Negative superheat

[Techsingh]

Hi guys, I have two Dunham Bush screw type 150 ton air cooled chillers model ACHFX 150-6s. We recently installed new compressors and the unit were set up by a Dunham Bush tech but lately i have getting zero or negative superheat on the suction, any help will be greatly appreciated.

[frank]

'Negative Superheat'

That's a new term to me........could you explain?

[Gary]

Hi guys, I have two Dunham Bush screw type 150 ton air cooled chillers model ACHFX 150-6s. We recently installed new compressors and the unit were set up by a Dunham Bush tech but lately i have getting zero or negative superheat on the suction, any help will be greatly appreciated.

Sounds like you need to calibrate your instruments.

[Josip]

Hi, Techsingh

Hi guys, I have two Dunham Bush screw type 150 ton air cooled chillers model ACHFX 150-6s. We recently installed new compressors and the unit were set up by a Dunham Bush tech but lately i have getting zero or negative superheat on the suction, any help will be greatly appreciated.

... suction temp sensor must be calibrated/changed .... it can be only 0K (zero) not possible to be negative....

... there is something wrong with setup of your PLC ... relation between suction pressure corresponding to temperature and actual suction temp ... can be a problem also with suction pressure sensor ... or maybe with PLC program

... anyhow, suction superheat can be 0K having flooded PHE with ammonia - then you must protect your compressor with discharge temperature .... with other refrigerants superheat must be set at higher value 5-8K min

... what about alarms-shutdown of your compressors?


Best regards, Josip

[coolhibby1875]

'Negative Superheat'

That's a new term to me........could you explain?

when your your suction temp is lower than your evaporator temp!!!

[Techsingh]

what i mean by negative superheat is that the suction temp. according to my IR meter is lower than the pt chart says it should be.

[Techsingh]

Only the occasional low suction alarm

[Gary]

what i mean by negative superheat is that the suction temp. according to my IR meter is lower than the pt chart says it should be.

My experience with IR has been somewhat negative. Get a real temperature meter.

[Brian_UK]

what i mean by negative superheat is that the suction temp. according to my IR meter is lower than the pt chart says it should be.

If by "IR" you mean Infra Red meter then I respectfully suggest that you do not use it for accurate measurements. You need a proper thermometer.

[Josip]

Hi, Techsingh

what i mean by negative superheat is that the suction temp. according to my IR meter is lower than the pt chart says it should be.

Only the occasional low suction alarm

IR meter and actual values sometimes not match ... anyhow ... recalibrate your pressure and temp sensor at compressor's suction .... both instruments must be OK .... IR meter is only for some control and usually not very accurate ... because you are measuring out of suction stream ...


Best regards, Josip

[bill1983]

ir thermometers are notoriously unreliable for this type of measurement. if the reflectivity of the areas where the readings are taken differ then you will not be able to compare the readings. you really need to invest in a higher quality thermometer with either strap or clamp type probes. or use a touch probe and a reliable calibrated compound gauge, digital if you can get one. use the ir as a guide to case temps only but remember, that reflective surfaces produce a certain amount of error.

[bill1983]

oops look like we all thinking the same but you can all type faster than me

[Josip]

Hi, coolhibby

'Negative Superheat'

That's a new term to me........could you explain?

when your your suction temp is lower than your evaporator temp!!!

suction temperature must always be lower then evaporator temperature ... but, even then we do not consider it like negative superheat ...

Best regards, Josip

[Techsingh]

Thanks for your advice guys, if what your saying is right then i have a big problem because the Dunham Bush tech and all of the refrigeration techs that are maintaining this piece of equipment currently are using IR meters to take readings and make adjustments based on those readings.

[coolhibby1875]

Hi, coolhibby





suction temperature must always be lower then evaporator temperature ... but, even then we do not consider it like negative superheat ...

Best regards, Josip

when you are setting up electronic controls in any supermarket, using any type of controler, you always have a positive supaheat usually between 4-8k and this is measured between your evaporator inlet and your evaporator outlet (suction).
in all controllers i have used if you have a negitive supaheat i,e your suction lower than your evaporator the eev closes and the case goes high temp, the warmer the suction goes over the evaporator the the diff increases and your eev opens.

i am a bit confused about what you mean regards your suction should always be colder than your evap, it goes against anything i have been taught?

[Josip]

Hi, Techsingh

Thanks for your advice guys, if what your saying is right then i have a big problem because the Dunham Bush tech and all of the refrigeration techs that are maintaining this piece of equipment currently are using IR meters to take readings and make adjustments based on those readings.

Modern times-modern guys ... sorry for you ... try to find some other way to setup all things better.... calibration of sensors ... better thermometers etc....

Best regards, Josip

[Techsingh]

My chiller was not manufactured with suction temp sensors, do chillers in general come with them?

[coolhibby1875]

My chiller was not manufactured with suction temp sensors, do chillers in general come with them?

hi techsingh not all but some do i look after a few husser plants (austrian) and these packs only look at suction temp then convert it to temperature to presure, the lower the suction temperature the lower the presure, so it shuts down compressors and pumps down, the warmer the suction temperature the higher the presure so it starts up compressors, the pack try's to maintain a constant -7 degrees c suction temp, if our suction temps were lower than the evaporator as stated above we would never remove heat from the evap!

[Techsingh]

I think my problem could be the two oil return lines that connect to the compressor suction line,i was told that these lines bring a mixture of liquid refrigerant and oil to the compressor, is this supposed to be like this and could this be affecting the superheat?

[Josip]

Hi, coolhibby1875

when you are setting up electronic controls in any supermarket, using any type of controler, you always have a positive supaheat usually between 4-8k and this is measured between your evaporator inlet and your evaporator outlet (suction).
in all controllers i have used if you have a negitive supaheat i,e your suction lower than your evaporator the eev closes and the case goes high temp, the warmer the suction goes over the evaporator the the diff increases and your eev opens.

i am a bit confused about what you mean regards your suction should always be colder than your evap, it goes against anything i have been taught?

First I do apologize because English is not my mother tongue and maybe I cannot express in a right way what I mean ... sorry... anyhow technically must be correct ...


.... I'm also confused a lot with your answer

but, let's try to solve that ....

.... first, you write down that superheat is usually 4-8K (Kelvin must be written in capitals, because it is a measuring unit according to international agreement ... also it is unit for absolute temperature value and for that reason there is no NEGATIVE VALUE for temperature expressed in K ... the lowes it can be only and only 0K (there is no temp below) and any higher value as you like... hope you agree with me...

then the second part of our dilemma or whatever...

to keep an evaporator temperature for example exactly at 263K (-10’C) and corresponding saturation pressure at compressor suction we need to maintain suction pressure lower corresponding to suction temperature of 262K or even 261K, but actual suction temperature will be higher depending on system and refrigerant ...

energy can flow only from higher level to lower level, compressor is just a transportation unit, thus ... at compressor suction we must have a lower temperature then in cold room to keep that room cold ....

(due to friction loss through the suction pipe i.e. suction pressure at compressor's suction is lower then saturation pressure at evaporator outlet ... that pressure expressed in temperature which is now higher ... i.e. temperature difference - between saturation temperature at evaporator outlet and actual temperature at suction of compressor we call suction superheat) we need this to protect compressor's of return of liquid

hope this is clear enough ... if not ... some other RE members speaking English since they were born can explain this matter in English much better...

Best regards, Josip

[Josip]

Hi, Techsingh

My chiller was not manufactured with suction temp sensors, do chillers in general come with them?

nowadays is must have specially with PLC control it is possible to install it but all depends on complete system control ... to be accurate and to control the system we need to have pressure and temp sensors otherwise is not possible to have PLC control ... or maybe they use some other way ... we do not know your system ... we are guessing all the time...

I think my problem could be the two oil return lines that connect to the compressor suction line,i was told that these lines bring a mixture of liquid refrigerant and oil to the compressor, is this supposed to be like this and could this be affecting the superheat?

No.... not possible ... only what you can have is the higher suction temperature (still you can have your suction line with very low suction superheat causing problems with low suction temp) and loss in compressor's capacity ... try to regulate regulating valve on oil return pipe ... pipe must be warm a little .... not hot at all ... then you will have oil return what is very important ... and no capacity loss...

Best regards, Josip

[coolhibby1875]

Hi, coolhibby1875




First I do apologize because English is not my mother tongue and maybe I cannot express in a right way what I mean ... sorry... anyhow technically must be correct ...


.... I'm also confused a lot with your answer

but, let's try to solve that ....

.... first, you write down that superheat is usually 4-8K (Kelvin must be written in capitals, because it is a measuring unit according to international agreement ... also it is unit for absolute temperature value and for that reason there is no NEGATIVE VALUE for temperature expressed in K ... the lowes it can be only and only 0K (there is no temp below) and any higher value as you like... hope you agree with me...

then the second part of our dilemma or whatever...

to keep an evaporator temperature for example exactly at 263K (-10’C) and corresponding saturation pressure at compressor suction we need to maintain suction pressure lower corresponding to suction temperature of 262K or even 261K, but actual suction temperature will be higher depending on system and refrigerant ...

energy can flow only from higher level to lower level, compressor is just a transportation unit, thus ... at compressor suction we must have a lower temperature then in cold room to keep that room cold ....

(due to friction loss through the suction pipe i.e. suction pressure at compressor's suction is lower then saturation pressure at evaporator outlet ... that pressure expressed in temperature which is now higher ... i.e. temperature difference - between saturation temperature at evaporator outlet and actual temperature at suction of compressor we call suction superheat) we need this to protect compressor's of return of liquid

hope this is clear enough ... if not ... some other RE members speaking English since they were born can explain this matter in English much better...

Best regards, Josip

firstly i believe what you have posted is correct, and i dont doubt any of it, however its getting far to scientific for the likes of me to understand, but to get back to the question posted, NEGITIVE SUPERHEAT! if your evaporator is -22 degrees c and your suction is - 28 degrees c that is acording to all my paperwork a negitive superheat, now normally when you come across this out in the field, this is not a good thing, and normally results in liquid floodback to the plant, i have dug out some docs on my pc and elm and rdm recommend a 6 degrees + superheat for optimum performance of the evaporator, and danfoss recommend a 4 degrees + superheat, sorry if i have forgot to use a capital k instaed of lower case, but being from scotland,english grammer is wasted on the likes of me, i know what i am trying to say in my head but after a 6 pack of tyskie , what my fingers are saying is completly diff

[Josip]

Hi, coolhibby1875

firstly i believe what you have posted is correct, and i dont doubt any of it, however its getting far to scientific for the likes of me to understand, but to get back to the question posted, NEGITIVE SUPERHEAT! if your evaporator is -22 degrees c and your suction is - 28 degrees c that is acording to all my paperwork a negitive superheat, now normally when you come across this out in the field, this is not a good thing, and normally results in liquid floodback to the plant, i have dug out some docs on my pc and elm and rdm recommend a 6 degrees + superheat for optimum performance of the evaporator, and danfoss recommend a 4 degrees + superheat, sorry if i have forgot to use a capital k instaed of lower case, but being from scotland,english grammer is wasted on the likes of me, i know what i am trying to say in my head but after a 6 pack of tyskie , what my fingers are saying is completly diff

I believe it will be much easier if we are together sharing your tyskie whatever is that

again I must say NO-negative superheat .. sorry ... let's try like this..

evaporator is -22'C and our suction at compressor's inlet is -28'C/and some x pressure (depending on refrigerant) you are reading this temp/press on the same gauge

... that mean we have pressure loss within suction pipe and to maintain -22C within evaporator we are running our compressor at -28'C what is ok .... but actual temperature is not -28'C at suction of compressor ... if suction pressure correspond to saturation temperature then we have liquid back to compressor .... actual temperature must bi higher thus we have only gas coming back to compressor suction... reading on the same gauge showing pressure and saturation temperature we do not have an actual suction temperature which must be higher

... but still below -22'C ...like -23'C ...-24'C...

... so, we have difference of 4 or 5K

.... what is difference between actual temperature minus saturation temperature at that suction pressure

-23'C=250K - (-28'C=245K) = 5K
-24'C=249K - (-28'C=245K) = 4K

please ... check RE forums for more explanation ... under suction superheat, superheat .... it is in original English and maybe easier to understood...


Best regards, Josip

[desA]

All sounds incredibly complicated.

If you want a more full picture of what is going on - some thoughts:
1. At evap, measure Pe,sat, convert to Te,sat - off LP gauge;
2. Measure Te,sup at evap discharge (temperature);
3. SH1 = (Te,sup - Te,sat) (SH1 at evap discharge);

4. Measure Pc,suc,sat at compressor suction - 2nd LP gauge;
5. Convert Pc,suc,sat to SST (saturated suction temp);
6. Measure Tc,suc (temp at suction);
7. SH2 = Tc,suc - SST

SH1 is nice to know as it tells you how well the evap is working - ie. if it is flooded, or not.
SH2 will tell you the superheat entering the compressor itself, relative to the inlet sat temp. This is the superheat value to be concerned with for a compressor, I would suggest.

[coolhibby1875]

Tyskie is very strong Polish lager,and after 8 my head is any better this morning

[750 Valve]

if your evaporator is -22 degrees c and your suction is - 28 degrees c that is acording to all my paperwork a negitive superheat, now normally when you come across this out in the field, this is not a good thing, and normally results in liquid floodback to the plant,

You mention 2 totally different measures of superheat, the first one you refer to is evap superheat and I assume the temp you refer to is the coil out temp (-22). The second one is being measured at the comp and is total system superheat (-28). The second encompasses the suction line pressure drop which has the effect of lowering the SST - therefore a valve flooding back bigtime will have a tendency to show up colder at the comp as there is a suction line PD between the coil and the comp - not to mention if a subsystem has an EPR then this will exaggerate the phenomena even more, however it is in NO WAY negative superheat, its just simple pressure temp relationship. The temps you refer to are just two separate measures of pipe temp and technically are not a valid method of measuring superheat, yes - usually you would expect the temp to rise between coil and compressor (additional superheat) but there are so many system variables which - when in a floodback scenario, can make these values reverse.

Also using coil in and coil out probes to calculate superheat is a very crude way of evap superheat measurement (yes I know all the supermarket controllers use it - that only because a probe is cheaper than a transducer), true superheat is SST minus vapour temp and applies to both evap and system superheat.

I have attached a basic diagram showing a floodback scenario where the suction line is not sized for the mass flow rate seen under floodback conditions

[iceman007]

You cannot have negative superheat, because it does not exist. Superheat is applied to a vapour above it's saturated evaporating temperature. If the suction line is colder than this, then this is liquid, and there is no superheat at all. It can only be a positive value.

As for your chiller, I think you will find a simple solution with either a sensor out of calibration, or similar. If you have one of the older control panels then the inputs may be at fault. Check your approach and log the chiller properly if you want to find your fault. Make sure your instruments are true as well.

[low0tech]

negative superheat means that liguid refrigerant reaches suction.Either something is happening with your chiller or you are getting a wron measurement

[coolhibby1875]

A method of controlling a refrigeration system employing an electronic controller for operating an electrically actuated refrigerant expansion valve supplying the evaporator or endothermic heat exchanger. The controller is of the type which modulates the valve for refrigerant flow to maintain a desired target level of superheated vapor discharging from the evaporator. When the compressor return, or evaporator discharge pressure (Pc) is within a preselected range the system employs a strategy to increment or decrement the target level of superheat depending upon whether the rate of change (ΔPc) of discharge pressure is positive or negative and thereby maximize pulldown by minimizing compressor cycling to achieve the superheat target.

[iceman007]

negative superheat means that liguid refrigerant reaches suction.Either something is happening with your chiller or you are getting a wron measurement

There is no negative superheat. There is either no superheat or a positive value of superheat. Use some thermocouples and insulate them.

[El Padre]

As has been previously stated, you cannot have negative superheat, plotting a PH chart might make more sense for those who are a bit confused.

Cheers

[750 Valve]

A method of controlling a refrigeration system employing an electronic controller for operating an electrically actuated refrigerant expansion valve supplying the evaporator or endothermic heat exchanger. The controller is of the type which modulates the valve for refrigerant flow to maintain a desired target level of superheated vapor discharging from the evaporator. When the compressor return, or evaporator discharge pressure (Pc) is within a preselected range the system employs a strategy to increment or decrement the target level of superheat depending upon whether the rate of change (ΔPc) of discharge pressure is positive or negative and thereby maximize pulldown by minimizing compressor cycling to achieve the superheat target.

As quoted above they mean the rate of change being negative or positive - not the superheat value itself. Positive rate of change being INCREASING superheat, negative rate of change being DECREASING superheat.

This is NOT negative superheat, there is no such thing.

[Major Phucup]

Impossible for that to happen

[Mike NZ]

IR hand held temperature measuring instruments can only be used on certain surface shapes and emissivities with any useful accuracy in the hands of the layman. Due to reflective surfaces and spherical shapes it takes a bit of know how to, and when to use this item effectively.

[mad fridgie]

Just to upset the apple cart, you can get "supercooled" liquid, with knowing 100% i presume you can also get "supercooled vapour" so if that being the case then you can get negative superheat!
Practically in a fridge system, no negative superheat. IR testers are great, only in the hands of a very very skilled IR operator, for the average joe blow engineer (including my self) IR are F*****G useless