Heat rejection: Air Cooled Chiller vs Cooling Tower 1

[TECMSC]

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!

 

[RossABQ]

What are your design WB conditions? What kind of towers do you have, what are their design approach and range, and are they in good condition? This sounds like a large chiller plant, how many tons?

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.

 

[sridhar1312]

If the water quality is good water cooled is best option.But many times depending on the application like for Hospitals cooing towers are avoided to prevent legionella.

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 .

 

[cdxx139]

per Ross, first step shuld always be increasing the efficiency of the plant.

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

 

[Compositepro]

I don't understand why there is even a question. You already have cooling towers so you already know how to operate and maintain them. Now you want to know if you should spend less money to increase cooling capacity by 1000 tons or spend even more money to increase it by only 300 tons. What am I missing? Why do people use cooling towers in the first place? They are basically the cheapest, most efficient form of cooling possible for many cases.

 

[KiwiMace]

If your goal is to provide redundancy, then you should select the option that provides redundancy.

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?

 

[TECMSC]

thanks everyone, i'll try to summarize:
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.

 

[RossABQ]

With that design wet bulb, I hope you are in the desert. Talk to you tower rep to see if a fan and/or fill replacement will get you where you want to be with your existing towers.

 

[TECMSC]

The desert would be nice! We inherited the site so had to make do with what was installed. Good point-will look into this as an option as well.

 

[sridhar1312]

68 Degree F is god option to incoporate indirect direct evaporative cooling where you can get tow to four degree below et bulb and you can maintain room at 72 to 78 with 100% fresh air.
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.

 

[HVAC68]

Carry out a life-cycle cost analysis. The result will guide you towards the right answer.

HVAC68

 

[cdxx139]

Any suggestion to a LCCA procedure, primer or template?

knowledge is power

 

[HVAC68]

Don't have a template that I can lay my hands on right now. Keep it simple. Calculate running costs (electricity, water and in addition, water treatment costs for the water-cooled option) and initial cost.

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

 

[cdxx139]

HVAC68- thanks for the quick lesson.

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

 

[TECMSC]

Thanks HVAC68 for the spreadsheet
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!

 

[HVAC68]

Some more questions !

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

 

[TECMSC]

HVAC68 - the design WB is 68°F. At 95°F DB and above the CWS temp starts to increase. I'm not sure of the WB temp but "typical" WB temps at 95°+ DB for our area.

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!

 

[sridhar1312]

Some thing is missing? At 95°F above i.e hotter days you have issue of performance.Normally the wet bulb depression(DB-WB)will be highest in summer and cooling tower ought to perform better during this season. From the various data it appears the cooling tower capacity is derated due to aging, scaling of fills piping and even the condenser. Most of the time the condenser is descaled but the pipes diameter and cooling tower fills gets deposited with scale impeding the performance of cooling tower consequently the chiller performance.
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.

 

[CTDon]

Add a tower.

Pipe it properly

http://www.expertctr.com/ch_5.php

 

[TECMSC]

sridhar1312 - . WB depression seems to make sense, but unfortunetly, i don't have the exact temps during the summer months.

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!