Cooling Tower Performance

1) If my ambient temperature is varying from 10 deg min to 50 deg Max. What philosophy we need wand to considered for CT operation for large (variation in ambient temperatures.

2) For ambient temperature coming below 23 to 25deg C, the CT operation is inefficient are not.
Already we did proceed the design details for cooling tower under below,
Flow rate of 1200 m^3/hr.
Water inlet temperature: 45 deg c,
Cold water outlet temperature: 35 deg c,
Wet Bulb Temp.32 deg c,

What type of cooling tower it is? IS it forced or induced or atmospheric type? Mass of water and air influence water cooling and affect eh CT coefficient which is derived based on simple heat transfer equations.

The temperature difference between the temperatures of water entering and leaving the tower is called the range of the C.T.

Another term is Wet bulb approach which is the difference between the temperature of water leaving the tower and the wet bulb temperature .

Load on the CT is the load of the condenser. Efficiency of the CT is the ratio of the actual cooling obtained to the theoretical cooling to be obtained.

100% efficiency would lower your water temperature to the ambient wetbulb temp. The approach is the difference betweem wb and actual leaving water temperature, usually 2-3'F. Cooling tower design is now very efficient, the main efficiency improvement to efficiency you can make is running fan motors on vfd's driven by actual/setpoint temps. If you have a heat exchanger such as a plate and frame type you can also run the tower pump on a vfd although this doesn't produce as much efficiency as the vfd on fan approach. 84-94F is ideal for most refrigerants, slightly higher on most lithium bromide absorbers. If you are using the tower for process water and the ambient wb is higher than your design cooling water temp you will probably need to incorporate a chiller.

You have asked a general question and a more specific question.

First- cooling towers operate by passing air over a media that breaks up water flow into a large number of relatively thin films with a large net area. The surface water on these films is evaporated by the air flow over them. The heat required to evaporate the water comes from the remaining water in the film and in the air flowing over the water, causing that both the air and the water to be cooled down. The air will generally be cooled to the wet-bulb temperature, where it will be 100% saturated and unable to accept any more water evaporation. The amount of evaporation that occurs is determined by the differential between vapor pressure of the water and the vapor pressure of the water vapor in the air- as the wet bulb temperature in the air falls, the air's vapor pressure falls and it can accept more evaporation water (to a point).

If the entering air is cooler than the water, a compound action occurs- the water heats the air by transmission contact (cooling the water a little, while the evaporation re-cools the air and further water cools the water.

The amount of air required to remove a specific amount of heat to lower the water temperature a defined value is determined by both the wet bulb of the air and by the design of the media materials. Based on that amount of air- if the wet bulb is lower, the tower can remove more heat- so, if the wet bulb is lower than the design point, less air is normally required to cool the water.

Going back the "to a point" comment earlier- as the ambient dry bulb temperature of the air falls, it takes less water per pound of air to reach saturation- which limits the amount of evaporation and corresponding heat exchange (water cooling) that is available. Because of this, even with the transmission heating from the water, cool/cold air usually cannot provide the same level of total heat rejection that occurs at the "warmer" design condition. IF FULL HEAT REJECTION IS REQUIRED AT MODERATELY LOW AMBIENT AIR TEMPERATURES- you need to have the tower designed and selected for that heat rejection at the lower expected temperatures.

The concept of tower "efficiency" basically is how much energy can be transferred / removed based on the energy required to operate the tower. Under virtually ANY normal condition, that value is very high- for example, a typical cross-flow tower can remove the waste energy from a 500-ton (1760 kW) cooling system- that is usually about 2160 kW- with a 40 HP fan motor (about 30 kW) for a rating of 72. Some other tower designs may require as much as 50 HO or a little as 35 HP, so the rating goes from 57.6 to 82.2- not bad under any circumstance compared to the cooling system operating at 0.8 kW per ton (3.52 kW) or a rating of 4.4.

Now- going to your second specific question-

What will happen, or need to happen over the wide range of ambient temperature that you have presented? Assuming that the Wet Bulb at the upper ambient temperature is at or below the tower's rated WB, the tower will operate at design in that situation. At very low ambient air temperatures, it is POSSIBLE that the tower may not be able to provide FULL rated heat rejection- only you tower manufacturer can provide that data- BUT, generally, you will need less air to provide the required heat rejection, so you should install a VFD drive controlled by the leaving water temperature of the tower to maintain stable operation.

IF the load is from a chiller, you might want to allow the water temperature to get cooler- to provide higher efficiency at the chiller by setting the design water temperature as low as the chiller is designed to support. As shown above, the small added fan energy for cooler water will be more than offset by the improved chiller efficiency- assuming that cool entering water can provide 0.5 kW per ton chiller performance (250 kW driving energy) vs. the normal 0.8 kW per ton (400 kW), the added 15 kW of "unnecessary" fan energy is totally irrelevant.

Check this ECODRY System http://www.frigel.com/pdf/EcoDrySystem_LR_EUR.pdf?bcsi_scan_189BCEB341B1E11B=WOqQNUt78XUkq/ryrLMAIjPh78AjAAAA+Q2IEQ==&bcsi_scan_filename=EcoDrySystem_LR_EUR.pdf