cooling tower design

For all cooling tower systems (open and closed-circuit), cooling water is pumped through the water-cooled equipment where it absorbs the heat rejected by the equipment. This water is then circulated to the cooling tower where it is cooled (directly or indirectly) mainly by the evaporation of a portion of the water that is in direct contact with the ambient air. For closed-circuit cooling towers, the water contained within the closed-loop heat exchanger coil is cooled by the evaporative effect of the open-loop cooling tower water. The greater the rate of evaporation, the closer the leaving water temperature will approach44 the wet bulb temperature of the ambient air. The rate at which the water evaporates in the cooling tower is proportional to the surface area of the water in the cooling tower and the airspeed, and is inversely proportional to the wet bulb temperature of the ambient air. Thus, an increase in the surface area of the water and/or airspeed in the cooling tower and/or a decrease in the wet bulb temperature of the ambient air results in a decreased approach temperature. A typical approach temperature for HVAC applications is between 5 and 10°F.

Range is defined as the difference between the water temperatures entering and leaving the cooling tower and is determined by the heat load on the cooling tower and the water flow rate.

Typical design temperatures for a cooling tower system are as follows:

• Cooling tower entering water temperature: 95°F
• Cooling tower leaving water temperature: 85°F
• Ambient wet bulb (wb) temperature: 78°F

From these design temperatures, it can be seen that the approach is 7°F and the range is 10°F.

Other design considerations are as follows:

  1. The entering and leaving water temperatures and the water flow rate through a cooling tower must be consistent with the water-cooled equipment that it serves.
  2. A typical flow rate for a cooling tower serving water-cooled refrigeration equipment is 3 gpm per ton of refrigeration capacity.
  3. The water flow rate through a cooling tower should be full flow to the distribution nozzles or full bypass to the cooling tower sump.
  4. Cooling tower capacity is modulated by varying the airflow through the cooling tower in order to maintain a constant leaving water (condenser water supply) temperature. This is accomplished by cycling the cooling tower fan(s) on and off, by varying the speed of the cooling tower fan(s) or by diverting condenser water from the cooling tower distribution nozzles to the cooling
    tower sump.
  5. During start-up of a water-cooled chiller, condenser water should be diverted from the distribution nozzles in the cooling tower to the cooling tower sump until the condenser water supply temperature rises to the desired temperature, which must be above the minimum condenser water supply temperature required by the chiller.
  6. The condenser water supply temperature is typically maintained at 85°F. However, because refrigeration equipment typically operates more efficiently at a lower condenser water supply temperature, one way to improve energy performance is to allow the condenser water temperature to float down to the minimum condenser water supply temperature (approximately 70°F) whenever possible.
  7. If winter operation of the cooling tower is required, the cooling tower and all exterior piping will need to be winterized. Winterization includes the installation of a heater in the cooling tower sump, which can be electric, steam, or hot water. The heater should be sized to keep the water in the sump at 40°F at the design winter outdoor temperature. Also, the exterior piping, including the condenser water piping, drain piping, and overflow piping, should be heat-taped and insulated to prevent it from freezing. Typically, heat tape is self-regulating and does not require any external controls, only an electrical power connection

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.