Open Chilled Water Tank with Hot/Cold Well

[Cooly]

Came across a 16'x8'x8' process chilled water tank open to atmosphere equipped with a hot and cold well. A 500 ton centrifugal chiller takes suction from the hot well and discharges to the cold well. Four 15HP pumps take suction from the cold well, distribute cooling water to the process and then return warmed water to the hot well.

Does anyone know why such a system would be used as opposed to a closed primary/secondary type of chilled water system?

Obvious disadvantages (to my thinking) of this system include 1) wasted process pump energy to overcome static head loss between the water level in the tank and the highest points in the system (40-50 feet high)and 2) the water is exposed to the atmoshere - dirt and corrosion.

Thanks for any input.

 

[ChrisConley]

Also, you've increased the amount of air that gets dissolved into the system that has to removed.

For a process system, I could think of a couple of bizzare reasons the tank is open, for an HVAC application... not many.

On the plus side, no expansion tank.

 

[quark]

That is a common set up for process cooling, particularly when you have instantaneous loads. I have operated many such type of systems in chemical and API plants.

You can think about the following advantages.

1. Supply temperature can be more or less constant (otherwise, for a constant dT across chiller, the outlet temperature will be high during the starting of the cooling process).

2. No mixing of hot and cold fluids (so, no averaging).

3. Chiller will be on constant load, as long as it runs, and expensive capacity controls are not required (optimum efficiency of the chiller).

4. You chillers need not be designed to the peek capacity, if you properly size the tank.

This is rather old design and the problem you mentioned can be offset with thermal storage systems that can take care of peak loads.

Dissolved air is not a problem in the practical sense, atleast I never faced any problems.

Dust and dirt can be a problem if these tanks are not covered from top.

 

[gepman]

Cooly
What kind of process is it?

 

[DrRTU]

Hot well / cold well designs work great as buffers. Carrier used to have sizing recommendation in the flowtronic chiller guide. The trick to the tank besides the size, is the holes in the baffle and height of the baffle plate. The simple designs, have the suction of close coupled pumps at the side of the tank. Elevating the tank increases NPSHr. This design works great with 2-way valves on the house pumps, a mini-flow line at the end of run and no VFDs. The tank can also allow for elevated house water temps.

http://www.aecinternet.com/index.cf...Reservoir Pumping Systems/productID/1350.html

 

[gepman]

The hot well/cold well tanks that you saw are also fairly common in the food industry. I have also seen this type of system in a pressure vessel (as opposed to an open vessel) with a baffle between them.

I am not sure if I agree with your statement that the pumping energy is wasted going to the highest 40-50' level since I believe that you would recover it on the return downhill path as long as it is not open channel flow. I have seen siphons work many times including in hydropower penstocks that were 6' in diameter.

A closed primary/secondary system that you advocate would have additional pumping losses due to the pressure drop of both the primary and secondary side in the heat exchanger. In addition the compressor would have to operate at a lower suction pressure in order to have a temperature difference across the primary/secondary heat exchanger. This would be a minimum of 2 deg. F lower and probably closer to 5 deg. F. depending on the type of heat exchanger. The lower suction pressure would result in lower energy efficiency.

 

[KenRad]

I had a customer whose process gave some very erratic loading with occasional high return temperatures, who insisted that they needed a large buffer tank. We did a system like you describe and it works fine.

Agreeing with ChrisConley, it does make for more difficult chemical control, since you cannot remove the O2. In a truly closed loop, you can remove it, and the chemistry becomes essentially maintenance free.

My guess is that the biggest fans of this kind of system are the water treatment companies.

---KenRad