Cooling Tower efficiency

[PagoMitch]

The accepted Cooling Tower efficiency value is defined as a function of inlet, outlet, and wet bulb temps. I can find no benchmark type of parameter that include power input for the fan.

I recently replaced a pair of small (physically) 30HP centrifugal fan towers with a pair of larger (physically) 5 HP prop type. Same system capacity, slightly increased efficiency, but an 83% reduction in power requirements. While the towers were about $100k, the savings in energy resulted in a less than 1 year payback!

I'm now trying to write some performance specs to define this process. Other than making prospective bidders perform a simple payback calculation and specify a timeframe, is there an industry accepted rating of some type that would incorporate tower efficiency AND power input?

 

[cry22]

You made a payback of one year for $100K invetsment from 50 HP of energy reduction?
I'd revise those economics before I attemp re-writing anything if I were you.
Usually when you select a centrifugal tower (the right terminology being forced draft) and compare it to the induced-draft tower, you see a small difference in HP (max 25%), not the kind of diference you mention.
May be your towers were way oversized at first?

I find it hard to believe that 50 HP energy consumption can amount to $100K pay back in one. I see a 5-year pay-back at best (i.e assuming you are in the 12 cent plus KWH area AND you operate at full load 8760 hours of the year).

 

[PagoMitch]

Thanks for addressing the question.

FYI,
Location is a small island in the pacific. Diesel fired power plant, with all fuel shipped in by tanker. Fuel surcharge almost 25% of utility bill. About a 10F degree differential in ambient temps, day, night, year round. Although sometimes the wind blows 10 mph, and other times 150 mph; but I digress. Tower motors were amp recorded, confirming almost max use.

Original Towers:
60hp x 0.746 kw/hp x 8760 hrs/yr x $0.30/kw-hr / 0.90e = $130,699 (actual power bill averages about $10,000 monthly)

New Towers:
10hp x 0.746 kw/hp x 8760 hrs/yr x $0.30/kw-hr / 0.875e = $22,406

Both Towers CTI Certified, with the new towers' footprint over twice that of the old towers.

In actuality, $0.30/kh-hr for small diesel power is quite reasonable. I visited several places in Alaska last year where diesel fired power was being billed at $0.50/kw-hr.

Back to the original question. IMO, It is professionally irresponsible to not take motor HP into account when selecting a cooling tower. But left to their own, most Design Build/Performance based Contractors and/or unthinking Engineers will select the smallest tower, spinning at the highest speed, requiring the largest motor HP, in order to minimize first costs and (for the Contractor) maximize profits. That is exactly what happened here, and is exactly what I am attempting to resolve. Over the past 5 years, our facility could have saved almost half a million dollars... This may be small change for you, but not for me or my facility. Unfortunately, for some reason CTI does not address motor HP in any type of overall cooling tower efficiency standard, say like a COP for heat pumps. Hence my original post.

 

[cry22]

Never heard of such an electrical rate, you live and learn I guess. It is a diferent world out there.

At $0.3/kwh, even a 5 HP motor is a good candidate for VFD while you're at it.

I actualy oversize cooling towers fans and spec VFD, that way, they always operate at low speed, getting quiete cooling towers.

Multiple modular towers such as from Towertech (they use prop fans under the tower) can be worth looking into in your next replacement.

I wonder if ASHRAE 90.1 addresses scuh a thing. It is indeed a good question, chillers have IPLV ratings, why not cooling towers.

Good luck though.

 

[AbbyNormal]

we are diesel generated here, its about US 35 cents a kWh right now

water is 3 cents a gallon and I got an 82 wet bulb for six months

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[marcoh]

I generally specify the maximum cooling tower motor size. The cheapest cooling towers will be the physically smallest, thus the 'profit maximisation' option offered by contractor is promptly rejected as non-compliant.

 

[RossABQ]

I think you are looking at a complete misapplication, or else the design criteria changed dramatically. Perhaps a low profile was important earlier?

I recently compared small (1,000 gpm) galvanized towers of many different configurations, and found similar differences in fan HP. The energy hogs are typically designed for installation where aesthetics is more important than energy cost.