Gas Temp Rise in Liquid-Flooded Compressors

[zdas04]

I've tried for many years to predict the liquid temperature rise for flooded screw compressors and (recently) liquid-ring compressors. The best I've come up with so far is a three-step process:
1. Calculate heat of compression as though the liquid were absent using T(out) = T(in)*(P(out)/P(in))^((k-1)/k). That gives delta T.
2. Convert volume flow rate to mass flow rate and then HeatOfCompression=m*c(p)*delta T.
3. Convert the liquid volume flow rate to a mass flow rate and solve for liquid delta T using the HeatOfCompression from "2" above.

Does anyone have any opinions on this technique or have a more straight-forward technique for predicting this temp rise?

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Torricelli]

David,

for the liquid ring compressors:

Qmot + Qcond = m.Cp.dT

Qmot [KW] power absobed of motor (acc. perf.curve)
Qcond [KW] heat of condensation of vapours (if applicable)
m [kg/s] seal water flow
Cp [KJ/kg.K] Cp of liquid as the Cp of gas will be neglectible
dT [K] temperature increase of liquid

Liquid ring compressor are usually designd for dT = 10 K
(i.e. 1/4 of gpm per HP or 75 ltr./h.KW)

hope it helps

PST

http://www.vacuum-guide.com

 

[zdas04]

Torricelli,
From the above, the amount of compression is immaterial to the oil temperature rise? That is difficult for me to accept.

Working with flooded screw compressors for many years, I've seen significant changes in the outlet oil temperature (with the same inlet temperature) when the conditions change from 5 ratios to 10 for example. That would say that predicting liquid temperature rise has to take into account the heat of compression of the gas without considering the liquid and then back into the liquid dT from the heat of compression and the liquid mass flow rate (everything I've done in this area has been with non-condensible gases).

The 1/4 gpm/hp seems like a rule of thumb that results is satisfactory performance over a wide range of conditions, but since there is no way of knowing what proportion of driver capacity will be used in the field my guess is that it results is very conservative designs and significantly oversized pumps.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[crjones]

In general on oil flooded screw compressors, the oil absorbs 82% of the input power.

 

[pmover]

david,

a few years ago, i prepared a request for proposal for a liquid-filled rotary screw compressor for natural gas service. the packager responded with a unit that required no air-cooler for gas steam after compression, even when operating at 100% recycle. surprisingly, the heat of compression was absorbed by the oil and the oil cooler provided sufficient cooling to maintain operation at 100% recycle. i questioned this offer/proposal with supplier and i had one of the process engr's conduct a hysim analysis to substantiate claim. the hysim results confirmed the packager's offer. needless to say, i was darn impressed with process engr's & hysim's results and the supplier's offer.

my analysis would be similar in that:
1) determine heat of compression, knowing process fluid,
2) obtain liquid oil flow rates and oil properties from equipment mfg,
3) know process stream flow rates,
4) assume inlet temperatures for both fluids - worst case scenario,
5) assume nearly identical mixing of two fluids (oil and process stream) and then determine resultant outlet stream temperature of oil/process fluid.

-pmover

 

[zdas04]

pmover,
I've found the dT across a screw to be a function of the oil mass flow rate also.

The only time I can see not having a gas after cooler is when you have dehydrated gas coming in. If the gas is saturated, then you need to get the oil temp high enough to cook the water off or the oil gets contaminated. Most sales points can't accept 200F gas. For dehydrated gas the manufacturers generally recommend about 130F oil outlet temp which is close enough to contract-sales temp that you'll likely be in spec without a "cooler" since the pipe will radiate much heat away.

David

 

[pmover]

yes, oil flow rates do impact outlet temps

um - i'm thinking,

it seems to me that if gas is saturated, i'd recommend dehydration before compression and not during the compression process. i envision potential corrosion and oil contamination problems; hence, a potentially messy environmental matter. but, if the system is designed to accomplish the dehydration during compression, then i suppose it might be acceptable. i'd certainly get references for similar applications.

-pmover

 

[zdas04]

Gas field, wellhead compression at less than 20 psig suction and 80 psig discharge. There is no contractual reason to dehydrate and low-pressure dehy units are very fuel and process inefficient. At these pressures and wellhead temperatures 3,000-4,000 lbm/MMCF water content is very common. If the compressor oil outlet runs at 205-215F the water is cooked off and stays with the gas into the gathering line. Much cooler than than that and the oil turns into a milky emulsion and stops lubricating very well.

David