take reading on suction line close to the evaporator to check system performance. I wish it could help

Best regards:)

[US Iceman]

...refers to the magnitude of the temperature of vapor entering Condenser (http://www.refrigeration-engineer.com/forums/glossary.php?do=viewglossary&term=61) not the superheat which is regularly cheeked to know the system performance, it just indicates if there is a liquid carry over.


Discharge superheat is also useful when the system application uses screw compressors. In this application, the oil is at discharge pressure.

This means you have to have sufficient temperature (superheat) to reduce the amount of refrigerant in suspension in the oil.

Very low or zero discharge superheat would indicate liquid carryover.

Discharge superheat should also be checked as it can point to other issues such as broken discharge valves, or high suction superheat also, and more.

[LRAC]

Here you go cretan

To measure evaporator superheat:
1) Record the actual temperature at the TXV bulb.
2) Record the evaporating pressure at the TXV bulb.
3) Convert the evaporating pressure to temperature by using those handy
pocket pressure/temperature cards. These cards/charts show the saturation
pressure/temperature relationship for those refrigerants.
4) Subtract the temperature you converted on the pressure/temperature card
from the actual temperature you recorded at the TXV bulb.
5) The difference is the actual evaporator superheat.

To measure subcooling:
1) Record the compressor discharge pressure at the condenser or receiver.
2) Convert the discharge pressure to temperature by using the
pressure/temperature cards.
3) Record the actual temperature of the liquid leaving the condenser.
4) Subtract the liquid temperature (at the condenser) from the converted
temperature (from the discharge pressure using the pressure/temperature
card).
5) The difference is the amount of subcooling.

Kind regards
Lrac

[carlos1975]

hi
where can i find a refrigeration cycle showing subcooling,, superheat temperature points etc

[olumide]

Hi Cretan,

Nomally any superheat, shoud be 6degrees diffrent. Btw the gass or vapoure temprature reading that come's out of the evaporator pipe passing thruogh expansion bulb on suction line and the temprature reading on the LP gauge,If the diffrent is below 6degrees the system is overcharge and if the system is above 6degrees the system is undercharge or short of refigerant.;)
OLUMIDE

[US Iceman]

where can i find a refrigeration cycle showing subcooling,, superheat temperature points etc


Review the article found at this link:
http://www.sporlan.com/10-135.pdf

This was posted not too long ago. It is is IP units, but you can certainlt figure out the right SI units to use in the same way.

[chillyblue]

Hi All

i can get my head round superheat, it's subcooling and sightglasses that screw me up.
The other day i was charging a system, the sight glass was flashing not vigourosly and i was getting a reading of 3 deg . C of subcooling, my gauge line was on the liquid reciever and my temperature gauge was on the liquid line outlet of the reciever.
I ignored this and continued to charge the system til the sight glass was full.
Surely if the sight glass is not full there must be gas present, in which case there should not be any subcooling????
Secondly have i in theory added too much refrigerant to the system.??
thirdly should the system be charged when it is under maximum demand or standard operation?

Cheers

CB

[lana]

Hi there,

Sub-cooling is the best indication for proper charge. Of course if the components are selected correctly.
If sub-cooling is low then it is under-charged, if sub-cooling is high means the system is over-charged.
Measure the sub-cooling by measuring the liquid outlet from the condenser not the receiver.
Bubbles in the sight glass do not necessarily mean lack of refrigerant. This could be from other faults too, like blocked filter or pre-expansion in the liquid line.

Cheers

[Pooh]

Hi
LRAC's method is spot on however be warned when using Zeotropic refrigerants ie 404a as it has a glide and if you read the wrong scale on the chart or comparator you can still have liquid back to the compressor, we use R401a in the department as a R12 drop in it has a 7 degC glide so if you get it wrong and set the valve to 6 degK you have no superheat. when setting superheat always use the higher of the temps indicated to work from.

Ian

[chillyblue]

Hi

Thanks for the explanations, what would be classed as a high subcooling an low subcooling.

Chilly Blue

[Pooh]

Subcooling is subcooling as long as you have some you should be ok but 10 to 12k is ideal to guarantee no flash gas at the metering device. The more the merrier in some systems, I have known 20k plus on some applications. If you look at an enthalpy diagram you can see the benefits of subcooling graphically.

Ian

[cretan]

so if I want a superheat of 10.on a freezer running 404a.do I take the temp reading on the suction side by the compressor.then the pressure reading on the low side.convert it with the p\t chart and subtract it from the temp reading?when is the best time to check superheat?does it matter the temperature of the freezer?should it be cold in the freezer warm in the freezer.what effect does the temp inside the compartment have when adjusting superheat?

[paul_h]

You want the pressure at the evap exit, and the temp at the evap exit for saturated evaporator superheat.
If you cant get the pressure at the actual evap, and only at the compressor, realise that the presure your reading might be slightly lower (by 1-2 degrees) due to pressure drop. I aim for 5k with both measurements at the evap, and 6-7k if the suction pressure is taken at the comp. that is, if the system is running good with 6k, I won't mess about with it, if it's 7k measuring at the compressor, I won't either, but if it's 7k at the evap I will.
It's best to get everything at the evaporator.
Superheats are best done with a proper refrig charge at normal operating temps. It's best to sort it out at the temp it's going to be running at, so you can see the pressures and temps at the compressor with your settings.

[Gary]

Subcooling is subcooling as long as you have some you should be ok but 10 to 12k is ideal to guarantee no flash gas at the metering device. The more the merrier in some systems, I have known 20k plus on some applications. If you look at an enthalpy diagram you can see the benefits of subcooling graphically.

Ian

At about 8.5K/15F subcooling, the flash gas is gone. From that point on you are filling the condenser with liquid, reducing its efficiency and unnecessarily raising the head pressure (condensing temperature). Sometimes you need more subcooling in order to lift the liquid for an elevated evaporator, but I would draw the line at 11K/20F (condenser outlet) subcooling.

Any benefits of subcooling come from cooling the liquid, not raising the condensing temperature (head pressure).

[Pooh]

Gary
agreed but what I failed to mention was the high subcooling levels were artifially produced using a subcooler.

Ian

[mohamed khamis]

At about 8.5K/15F subcooling, the flash gas is gone. From that point on you are filling the condenser with liquid, reducing its efficiency and unnecessarily raising the head pressure (condensing temperature). Sometimes you need more subcooling in order to lift the liquid for an elevated evaporator, but I would draw the line at 11K/20F (condenser outlet) subcooling.

Any benefits of subcooling come from cooling the liquid, not raising the condensing temperature (head pressure).

Hi Gary

the subcooling load in the condenser is almost about 5 to 10% of the condenser frontal area. Although this portion but the subcooling area takes place a rather portion in the condenser area due to the smaller heat tranfser coefficient of the subcooled liquid comapring to the two-phase coefficient and this portion at the end of the condenser. The more subcooling the more system efficiency but on expense of the more augmentation in condenser area, this is only the drawback of the subcooling. however as u mentioned "you are filling the condenser with liquid, reducing its efficiency and unnecessarily raising the head pressure (condensing temperature)" this is only occured when the liquid which is settled in the receiver is flashback to the condenser if the reciever is full by the liquid and there is restriction or quite bloackage in the liquid line or oil slugging on the TXV. at this moment the liquid goes back to the condenser and does what u mentioned. In contrary, the more subcooling assist the system performance when the condenseing pressure drops responding to the drop in ambinet temperture where is the latent heat is increased and need more area to occur that is provided by the subcooling area at the design condition.

cheers:)

[mohamed khamis]

I will emphasize on that the more subcooling is the more enhancement in system performance, however there is a limit for the subcooling which should be clarified. every thing in the real life has a aprons and cons. the more subcooling translates to the more refrigeration effect and in turn the refigernat mass flow rate will be smaller for the same cooling capacity resulting in the compressor input power is decreased harmonizing with the less refrigerant mass charge, thus the COP is enhanced. Alternatively, the low mass flow rate for the same evaporator and condenser geometrical configuration leads to the low refrigerant velocity inside the evaporator and condenser. Accordingly, in the evaporator the refrigerant boiling off will generate a clumsy refrigerant vapor clinging inside the tube surface and makes as a blanket causes a significant reduction in refrigerant heat transfer coefficient and hence the system capacity will injured particularly at partial load operation. Furthermore, the ability of refrigerant to scrub or scavenge the oil to return it to the compressor will be very weak causes a drastic losing in compressor oil with long time service and in turn the oil will settle in the evaporator and condenser adding a negative credit in the coils performance, the same is correct for suction line oil return. the same way in the condenser but not significant as evaporator as the change from vapor to liquid attenuate the dilemma of slowing of refrigerant flow. In short, the subcooling and supheating aslo have a limitation border can not be overstepped.

Cheers

[ewart]

The proper way to check for superheat is to check both pressure and temp. at the same point of the circuit. Check the pressure you get from the system to the PT CHART the corrinsponding temp. on the chart you get should be lower than the temp. you measure. The difference between measured temp. and temp. on the chart is the superheat.

[chillyblue]

Hi all

Re subcooling
is it possible that you can have good subcooling at the condensor outlet, but not enough liquid in the reicever to fully supply the liquid line/tev???

Cheers Chilly Blue

[expat]

Let's be logical here. I'm no veteran but it would seem that if in your cycle you have liquide ariving at the tev then evaporating in the evapoator with the correct superheat then returning to the compressor then compressing into the condensor and leaving with the correct subcooling :you have a correctly balanced system don't you?

[expat]

Gary, mohamed khamis, paul_h, Us-Iceman, Lana...I wasn't, stricktly speaking answering. More, seeking asurance:o

ps. if I forgot anyone, no offense..., it's late.

[frank]

Let's be logical here. I'm no veteran but it would seem that if in your cycle you have liquide ariving at the tev then evaporating in the evapoator with the correct superheat then returning to the compressor then compressing into the condensor and leaving with the correct subcooling :you have a correctly balanced system don't you?
A system is only in balance when the heat of evaporation plus the heat of compression is rejected at the condenser. (not heat pumps in heating).

Tempurature measurements around the system will confirm (or not) if the system is in balance when plotted on a Mollier Chart

[AU Fridgy]

hi
where can i find a refrigeration cycle showing subcooling,, superheat temperature points etc

Get youself a copy of Coolpack 1.46 you can download it here: :mad:sorry can't post a link cos I have not made 15 posts. If you have trouble finding it message me with an email address and I'll send you the link

you can plot your own points for the gas you want and see your own subcooling/superheat/pressure drops etc.:rolleyes:
regards

[The MG Pony]

Cool pack is in the tools and calculaters section.