Help needed!

[gk2108]

Hello friends!
Gr8 to be a part of this forum..
A few queries as far as the R-22 refrigeratrn system is concerned( 81TR @ -32 deg C brine outlet )
Our system is designed for a sunction superheat of -35deg C . THe system is also having an economiser .The refrigerant vapour pressure before sucntion is ~0.2 kg/cm2. Brine leaving is at ~-31.3 deg C , but the temp of the suction vapour is as low as -42 deg C.THe system is also having an economiser .
Is this situation normal??( COnsidering a delta between refrigerant and brineas high as 9 deg C ???)
Wld appreciate your valuable feedback . SOme reference material links in this regard would also help

Cheers !!

Govind

[Lowrider]

First things first, superheat can only be a possitve nunber, other wise you'll get liquid back to the compressor.

Than a 9K approach (sat. evaporator temp - leaving brine temp) sounds about right to me.

Brine has a lower specific heat than water and because it's more oil like will also thansfer the heat far worse then water.

The specific heat for water is, rounded up, 4,2 kJ/kg*K, for 30% ethlyleenglycol, off the top off my head, it's 3,85 kJ/kg*K.

[hendry]

Hello friends!
Gr8 to be a part of this forum..
A few queries as far as the R-22 refrigeratrn system is concerned( 81TR @ -32 deg C brine outlet )
Our system is designed for a sunction superheat of -35deg C . THe system is also having an economiser .The refrigerant vapour pressure before sucntion is ~0.2 kg/cm2. Brine leaving is at ~-31.3 deg C , but the temp of the suction vapour is as low as -42 deg C.THe system is also having an economiser .
Is this situation normal??( COnsidering a delta between refrigerant and brineas high as 9 deg C ???)
Wld appreciate your valuable feedback . SOme reference material links in this regard would also help

Cheers !!

Govind

Govind,
you means your saturated suction temp (SST) = -35degC?

yes, below -25degC needs economizer.

suction pressure is ok [within range].

if dT is 9K, your SST = -32-9 = -41degC.
but, you may still get lower Suction Temperature due to actual superheat and piping design.

hope that helps.

[gk2108]

Sanderh,
What i meant was that our chiller system is designed for a brine outlet temp of -32 deg C , and the suction at the compressor at -35 deg C . That makes it +3 deg C suction superheat.apologies for penning down wrongly.
Now ,in practice, what i am observing is that the refrigerant gas suction temp is around -41 deg C and the brine leaving the chiller is at -31 deg C.This is a DX type of a chiller.and i wanna know whether such a high dT( between the refrigerant gas and the brine ) is allowable , and is this efficient operation??????
Regards

Govind

[Lowrider]

I understood your question, hence the numbers to explain it, imo, better.


Can you tell me what brand it is?

[hendry]

Sanderh,
What i meant was that our chiller system is designed for a brine outlet temp of -32 deg C , and the suction at the compressor at -35 deg C . That makes it +3 deg C suction superheat.apologies for penning down wrongly.
Now ,in practice, what i am observing is that the refrigerant gas suction temp is around -41 deg C and the brine leaving the chiller is at -31 deg C.This is a DX type of a chiller.and i wanna know whether such a high dT( between the refrigerant gas and the brine ) is allowable , and is this efficient operation??????
Regards

Govind

Govind, efficient in our context is low input higher output.

currently, your data explains that you need dT of 10K to produce -31degC brine.
normally, in our design, we use merely 8K max. to do so.
probably, there might be some other issue/s in your system which you should consider.

sometimes, in our troubleshooting, we seek beyond the arising problem/s to eliminate the issue/s.

[Lowrider]

Ok, first the suction superheat is the difference between the saturated vapour temperature and the suction temperature. The other value to use is the difference between the evaporator outlet temperature and the saturated vapour temperature, or approach.

Super heat should be, dependend on the type of expansion valve, between 3 and 10 K. The approach for a shell/tube heatexchanger normally 1 to 5 and up to 8 for brine. Brazed plate heatexchangers slightly higher.

If superheat is too high either the charge isn't right, the valve isn't openeing enough or there could be oil in the evaporator.

If approach is too high the same goes for the oil or the evaporator is fouled!

For some reason you won't tell what brand it is, this however would help a lot!

[gk2108]

The chiller package is basically an assembled one.Mfg by Systems and Components . the compressor is Hallscrew( twin lobe , HS2028 series). expansion valves are danfoss make( solenoid operated exp valve). And the evaporator is of copper metallurgy.
Regards
Govind

[gk2108]

Sanderh,
Can you suggest sum ebooks which could make me understand these fundamentals in a better way>??Basically my job profile is to take care of the utility operations, without much more stress on all these design aspects..but i guess this wld direclty affect our overall system performance..

[Lowrider]

I don't know of any ebooks. But perhaps someone in your country with experiance in chillers can help and have a look!

[Gary]

Sanderh,
What i meant was that our chiller system is designed for a brine outlet temp of -32 deg C , and the suction at the compressor at -35 deg C . That makes it +3 deg C suction superheat.apologies for penning down wrongly.
Now ,in practice, what i am observing is that the refrigerant gas suction temp is around -41 deg C and the brine leaving the chiller is at -31 deg C.This is a DX type of a chiller.and i wanna know whether such a high dT( between the refrigerant gas and the brine ) is allowable , and is this efficient operation??????
Regards

Govind

Possibly some definitions would be helpful, in the interests of improving communications.

A "delta-T or dT" is a change in temperature of a single substance or flow of substance. For example, if the entering brine is -37C and the brine outlet temperature is -32C, this is a change in temperature of 5K or "5K delta-T".

A "temperature difference or TD" is a comparison of the temperatures of two different substances or flows of substances. For example, if the brine inlet temperature is -37C and the saturated suction temperature is -41C, then the difference in the temperatures of the two flows is 4K or "4K TD".

The "approach temperature" is a particular type of TD, specifically the difference between the brine leaving temperature and the saturated suction temperature. As heat is transferred between these two substances, the temperatures come closer to each other, or "approach" each other. If the brine leaving temperature is -32C and the saturated suction temperature is -41C, then this gives us "9K approach".

The "superheat" is a particular type of delta-T, specifically the difference between the saturated suction temperature of the refrigerant and the temperature of the refrigerant in the suction line. If the saturated suction temperature is -41C and the suction line temperature near the compressor is -35C, then we have "6K superheat" at the compressor inlet.

[Lowrider]

Thanks for explaining it Gary, I was busy the last few weeks and finely have 3 weeks of holiday now!!

Anyway, as I said before an approach of 9K would be reasonbly common on a plate heat exchanger when using brine. However a shell/tube one would have a lower one. Reasons for the approach to become higher can be found in my previous post.