RSW calculation

[sushibar]

Hello,

I just found this forum and I am new. I am not an refrigeration engineer but know enough to keep our fishing boat running.

We have an Glycol based RSW system. The system has 4" water plumbing and 6" glycol welded to the outside of the 4" chiller. The rest of the circulation system is 3". I have 2 Carlyle O5T 50 hp open drive screw compressors, that can run single or in tandem.


I do I know how many TR I am actually getting. I know it is volume of water, time , temperature change in that time. How do I make the formula.
I have about 1700cuft of sea water to chill. Right now I tested it the other day. I got 45f start 32F finish in 3 hours.

The other thing is how do I speed up the chilling time.
Lower Suction Pressure?
More water Flow?
More water Pressure?

My water flow is about 250gpm and maybe 375 with the other water pump

Any help is appreciated

[gregd1401]

Need to know SG of glycol to be able to calculate capacity.

[sushibar]

SG Propylene Glycol 1.036

[sushibar]

condenser temp inlet is 55 F outlet 60F.

[gregd1401]

O.K. 1btu=1lb water/oF.
Or if you can handle metric
flow(l/s) x td(oC) x 4.1817 =KW's cooling

[michaelm]

B”H

Hi Sushibar,
I do not know how accurate do you need the numbers. I total in three hours you system withdraw ~118 TR. I would be very carefull with the lowing suction pressure without knowing the design of the system. I can freeze up and damage your evaporator. Good Luck.

[sushibar]

I don't need the numbers to be perfect.
Thanks for the help.

My Suction pressure is about 23-26 psi with one compressor at the target water temperature. Two compressors I'm at about 10 psi. The coldest I could ever get the glycol was -30 C. The water was 29 F.

My glycol tests out 60 %, that puts me in the -55 freeze up range.

I have safety flow meters in the system that will shut down the system if no flow is indicated.

Anyone know why I can only get to 29F on the water side and not any colder.
I'm assuming the inside of the chiller pipes are freezing and not allowing much heat transfer anymore.

I would like to get the water down as cold as possible as its better for a large volume of fish being added.

[sterl]

I think you hit it....The water is starting to freeeze and either the flow gets restricted or the ice films the heat exchanger surface and retards continuing heat transfer. Lowering the suction would be dangerous to the heat exchanger unless its of a falling film variety.

The way to Bank Cold is to make sea water ice; most of those on board fishing vessels are drum types making particle or shaved ice.

[charlie n]

I recommend hot gas bypass to keep the suction pressure from getting too low. Sterl is right, at 29F the seawater is beginning to freeze.

[sushibar]

1)refrigerant chills the propylene glycol
2)the chilled glycol is circulated around the sea water. the sea water is pumped thru 120' of 4" pipe. The 4" pipe is inserted into a 6" pipe that is welded to the 4" pipe. The glycol is circulated thru the 6" part.


I have had the water down to 27.5 (rare) degrees but it was getting slushy which is ok.

Sterl if I increase water flow would this help any or would it be minimal.

I am trying to find an inexpensive way to help speed up chill time and or get below the 29 f mark.

This might be a moot point though.

Thanks for the info so far

[sushibar]

I am pumping about 260 gallons per minute water flow, and about 350 gpm glycol.