Cooling Tower Temperature Control

[krb]

Does anybody have any suggestions on how to control cooling tower leaving water temperature to +/- 1 deg F? We have a two cell tower, each fan has a VFD, and the tower serves about 50 smaller closed loop water systems cooling process loads. Our operation currently is up and down, so process loads do vary somewhat throughout the day. However we want to control leaving water temp to 81 deg F. The water systems serve research equipment, so even though they are concerned about energy efficiency, they are more concerned about stable temperatures. As far as I know, all of the closed loop systems have control valves.

Any ideas? I'll be glad to post additional information as needed.

Thanks in advance

KRB

 

[walkes]

Do you have a 3-way valve to bypass the tower and to mix entering water and leaving water to maintain the 81°F?

 

[katmar]

KRB,

Are the VFD drives controlled by the temperature at the moment? I am not sure whether you are saying that you do not get good enough control with the VFD's, or whether you are asking if the VFD's should be linked to the temperature.

How much spare capacity is there between the tower's rated capacity and the average and peak loads actually experienced?

katmar

 

[krb]

Walkes,

We have a bypass valve, although it is not a 3 way valve. I have been told this valve is unreliable (I have only been here a few weeks) so this sounds like an issue to be resolved.

Katmar
Yes, the VFD's are controlled by the leaving water temperature, but our water demand significantly exceeds out thermal demand and therefore we using about 25% of the towers thermal capabilty. Our problem is the current control configuration runs both fans at the same speed, and the VFD's turn off below 20hz, the lowest speed recommended to run. We see alot of cycling at this point, fans off, fans on, and swings in the temperature along with this.

Thanks for the questions, they always spur additional ideas.

KB

 

[katmar]

If the VFD's are cutting in and out you will struggle to get good control. I think you need to concentrate on the bypass valve and get it working well. As you implied before, this may not be the most energy efficient method, but it should give you tight temperature control.

 

[SAK9]

I am yet to come across an installation where substantial energy saving has been achieved with VFDs on tower fans.
Multiple cell CT is the way to go for this kind of variable loads.But that is beside the point in our discussion.

In my opinion,you need to put your bypass valve under pressure control(sensed by a DP transmitter across the headers) so that tower flow rates may remain constant( I am assuming you have constant speed pumps)irrespective of the load flow rates.How is the flow controlled at the loads? two way valves?

As for the tower outlet temperature control,you need to examine the mechanical side of the things ie how the fan air flow varies with speed variation.This will give you an indication of the nature of the tower capacity variation and may explain why the fans cycle on/off.I believe the relationship may not be linear which would mean with your existing system a +/-1 F temperature control may not be practical.

As suggested by Walkes,for stable control of temp,you need a 3 way valve which would modulate to give a steady 81F.

 

[LIKALA7]

Firstly you can use the cooling tower VFDs to control the temperature leaving the cooling tower to the prevailing wetbulb plus 3.5 deg C. (so you will have to measure wet bulb)This will make the system more efficient. Next, you will need to modulate the control valves in the process water loops to maintain a constant leaving temperature. If these valves are 2-way then you would require a VFD for the condenser water pump. This strategy will make the system as energy efficient as possible given the constant leaving process water requirement.

However if the cooling tower is of the spray type (i.e uses nozzles) then you may need to keep the flow constant to avoid loss of cooling tower capacity. In this case you cannot use a VFD for the condenser water pump but you can use a central bypass which will keep the flow around the cooling tower constant.