Cooling Tower Pipe Sizing

[pavlik]

I know that the rule of thumb when sizing water pipes in a cooling tower is 3 GPM per 1 TON for condensed water (i.e. return) and 2.4 GPM per 1 TON for chilled water (i.e. supply).
Who came up with these figures, and don't these numbers depend on Temperature differences anyway?

 

[briand2]

Because of the ratio of latent heat of evaporation to specific heat capacity of water, evaporating about 1% of the cooling tower circulation rate will cool the remaining circulating water by about 5.5 degC (say 10 degF). Everything else to do with cooling tower sizing just follows from that!

Regards,

Brian

 

[ClydeMule]

These numbers come from algebra and the semi arbitrary choice of having a 10 F DT across the evaporator, condenser, and fan coil.

If you plug 10 F in the equation 500 * GPM * DT and then divide by 12,000 you get 2.4 GPM / TON.

The heat of rejection out the condenser is greater then what is absorbed by the evaporator. For a water cooled chiller the total heat of rejection is 1.25 times the evaporator capacity. So when you say 2.4 or 3.0 GPM per ton you are reffering always to EVAPORATOR tons. 2.4 GPM * 1.25 = 3.0 GPM.


Most standard evaporators and condensers are sized using 2.4 GPM/TON (3.0 for condensers), 10 F DT, 9-10 F approach etc.

A given heat exahcnger has an infinite (within limits of course) number of capacities, therefore to give a given part a name it must be sizedd according to standard conditions.

Whether or not they apply to what you are doing is up to you to decide.

Clyde

 

[imok2]

Water cooled condenser ratings are usually published in tons of heat rejection at specified entering water temperature, water flow rate per ton, and condensing temperature. ARI Standard 450 specifies 85°F water entering, 95°F water leaving, and 105°F condensing temperature as the basis for ratings, but does not define a ton of heat rejection. So if we use 15,000 Btu/hr which is the commonly used ton of HR in water cooled condenser. On this basis, the standard water flow rate per ton can be calculated as follows:
CHR (Btu/hr) = gpm x 500 x °F water temperature rise, or delta “T”
For 15,000 Btu/hr CHR per ton, and 10° water temperature rise, 3 gpm per ton is the required flow rate. Example: 15000btu/hr/500x10degrees = 15,000/ 5,000 = 3gpm
Now with closed circuit coolers(evaporator) operation of the system is calculated a little different because it’s based on a heat rejection of 12,000btu/hr, so using the same formula it becomes 12,000btu/hr/500x10 degrees = 2.4gpm. The extra 3,000 btu/hr in the condenser is a gain mostly from the heat of compression of the refrigerant in the compressor. Now you may ask where does the 500 number come from ,well 500 represents the number of lbs of water circulated in 1 hour or 8.333lbs x 60 minutes = 500 lbs. Hope this is helpful