cooling tower selection

[Nivrah]

Hi guys,
I am in the process of obtaining quotes for cooling towers. Do you guys know what info I must give the supplier to size and select according to my needs?
So far I have:
Design heat load
design wet bulb temperature
range
Approach
make-up water
desired cycles of operation.

are there anything else I have missed?
Should i specify parameters fora winter case or summer case given taht the wet bulb temperature will be higher in summer?

 

[MortenA]

You could ask some of the vendors to send you their input sheet. They may have that for your inspiration.

Best regards

Morten

 

[davefitz]

I think that the CTI cooling tower institute has a standard specification sheet that you could fill out.

 

[SNORGY]

Nivrah,

By "cycles of operation", I assume you meant to say "cycles of concentration".

There are a number of things you need to specify in addition to the above. In no particular order:

* maximum and minimum wet and dry bulb temperatures
* seismic zone and wind loading
* induced draft - single crossflow / double crossflow
* type of fill (film versus splash versus wood versus PVC)
* how many cells (as it relates to turndown / redundancy / etc.)
* limitations on blowdown (and whether it is continuous or intermittent)
* limitations on evaporation and drift
* flow balancing and distribution across the top of the tower
* provisions (if required) for degassing the return water
* provisions for winterization (if applicable) - usually by fan control
* any requirements for plume abatement

Marley / SPX would be a good resource.

 

[davefitz]

other items may be :
noise levels at ( 30 meters)
heat duty ( MWth on design day)
heat duty during steam turbine bypass operation
fan motor details ( max permitted motor size, etc)


The sizing is almost wholly dependent on the hottest design day; the unit is oversized for all other days. So the client must be careful of the details specified for such an occurrence.

 

[Nivrah]

Snorgy,
Sorry for the typo..i meant cycles of concentration. .
the cold water would be used for cooling our turbines and compressors. curently, they use untreated water at 15 degrees C.
Our mill uses a conservative wet bulb temp of 24 deg C for new cooling tower selections. this is for the absolute hottest day annually. it would also mean that on very hot days, the water out will be around 25's Deg C or higher. i wonder if the turbines and compressors can handle that!

 

[rmw]

You will have to find out from your turbine supplier what their back pressure limit is and then find out from yur condenser supplier what the HEI predicted performance will be at that inlet water temperture. When you can answer those two questions, you will have the answer to what you asked.

rmw

 

[SNORGY]

I am not a turbine specialist, but I would initially suspect that for a turbine, even at 25 C water, you probably have enough approach temperature that cooling shouldn't be an issue. A good question for the turbine supplier. Is it just the surface condenser that is the issue?

 

[rmw]

No, it is a combination of the two. For one thing, the turbine will have some discreet back pressure limit regardless of what kind of condenser it has under it, good bad or very bad. A major turbine manufacturer that I am very familiar with has a 5.5 in Hg back pressure limit. That is set by a lot of factors way too deep to go into here.

So a condenser designed to produce a back pressure much lower than 5.5" Hg, say 2.5" Hg with 25C water probably wouldn't have a problem. On the other hand, same turbine with a condenser designed to have 2.5" Hg with 15C water (thinking of a real case) is approaching getting in serious trouble with 25C water and probably at 5.5" Hg at 29C water.

rmw

 

[Nivrah]

rmw and snorgy,
he turbine spec sheet says that the max inlet temperature in the generator and oil coolers are 25 Deg c . The leak off condenser can take a maximum of 27 deg C. So I guess, the condenser back pressure limit has been set up in such a way that it works fine as long as the inlet water is less or equal to 27 deg C.

 

[stgrme]

Generally, the generator and oil coolers are supplied with cooling water from a closed cooling water system, which will have a higher temperature than the circulating water supplied to the condenser. The leak-off (gland steam) condenser is usually located in the condensate (feedwater) system downstream of the condensate pumps. Neither one of these systems set the limits for the backpressure on the turbine. Look further in the turbine spec sheets or ask the turbine manufacturer for the limits on backpressure.

Best of luck!

 

[SNORGY]

Nivrah, rmw,

When it was stated:

"the cold water would be used for cooling our turbines and compressors. curently, they use untreated water at 15 degrees C."

I had at first glance thought that this was a simple cooling issue, rather than a surface condenser issue.

rmw's observations re: surface condenser vs turbine cack pressure are correct.

Nobody needs the potential for anything other than completely dry steam in contact with turbine blades.

 

[rmw]

Turbines have low pressure limits too.

Actually the lower a condenser back pressure gets, the wetter the last stages of the turbine get and the further up the steam path the wet zone travels. Several of the last rows are "armoured" so to speak with wear shields to handle the moisture. The rows further up the steam path aren't.

In non-reheat turbines and in Nuclear turbines, the entire LP section can be in the moisture zone.


rmw

 

[Nivrah]

gents,
the objective here is to maintaing cooling water at lower than 25 deg C.There are alarm limits in the turbine controls in that inlet cooling water > 25 deg C will send operators in panic mode. So, it is a simple cooling issue as SNORGY pointed out.

Here in Australia, summer days can be very hot. So since the alarm limits have already been set up at 25 Deg C max, i will have to stick to providing cooling water equal to or less than 25 Deg C.

My superiors have agreed on the issues of hot days in given our geographic location and we have decided to use a S/T heat exchanger rather than a cooling tower. Two reasons follow: A S/T HX will have 20 deg C max cold inlet water on hot summer days --> easy to keep oulet water to less than 25 deg C.
Secondly, had we use a cooling tower, we would have run into corrosion issues. this is because our mill uses high dosages of Sodium hypo for water treatment in CT.this promotes a corrosion intensive environment.

 

All you need to provide is the design conditions required; GPM, EWT, LWT. If you don't know this, then the BTUs needed to be cooled is needed.