Running Cooling Towers "too dry"

[bigAlittlee]

So I am doing some retro-commissioning work on an older building that currently wets all 4 cooling tower cells at all times. Control valves do not exist. Single speed fans are sequenced by thermostats in each basin, each of which control their respective fan to operate at varying basin temperatures. For example at basin temp = 70F fan 1 runs, at 72F fans 1 and 2 run, at 74F fans 1, 2 and 3 run, and at 76F all 4 fans run.

Obviously there are improvements to be made to the current sequence of operation, and VFDs for the fan motors will be installed before cooling season, which will help a lot.

Engineers have made comments about running cooling towers "too dry" and by too dry in this case, that means 25% of design water flow. The majority of the time a single pump of 4 (matched to the 4 towers) is ran, over 4 towers, so they run at 25% flow each. One concern with running too dry, that I understand and agree with, is that when fill is dry, it is more prone to the accumulation of scaling, but in this case the water quality is very well maintained so I don't see this as a problem. I don't see any other issues.

Basically I see two options:

1. Continue to circulate over all 4 cells in all circumstances, but change fan control to stage fans simultaneously and adjust speed to maintain a 7F approach. With 4 cells there is more heat transfer area, and I would expect this equates to a lower ratio of fan power to heat rejected.
2. Define a minimum tower flow, add control valves for each tower, and sequence towers in a way to ensure that this minimum flow is maintained.

I think option 1 is the favored option. Does anyone disagree? Thanks.

 

[Drazen]

i believe it would be worthwhile to investigate efficiency-water flow dependency for towers.

if you could possibly find that one chiller on max water flow is much more efficient than 4 chiller on 25%, you should go for it.

picv valves set on highest possible min. water flow together with variable speed pump and towers sequencing could get to that.

having towers occasionally dry should not be an issue if we assume that good water management also means good overall maintenance management that includes regular checks of water basins surfaces.

 

[DubMac]

Add the control valves, forget about the VFD's.

 

[macmet]

I'm curious DubMac... why you would get rid of the VFDs.

 

[DubMac]

Just rattlin' the cages; thats what I do...

Actually mean before doing anything, get control valves on the system. Like a car without a steering wheel w/o them.

 

[Waramanga]

Do you mean that you have 4 cooling towers? I am not sure how much cooling you get out of a tower without the fan going but I would think it is significant. A good cooling tower rep will be able to tell you what the capacity is with the fan off, then you just need to look at your pump head to see if it is better to run the tower without the fan or run one of the four towers flat out.

 

[bigAlittlee]

Yes 4 towers each usually at 25% of their design flow, except extreme summer conditions.

I don't know if you do care about control valves that much once you add the VFD. I feel running the fans at low speed and using as much surface area as possible would be the optimal scenario. Also understand when operating within utility incentive programs, which I am, control valves aren't on the list of covered items, but VFDs are, so that is a driving factor. I am actually not in a position to add control valves - I can basically add new sensors and rewrite control sequences. The VFDs just happen to be getting added, separate from the budget I am working under, but the result is that they become my responsibility in performing energy calcs.

Waramanga you're right. Too many assumptions to make to feel good about it, and calcs must be reviewable, and therefore justified. Working on getting model/serial numbers to SPX and they said they would help.

 

[RobsVette]

Another concern would be if they are forced draft towers, and you reduce the flow too much you can get excessive water carryover. With induced draft towers this isnt so much of a problem. I assume they are cross flow, induced draft, since you said SPX, so they are probably SPX Marley Towers. In my experience operating as you stated has never been a problem. I have always said (as you did) that more surface area is better.