Cooling Tower vs Geothermal Chiller condesing loop

[tlona]

My client has a 255 Ton chiller with a 795 gpm condensing loop to a cooling tower. The cooling tower is rated for 95F EWT & 85F LWT. The Chiller Condensing loop is rated for 85F EWT & 95LWT. The client has a well that he would like to use for the condensing cooling as opposed to the cooling towers. We are trying to size the well pump. Would it be the same as the condensing pumps at 795gpm or can it be small due to the fact that the Geothermal well will provide 50F.

 

[KiwiMace]

Are you going to dump the water somewhere on a one-pass system? If you planned on doing this with a direct feed into your chiller, then no, you don't get savings on the flow. This is highly wasteful and you will probably have water quality and compressor surge issues.

You will need a tertiary loop, basically replacing your cooling towers with another closed heat exchanger - maintaining the condenser water at the rated or possibly lower temps, but same flow. Still wasteful, but water quality issues can be dealt with at the heat exchanger. You'd still need a dirty resource consent officer with a big mortgage and a taste for exotic cars/handbags.

The well water flowrate depends on what kind of heat exchanger you are using - you will likely be able to use a reduced flow with a shell and tube - probably the better choice for dirty well water anyway. Talk to a HEX vendor for this part.

 

[GMcD]

Way too many questions first:

1. What is the water capacity and recharge rate of the well?
2. How deep will the well pump be (could eat up any chiller electrical savings on well pump energy if it's deep)?
3. What is the water quality of the well water?
4. Are there environmental regulations that have to be applied for to use well water at the rate you intend?
5. As above- where will the used well water be dumped, and what are the environmental regulations and technical issues related to where and how the waste water is dumped?
6. What does the chiller manufacturer think about using lower condenser water temperature? How will the compressor controls and refrigerant charge be adjusted to suit?
7. If the well water is a nice constant 50F temperature why not use it directly as the chilled water in the building cooling system?

 

[317069]

your question is about flow rate in condensing loop.
the flow rate is related to the temperature difference across the condencer and it is about 3GPM/Ton. so either you use tower or anything else the flow that has to pass the condenser is the same as long as you want to keep delta T across the condenser.

 

[walkes]

Agree with GMcD, we have regulations that limits the temperature differential for a ground water system, so your flow may go up. The water quality would be a major concern for scale, corrosion etc. We are replacing systems after 5 years where people forgot to check that part.

 

[tlona]

Thanx all. This is well system is existing had has been used in the past as a back-up to the cooling towers. The well is currently in operation providing make-up water to the cooling towers. The existing well pump (400gpm) has died and they are replacing. They were wondering if they really need a 400gpm pump. My suggesstion would be to put the pump on a VFD so when it is only being used for make-up water the flow rate can be reduced. So I wasnt sure if I could reduce the flow rate since the well water is 50F

 

[cry22]

400 GPM for make-up water on a 255 ton load?
You need 10 GPM of make-up water for such a load.
There is something you're not telling us or you don't know about or you do not understand.
seems to me that the well pump is a back-up or a supplemental pump to the cooling tower condenser water system, NOT make-up water.
If the condenser water (400 GPM) is pumped back into the well, the well water temp. cannot stay at 50F constant.

You mention VFD? for a 10 GPM load for make-up? what would control your VFD?
Even for condenser water purposes - what would control your VFD?you have one cooling tower with most likely a tower by-pass (3-way valve) at inlet to keep condenser water to the chiller at 70F, meaning constant flow. FYI VFD's work with variable flow systems, BUT you must have means to get variable flow, i.e 2-way modulating valves. NOT 2-position control valves.

There is no use for your proposed VFD at all, even if it were a condenser water pump and not a make-up water pump.

But you seem to be mixed up with terminology, so we cannot really understand your problem.

Your discussion in the field "between you girls" is nothing more than a Barber's discussion, hire an Engineer to give you a professional opinion.

 

[tlona]

sorry. I was just trying to keep it simple and was just curious if you could reduce condenser flow rate by providing a colder source of cooling as I do not understand the internal thermodynamics of a screw chiller condenser. And maybe the question is too generic. The existing well has a 400gpm pump that pumps to a 5000 gallon holding tank (on off control). The holding tank is pressurized and provides the cooling tower make-up water. The well is also used as a back-up for the condenser cooling loop in the event of a cooling tower failure. The well pump has recently died so rather than just putting in another 400gpm the question was raised whether they really need a 400gpm pump (is it too small, is it too big or is it just right) keeping in mind the well will still act as back-up in the event of a cooling tower failure.

 

[tys90]

I would like to see how the current system even works. It would be far simpler to use a 795 GPM pump in the well as a direct backup with a couple of 2-position changeover valves. If the condenser water system goes down you just switch the feed to well water and the system runs (disregarding water quality issues). You must have a 3-way valve on the condenser loop to mix for this to work though, as 50F is usually too cold for condensing water. This way you would essentially have an identical back up system.

However, if you currently run this system as a backup then it must be some sort of primary-secondary setup? Or does your pressurized tank become your source of cooling? If so, that probably doesn't last very long before heating up the 5000 gallons of water (roughly 22 mins). If the well pump provides the condenser water I doubt the chiller would run on 400 GPM of flow. So it would have to feed into some sort of pri-sec loop. I can't imagine that's what is happening as that would be pretty messy.

 

[tlona]

There are two condensing loop. The primary which consists of two recirculating pumps (one for backup) and two cooling towers. The well loop is backup which is open loop. When the towers are in operation the well system always provides make-up water to the towers via the pressurized tank as mentioned before. When the well system is placed in operation, the condensing loops are shut off, the operators do a manual valve alignment so the well system is once through. The chillers are not absorption. I thought the existing 400gpm pump was undersized as well. This is what begged my initial question. I first assumed that is was small based on the low supply temp. thanx for all of your help