Heat rejection: Air Cooled Chiller vs Cooling Tower
Heat rejection: Air Cooled Chiller vs Cooling Tower
(OP)
We're considering two options for additional heat rejection. The problem is that on a design day our cooling towers run at their limits and we lose redundancy with them. The thought is to either add a 1000ton cooling tower to the existing ones (largest we can fit); or, add a 350 nominal ton air cooled chiller (largest we can use). Either option will take some load off the existing chiller plant.
We're limited to the 350T chiller but have flexibility on size of cooling tower. Prelim budget indicates cooling tower is less expensive. My first thought without doing any calcs is that the cooling tower is the way to go.
any thoughts? (is it this simple?)
Thanks in advance for any input!
We're limited to the 350T chiller but have flexibility on size of cooling tower. Prelim budget indicates cooling tower is less expensive. My first thought without doing any calcs is that the cooling tower is the way to go.
any thoughts? (is it this simple?)
Thanks in advance for any input!





RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
I'd guess these are pretty standard counterflow towers with 10 deg approach and range. Tower condition can be deceptive, clean looking towers can have huge air leaks around the case and be wasting airflow.
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
I have come across many places the water cooled and air cooled both are of same capacity working with same discharge pressure.
water related maintenance is always cumbersome compared to air .
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
1. How many tons/age is the existing plant? chiller and cooling tower.
2. Do both options cover the required load? or will you still be short with one of the options?
3. Any future plans for expansion?
If future expansion is expected, and you cant replace one of the existing chiller to a larger more efficient chiller, then upgrade with the 350 ton chiller.
If not and if the existing chillers cover the current cooling load, then upgrade the cooling tower, it will increase the efficiency of the plant as a whole.
If the chillers do not cover the current load, it sounds like you will have to do both the chiller and cooling tower.
Chillers suggestion - compare the VFD scroll vs centrifugal
CT suggestion - dont use centrifugal type if you can help it (even if pressured by the architect)
knowledge is power
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
You say 1000 tons is the largest you can fit, so if this is big enough to allow you to stand down another tower then this is clearly the best solution.
If a 1000 ton tower is not big enough, then a 350 ton chiller certainly isn't either. If this is a flow capacity issue, you must also be running your redundant chiller & pump which is a whole other problem.
Where is the decision here?
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
the plant is less thatn 10years old and is a total of 2000 tons (3+1 with 500T chillers/towers). The cooling towers are sized for 68°F WB (a little undersized). maintenance is good and the plant is running well, the CWS temp starts to rise when OAT rises above ~95°F DB (and typical WB) temps cutting into the tower redundnacy.
The plant can handle the load except for the hotter days due to a lack of heat rejection. since the towers are 500T we figured that an additional 350T air cooled chiller should reduce the load enough so that we don't lose tower redundancy. Also, we were considering a magnetic bearing chiller to get some experience with it (this is why we considered the chiller).
so, at this point it seems the 1000T tower is the appropiate choice. it gains 500T added heat rejection capacity (to match exiting operation on hottest days ie w/o redundancy)and adds 500T for redundancy. Less expensive as well.
I appreciate all of the comments, it hepled me focus in on the issue.
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
Indoor air quality will improve and your power will be reduced by over 50% compared to AC.
If not for all areas it can be deployed for some of the areas.
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
HVAC68
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
knowledge is power
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
Include all that is required for the initial cost - the cost of bigger cable or breaker size for the air-cooled option, cost of building a bigger tank for storing water (if requried) for the water-cooled option, cost of water treatment equipment for the water-cooled option, etc., etc.
This needs to be done in detail for you to arrive at the right answer.
Good luck.
HVAC68
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
I also found a spreadsheet at:
http://www.ga.wa.gov/eas/elcca/sheets.html
TECMSC-didnt mean to get off topic.
Since you stated:
"The plant can handle the load except for the hotter days due to a lack of heat rejection."
then I think the cooling tower is the correct option, and will only help in the overall efficiency of the plant.
Good luck
knowledge is power
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
cdxx139 - no problem with topic. this is good info and could figure into the decision in the long run (not to mention other projects!).
for a little more info: we ended up adding louvers in adjacent walls and velocity stacks just to maximize performance of existing cooling towers. The 68°F WB seems to be the main issue!
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
What's your design wet bulb ?
How much more does it go than the design ? and for how long ?
Is your cooling tower designed for the design wet bulb ?
In one of the earlier posts, you had indicated that the issue comes up only when the ambient temperature goes up. If this is for only a short period, consider adding a cooling tower module only to increase the heat rejection area. Maybe you don't even have to add an additional chiller.
HVAC68
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
I think the issue is really a load based problem (need more heat rejection area as you indicated) at the towers and not so much temperature issue since the towers work, but only up to a certain point and they can't keep up the required heat rejection.
So, we need to reject more load (cooling tower)and not add to the cooling capacity (chiller)
clear as mud now!
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
We can analyze better if we have the following parameters:
Cooling water temperature entering condenser
Cooling water temperature leaving condenser
Wet bulb temperature
Sump temperature of cooling tower.
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
Pipe it properly www.expertctr.com/ch_5.php
RE: Heat rejection: Air Cooled Chiller vs Cooling Tower
CTDon - Thanks for the link - good info. Equalizer lines have been an issue in the past as well e.g. two cell towers -- 10"-10" into 10" then into 10" common equalizer for three additional towers!