EC and Air cooled condensers working in parallel

Hi,

Receive one project for reconstruction (to make an offer for installation works)

Ammonia system, with old air cooled condensers (8 pcs working in parallel), due to problems caused with high condensing pressure new reconstruction design assume to remove 3 old air cooled condensers and install 3 new evaporative condensers also in parallel connection.

Size of new condensers is not important now (have no data but believe new EC are bigger ;) ) but idea...the rest of refrigeration plant is pure classic;)

Screw compressors with refrigerant cooled oil coolers.
Climate is extra dry and hot (Saudi Arabia).

I have no expirience with this design (I do not like such design too much, but I do not know why:confused: ) what about you?

Best regards, Josip :)

Hi Josip,

Here are my thoughts.

The parallel condensers could be tricky. The air-cooled condensers will have a much higher pressure drop than the evaporative condenser coils.

When you try to gravity drain all of the condenser in parallel, the coils with the highest pressure drop may have a tendency to hold up liquid in those coils.

This would be my biggest concern.

Since the location is Saudia Arabia the low wet bulb temperatures should really help with sizing the evaporative condensers. Someone may suggest sizing the new condensers for a condensing temperature higher than 35C, but I would not recommend that practice.

Depening on the actual heat rejection of the system I might try to see if all of the air-cooled condensers could be replaced with evaporative condenser.

At 35C condensing, you could sell the idea of much lower compressor input power and mich less fan power too.

I don't know what they are paying for electricity, but this might pay for itself quickly.

When you say the oil coolers are refrigerant cooled, do you mean thermosiphon? If this is correct, that adds another layer of complexity to the condenser issue too.

Is the high-pressure receiver indoors, or outside in the sun?

This sounds really interesting.

Quote:

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Originally Posted by US Iceman

Hi Josip,

Here are my thoughts.

The parallel condensers could be tricky. The air-cooled condensers will have a much higher pressure drop than the evaporative condenser coils.

When you try to gravity drain all of the condenser in parallel, the coils with the highest pressure drop may have a tendency to hold up liquid in those coils.

This would be my biggest concern.

Since the location is Saudia Arabia the low wet bulb temperatures should really help with sizing the evaporative condensers. Someone may suggest sizing the new condensers for a condensing temperature higher than 35C, but I would not recommend that practice.

Depening on the actual heat rejection of the system I might try to see if all of the air-cooled condensers could be replaced with evaporative condenser.

At 35C condensing, you could sell the idea of much lower compressor input power and mich less fan power too.

I don't know what they are paying for electricity, but this might pay for itself quickly.

When you say the oil coolers are refrigerant cooled, do you mean thermosiphon? If this is correct, that adds another layer of complexity to the condenser issue too.

Is the high-pressure receiver indoors, or outside in the sun?

This sounds really interesting.

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:) Electricity is almost free 0.12 rials/kWh or something (4.8 SR=1Euro=1.28US$)

Yes, thermosiphon oil cooling...

2 pcs of receivers are indoors....

Some of my colleagues are telling me that all will be ok but we are still in :confused: :eek: discussion.

Best regards, Josip :)

Hi Josip:)

We did a site recently, had to pipe an aircooled with an evaporative. I got it to work, but on the second attempt,it worked when I drained both condensers into the receiver separately, with the condensers 10' above the receiver. Unless you have a connection for each condenser into the receiver separately it won't work (or work well).

As for the condensing, what is the wet bulb in Saudi, from memory you would be selecting a 40 or 45 condensing temperature on evaporative.

Hope this helps.
Kind Regards Andy:)

Hi Guys,

I think it is possible, but...there will have to be some special precautions with the condensers draining into the receiver. This is the tricky part I mentioned before.

Adding in the thermosiphon system does add some potential problems. I'm not saying it is impossible, you just have to be very careful with the piping a and pipe sizes.

Andy, why use 40C to 45C for the condensing temperature? It is true that would reduce the condenser size, especially in a very dry climate. I have seen some water treatment problems with high condensing temperatures on evaporative condensers.

If I could, I would use a max. of 32.2C, instead of the ususal 35C condensing for ammonia.

You may want to consider the use of a surge receiver in lieu of a through-flow receiver. Take the thermosiphon feed off the low point of the liquid header under the receiver and return the overfeed to the receiver. Use a low level switch on the receiver to close the liquid feed line to the low side in the event of a low receiver level to protect the compressor oil cooling circuit.

With the surge receiver, the liquid trap out of each condenser becomes less of an issue and you usually will have enough height to trap for the higher pressure drop of the air cooled condenser.

A big issue for counties with a limited water supply is the required quanity of water needed for evaporation as well as blow down. Water in this area is usually pretty hard and you will have to use a lot of chemicals and a high number of cycles (blow down) to keep the condenser in the proper water range.

Ken

what is the meening of condancing temparature in air cooled condenser

What about air condensers first serial connected with EC?
Air condensers working as desuperheaters with control of speed of fans connected to a PT

Quote:

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Originally Posted by davidlleida

What about air condensers first serial connected with EC?
Air condensers working as desuperheaters with control of speed of fans connected to a PT

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You have to be careful of the pressure drop through the heat exchangers. As you decrease the refrigerant pressure, you change the saturation temperature (condensing temperature) of the vapor. A lower condensing temperature usually requires a larger heat exchanger to reject the same amount of heat from the system.

"You have to be careful of the pressure drop through the heat exchangers."
I can only be agree with you (as you can see I need a pressure drop lesson :))

Thank you so much