Maximizing Evaporation Loss in a Cooling Tower

I see a lot of scale building up......thats going to be a problem. you didn't provide any info on the quality of water you propose on using, were you planning on flushing large amounts of water down the drain or using chemicals to hold them in suspension?

The water will be pumped from any available seawater, or brackish water supply. It will be run through once and then returned to its source. How do you recommend best dealing with the scaling?

1) Hinder

2) aCooling Tower Design | Cooling Tower Construction

2)b Cooling Tower Contractors

3) "working outside the normal tower parameters"

4) Designer or location or incoming water qualities

also, for your best transfer of heat via evaporation you need airflow! you cant skimp here and rely on slow flow, it wont work. most towers have stages, the first stage is a blower, then comes water...afte that you can utilize VFDs to throttle flow of both. I'd suggest you make an initial investment and use copper tubing in the tower, galvanized steel is cheaper but with every nic the gal coating comes off and a cancerous rust sets up shop and in a while you have a pile of rust

Yes, the low airflow rate is my concern with such a large diameter cooling tower (well, at least large by our standards). Unless you are aware of any cooling towers with a diameter smaller than 38 inches, this is what I must contend with. A thought we have is to place some sort of insulation on the inside wall, with the intent of creating a smaller diameter, and thus increasing the airflow. It's not my favorite solution, but at the moment looks like the best one.

Where are you finding 40°C wet-bulb air? The worst design number I have seen thus far is 30°C (in Dahran).

Quite. There has to be a mistake there.

No mistake - The 40 C air is coming from the condenser.

The 40 C air is coming from the condenser.

You want the air leaving to be saturated at as near the water incoming temperature as possible. Problems I can see with 'standard' cooling tower: a) 8.5 gpm is a small fraction of the normal design water flow for a 20-50 ton cooling tower. This will result in poor water distribution and probable air bypassing, b) your application calls for a counter-flow arrangement, and many cooling towers use a cross-flow arrangement , c) seawater will be corrosive, so galvanized steel will be a poor material choice. And at 88 degC, many non-metallics will lose much of their strength, so that must be considered, and d) as Fredski has pointed out, this will tend to be a scaling service, so structured fill will tend to plug, and is difficult to de-scale.

I think you would be better served with a packed bed (or beds) of random packing with properly designed distributor(s). This should be achievable with a low air pressure loss. Then the packing could be dumped to clear/replace it. Something similar to this:

That is exactly how the evaporator was built for our 20 gallon a day prototype. To upscale to a larger production rate, we were hoping to purchase off-the-shelf cooling towers. All of the plumbing and casings will be non-metallic.

How did it perform? My understanding of packed columns is that they scale-up quite readily. Googling 'packed column scale-up' found several papers.

Googling 'non-metallic packed scrubber' found several potential vendors you could contact.

You might be able to adapt a cooling tower, but as I noted in my first post, there are several areas of off-the-shelf cooling towers that are sub-optimal for your design.

FRP tank/vessel and packing manufacturers might be able to assist with the vessel, support plate, distributor and packing.

We are in the process of scaling up, thus the need for an off-the-shelf cooling tower. I will let you know how it works out.

From humidity tables for the conditions specified, even assuming the air exits at a wet bulb temperature of 85 degC it appears that you get only about 26 USG condensate/Hr.

Is this what you are looking at?

Yes, that is correct.

Something doesn't add up here. The only way that an evaporative cooling tower works is when the wet-bulb temperature of the air coming into it is lower than the temperature of the water coming into it. Evaporation is inevitable, and the more of it there is, the more effective the cooling will be. The amount of it is a function of the heat load on the tower.

There's nothing in the original post about wet-bulb temperatures, which renders calculation well-nigh impossible.

Published psychrometric charts are readily used to predict the performance of cooling towers.

There's nothing in the original post about the quality of the water to be presented for evaporation other than <...87.5 C seawater...>, not even the total dissolved solids level of the water, which varies from place to place on the globe. The blowdown point of the tower is not mentioned either, nor the quality of the make-up water. So, expect plenty of deposits to build-up on the wetted surfaces inside the tower, which will require frequent cleaning. One hopes that the pipework, pumps and heat exchange surfaces in the utility system can withstand attack from the concentrated salt solution it will all be expected to carry.

Cooling towers are renowned for being breeding grounds for all sorts of unwanted biology. <...87.5 C...> is, fortunately, high enough to kill off most of this stuff. However, there is no mention of the treatment regime to be provided to the water to make sure of the absence of biology at partial load.

Is there something wrong with simply rejecting the <...87.5 C seawater...> into some settlement lagoon, and recovering the solid salt it produces for sale as a by-product?

Energy conservation principles would seem to suggest that such a high delta-temperature across the tower would not be the most economical way to operate the facility. Using the <...87.5 C seawater...> to partially pre-heat another medium for use elsewhere in the facility before offering the water for heat rejection cooling by evaporation would seem to be a preferable way to go.

The simple recommendation here at this moment is to stop working on such an abstruse concept of cooling tower and start consulting a qualified Process Engineer to consider better ways of using the heat and the salt in the first instance, on the basis of the value of them to the facility. Once all these avenues have been explored, then is the time to start talking to cooling tower specialist companies to provide a viable cooling tower service using less demanding fluids than <...87.5 C seawater...> should the need remain.

_<subscribes>

Correction: 40degC wet-bulb doesn't make sense.

Evaporation and boiling are the same thing. Why not collect the vapour that has been evaporated from the seawater, and condense it as product water instead of blowing it off in the cooling tower?

I think that's what he's doing, although his thread title is misleading. OP didn't explain the abbreviation, HDH, which apparently stands for Humidification-Dehumidification. You can learn more about it here: http://web.mit.edu/lienhard/www/IDA_Perth_Presentation_2011.pdf, or here: http://www.sciencedirect.com/science/article/pii/S0196890499000187, or Google 'HDH desalination'. The air is recirculated from the condenser, that is the reason the wet-bulb temp of the air entering the evaporator (cooling tower) is so high.

[Applies handbrake.] Awaiting clarification from the original poster.

Yes, thank you for filling in my omissions.

The cooling tower is being used as an evaporator to vaporize the seawater, which is then sent off to a condenser.

In the early seventies we did a lot of work on solar stills. A totally passive structure with a sloping transparent cover over a black cement pan for holding a 1.5" depth of water with a passive level maintaining syphon and provision for draining the concentrated brine before crystallization produced about 1.5 to 2 USG fresh water on a sunny day for 10 sq.ft. area.

No power or blowers used.

This was at Bangalore, India (13 degree North latitude). The cost at that time was about us$14/10 sq.ft. of collection area including materials and labour. This is valid even now.

Yes, I am familiar with such stills, and the evolution of the technology. We have achieved many orders of magnitude higher fresh water production, while recovering the latent heat of condensation, which was not utilized in these early solar stills. Thank you for the post.

since you plan on heating the water to relatively near boiling ~80C, why not consider this to be a low pressure "boiler" or rather steam generator, and condense the steam under vacuum on the exhaust side of a single stage steam turbine (of the appropriate size and blading for the pressure/mass flow rate? The incoming water might just be cool enough to serve as a primary coolant for the condenser, and if you wanted to you could consider using the cooling tower to re-use this water to an extent. Just a thought.

Excellent thoughts. We want to keep the design simple, inexpensive, and not energy intensive (for off grid operation), so even though I would love to use a vacuum, it is not practical for our situation. Yes, the incoming seawater is used to cool the condenser water, which incidentally preheats it before the evaporation process, giving us a "2 for 1" heat energy utilization.

If you use the correct condenser design, and 25-30 ºC seawater as coolant (not sure if best idea, but works), any steam turbine exhaust into this condenser would be under vacuum naturally as a consequence of the properties of steam/water vapor in equilibrium with liquid.

Your best bet if you want to maximize de-salted water for consumption, etc.:

(1) scrap the cooling tower unless needed to cool the condenser on a separate stream

(2) It is not necessary to heat the water all the way to 85 C to get usefull quantity of vapor. Use a vapor compression still instead of a 20-30 ton chiller.

(3) Alternate: solar heat the seawater to the temperature you desire, but first use membrane technology to (1) microfilter the water, and then (2) nanofilter the water (removes hardness ions, whilst maintaining at or slightly lower sodium chloride content.) Now the water is conditioned for high intensity evaporation purposes. Use the cooling tower to provide the cold temperature where condensation will take place, use seawater to supply the tower, or scrap that and use once through cooling, from the feed stream. Increase output by partial air cooling of the first vapor? Application of even a slight vacuum will increase throughput of the system immensely.

Some clarification: Solar energy is collected via thermal collectors and the heat is transferred to seawater (87.5 C) via a heat exchanger. A portion of the seawater is turned to vapor by utilizing this cooling tower. This vapor is what I want to maximize! The vapor is then sent to another cooling tower to be condensed into freshwater distillate. The seawater exiting the first cooling tower is sent back to the ocean. Heat is recovered throughout the system. This system is to be used as a small scale, off-grid freshwater production plant. The seawater chemistry will vary based upon where the system is deployed, so average seawater numbers are all I can assume at this point.

Why re-invent the wheel? There are salt spray chillers that will (1) produce vapor and (2) condense vapor from a high temperature salt water solution. I truly hope you are able to invent a "rounder" wheel, good luck.

You might take a look at other process plants that recover such salts as sodium sulfate from high salt playa lakes - these use a regenerative evaporation process in conjunction with a crystalizer. Net products are dry salt and relatively low TDS water.

Thank you James - I am huge proponent of not reinventing wheels, and for keeping them as round as possible. I am not familiar with these salt spray chillers and will research them further. I wasn't planning on going into the salt selling business, but it may make economic sense as well. Thanks again!

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Good luck with the scheme.