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Air Cooled vs. Water Cooled condenser tonnage...are they equal???

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Air Cooled vs. Water Cooled condenser tonnage...are they equal???

seetwo

(Specifier/Regulator)

(OP)

30 Jun 03 12:01

Hello,

I am in the process of trying to decide if our company should switch from: a 60 ton water cooled condenser to: a 60 ton air cooled condenser.

My problem is that I have been unable to get a firm answer on whether a 60 ton air cooled condenser will give the same cooling power as our current 60 ton water cooled????

This if for an apartment complex in the state of Kentucky, USA. Temps typically peak in the mid 90's, occasionally 100+ degrees F. Average yearly humidity is 58% @ 1:00 p.m. to 81% @ 7:00 a.m.

We have been able to maintain a 42 degree chiller water temp pretty consistently. If we could get into the 38 or 40 degree range though, that would be a nice bonus, but that is not critical.

The existing water cooled tower is corroding and must be replaced soon.

It seems that in the long run, an air cooled condenser would be cost beneficial as there would be no water consumption and treatment costs. That seems to offset the increased electrical usage of the air cooled type.

We are using a chiller with 2 reciprocating compressors located indoors and the condenser nearby outdoors.

Any advise on the cooling power of the same size water cooled vs. air cooled condensers would be greatly appreciated.

Thanks. SeeTwo

tonnage

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

MintJulep

(Mechanical)

30 Jun 03 15:01

The answer is "it depends".

Condenser ratings are provide at specific conditions, and these conditions should be clearly stated by the manufacturer. The conditions include things like mass flow rates for the primary and secondary fluids, the entering and leaving temperatures for the primary and secondary fluids, pressure drop through the primary and secondary sides, etc.

Since you are considering going from water cooled to air cooled you cannot really make a direct comparison. You need to figure out what your actual capacity needs to be, and select a new condenser that will be able to provide that capacity at your location under whatever "design" conditions you elect to use.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

Cooky

(Mechanical)

30 Jun 03 19:37

SeeToo,

The flow and return condenser water temperatures from the cooling tower will always be lower than the ambient dry bulb temperature (I would estimate by 10 deg. F).

If you replaced the cooling tower with a dry cooler of equivalent duty, the flow and return condenser water temperatures would be approximately 10 deg. F higher than the ambient dry bulb temperature.

Consequently, the chiller will reduce in capacity by at least 15% (probably more), it will also be more prone to trip-out on high pressure during the hottest days.

Assuming your company does not wish to replace the existing chiller, I can suggest three options for your problem.

1. Replace the existing cooling tower with a new cooling tower - there will be no adverse affects on the chiller.

2. Replace the existing cooling tower with a dry cooler - the chiller will be derated and may trip-out more frequently on the hottest days.

3. Replace the existing cooling tower with an adiabatic cooler, this is a dry cooler with water spray fitted - It will give performance very similar to the cooling tower, but without the water treatment costs.

I hope this helps.

Cooky

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

rbrunt

(Specifier/Regulator)

1 Jul 03 10:41

I don't think you will be happy with the performance of an air cooled condenser unless you have capacity to spare. Water temps will be a good bit higher. But frankly considering you mention chill water temps below 40 and a reciprocating chiller I really think you need to consider a new chiller and some AHU coil replacement. There is no need to run that low of chill water temps. A new chiller would pay for itself in a very short time. It also sounds like you need a system that requires little maintenance. Tower Tech in OK. makes a virtually maintenance free tower that is all FRP so it won't rust. Nalco chemical sells a computer controlled water treatment system that can be remotely check via a modem. Often one can hook up with an energy management company who will finance the whole thing with money saved in power consumption and there will be no out of pocket costs.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

2

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lilliput1

(Mechanical)

1 Jul 03 17:55

Air cooled units are noisy and need more space for condensers. They also require more power. Air cooled receip & screw = 1.20 to 1.40 Kw/ton (incl compressor, cond. fan & CHW pump). Water cooled would be about 1.05 to 1.1. The air cooled unit will require less maintenance. If you can provide the additional space for condensers locate them so noise is not a problem, then go air cooled. Note that in the 200 ton & larger size the water cooled system using centrifugal chillers can go down to about 0.8 kw/ton (includes compressor, cooling tower fan, condenser pump & chilled water pump.
CHW pump = 0.05 kw/Ton
Cond Pump = 0.10 kw/Ton
Cooling Tower Fan = 0.05 kw/Ton
Condenser Fan = 0.10 to 0.15 kw/Ton
Recip air Cooled Chiller Compressor = 1.05 Kw/Ton
Air cooled screw chiller 1.2 Kw/Ton
Rotary water cooled chiller compressor = .8 to .9 kw/ton
water cooled centril compressor (200 Tons 7 up) = 0.6 kw/ton

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

with

(Mechanical)

5 Jul 03 01:38

You will get a temperature of ambient plus 8-10 deg C in an air cooled condenser, and with time and conditions this may further go up by 2 deg C. This may reduce the overall tonnage of the chilling plant.
krk

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

r22

(Electrical)

6 Jul 03 16:36

First of all is the existing an evaporative condenser with refrigerant flowing through it or a water loop that cools water and pumps it inside to be used to condense refrigerant. If it is the latter I would not do it. I would replace the tower with a new one. I can tell you that with small condensing units a tube in tube water condenser can reject much more heat than a refigerant air condenser when changed out like for like(tonnage rating). If the air condenser is undersized you're beat, you can't control variables like water temperature and flow rates to squeeze more capacity out.

With an air condenser you have dirty coils to deal with and refrigerant leaks. Make sure the piping is done corectly to account for compressor capacity control or you could suddenly start losing compressors when you never did before.

Best would be a cooling tower to keep the refrigerant charge localized near the compressors. Don't trade short term savings for long term problems.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

seetwo

(Specifier/Regulator)

(OP)

10 Jul 03 15:10

To: All,

Many thanks for all the advise from all the repliers!

I will monitor the water temps we are now experiencing and update this thread after I have enough data.

It sounds like going to the same size air cooled condenser on a water cooled system that is currently adequate, but barely so (not adequate on 100+ days*) would not be advisable because of the loss of cooling capacity.

* However, this spring we did some maintenance work on the coils in the apartments that were not cooling adequately in years past. We replaced isolating valves and flushed coils. So far, there has been much improvement.

So, chiller/tower capacity size is possibly okay. We need more time to test the system in the hottest weather.

To: Cooky,

You are correct that we do not want to replace the reciprocating chiller at this time. We recently re-built both compressors. However, at some point in the future we would consider upgrading to a modern scroll type chiller in our indoor boiler room and continue to have a outdoor condenser (either water or air cooled). I've heard they are less noisy and more efficient with fewer moving parts.

Re: your suggestion #3, wouldn't the evaporating water sprayed into an adiabatic cooler leave mineral deposits behind (and degrade performance in time)? That is why our current water cooled tower is set to bleed off water onto the ground isn't it (to drain off the minerals left by evaporating water)?


To: r22,

The condenser set up we use is the 2nd one you mentioned: A water cooled condenser tower with a water loop that is pumped inside to the chiller.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

vanspall

(Mechanical)

31 Jul 03 12:41

Suggest that you replace cooling tower with an Adiabatic Dry air cooler. there are such coolers on the world market which can acheive a recooled liquid temp of 7 deg C above wet bulb temp. Spray water is once through and is only operating when ambient temps are high rest of year operates as standard dry cooler. Also sprays are not directed at coil but into incoming air stream thus saturating air.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

PSucher

(Mechanical)

3 Aug 03 12:13

Depending on where the "Design" loading of your compressors are, you may be able to switch. You probably know that the evap coolers lower the condensing temperature of the refrigerant. If you use straight air-cooled HX (no evap), then you lessen the "tonnage" (you have reduced the delta-enthalphy). To get the same "tonnage" of refrigeration (which is the basic goal), then you need to move more refrigerant (i.e. mas flow).

Another effect you'll see by going with Air Cooling is that you need to compress to a higher pressure. SO you may have a motor problem on your compressor.

The best thing to do is to go to your compressor manufacturer or local system salesmen and ask him/her to look into this issue...

For the most part, you will not likely be able to do what you are asking to do without major changes (i.e. costs).

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