CHAPTER 15 | Typical Applications

AIR CONDITIONING:

The most common cooling tower application is for air conditioning with electric chillers. Typical conditions are 95 degreeF inlet water temperature, 85 degreeF water outlet temperature and a water flow rate of 3gpm /ton. With today’s easy to make computer selections, however, the astute designer should consider optimizing the chiller/ cooling tower combination. This is especially beneficial where the design wet bulb temperature is less than 78 degreesF. In Denver, for example, the design wet bulb is typically 68 degreesF and the cooling tower selected for 95 in and 85 out will be very small. It is clearly advantageous to reduce the condenser water temperature to 90/80, 88/78, 85/75, etc.. The tower will grow in size but the chiller- more importantly its compressor horsepower- will decline making a more economical selection. Operational savings can be substantial.

Absorption refrigeration was discussed in chapter 1. The special cooling tower selection considerations for absorption systems deserve repeating here. Occasionally, cooling towers intended for absorption systems are selected out of habit for the nominal 3gpm/ton. 95 degree in, 85 degree out conditions intended for mechanical compression systems. The result has the cooling tower only about 2/3 the required size and the designer wishing he/she had taken another career path. Don’t be one of them.

The application of closed circuit fluid coolers to water source heat pump systems was raised in Ch. 14. Such systems utilize numerous stand alone heat pump units that cool or heat their assigned space as required.

Each heat pump unit has its own refrigeration system. While cooling, room air travels over the heat pump’s evaporator coil and heat is passed from the air to the refrigerant. System water circulates through the condenser coil and accepts the heat plus that added by the compressor.

In the heating mode, the evaporator and condenser are switched. Room air takes on heat from the warm condenser coil and the evaporator takes heat from the recirculated water.

It is common for some heat pump units to be cooling while others are heating. This is, in fact, when such systems are most efficient.

The water loop connecting the heat pumps can get too cold when the heat pumps are predominantly heating and a boiler is placed into service to warm the loop. Similarly, a fluid cooler is in the loop to reject heat should most of the heat pumps be in the cooling mode.

Occasionally, southern califorians are attracted by the price of a conventional cooling tower- or don’t know the difference between closed and open cooling towers- and will apply an open cooling tower to a heat pump project. This is an invitation to disaster:

- The open tower’s poor water quality would cause unacceptable corrosion and clogging strangling the system and making it inoperable. (The water passageways of the heat pump cannot be mechanically cleaned.)
- Since water would pass through the tower at all times, it would be successfully rejecting heat even when the loop requires heating.
- Finally, there could be hydraulic problems with an open loop.

I understand open towers are commonly applied to 'Cooling Only' heat pumps in the south east.

AIR COMPRESSOR COOLING:
(UNDER CONSTRUCTION)

DYNOMOMETERS:
(UNDER CONSTRUCTION)

PLASTIC INJECTION MOLDING:

Plastic injection molding machines have different cooling requirements. The specific mold manufacturer should provide the optimum flow rates and water temperatures for the piece being manufactured.

If the necessary inlet cooling water temperature minus the ambient wet bulb temperature is less than ten degrees F, a chiller is typically required to provide reliable cooling. Such chillers are generally air cooled and do not need a cooling tower. Naturally, if the chiller was water cooled, the tower would be connected to the chiller and be selected to meet the heat rejection requirements of the chiller.

Cooling towers for direct mold cooling are generally of the 'Closed Circuit' type or of the 'Open' type in conjunction with a heat exchanger. This way, the mold is protected from airborne debris, scale and the like.

If the tower is 'almost' able to do the job without a chiller, ask the mold manufacturer if it would be ok to increase the flow rate and operate at a slightly higher inlet temperature. This could be an acceptable compromise that would restore the average mold temperature cycle time.

The best approach is to select a tower with a capacity rating that meets or exceeds the most severe anticipated duty.

IN GENERAL

When faced with specialized applications, contact the manufacturer of the equipment requiring cooling water. Often, unrealistically low water temperatures are stated by the manufacturer. This may be due to his desire to undersize the heat exchanger on his machine for cost savings. Or, perhaps because once-thru city water was anticipated. Whatever the case, ask about the rate at which heat must be rejected, desired flow rates and temperatures.

The system designer should try to get the cooling water temperature as high as practical for the most economical cooling tower selection. There should be at least seven degreesF between the cooling tower's leaving water temperature and the design wet bulb temperature. The greater this 'approach' the more economical the tower selection.

The following equation will help you find the correct balance of design parameters:

GPM = Heat rejection Btu/hr / (500 x RangeDegF)

Where 'Range' = The difference between the cooling tower's water inlet temp minus its outlet temp

The flow should be sufficiently high to be turbulent (for good heat transfer), yet not so high as to require expensive pumping due to excessive pressure drop. Example: A machine requires the removal of heat at the rate of 550,000BTU/HR. The manufacturer recommends 100gpm cooling water entering the machine at 90 degreesF. Solving for the 'Range' from the equation above yields 11 degrees. The cooling tower would be selected to cool 100gpm fron 101 degrees to 90 degrees at the appropriate design wet bulb for the location. Generally, a happy medium can be found that satisfies everyone’s requirements.

HOT WELL/COLD WELL:

Certain industrial applications are cyclical in nature imposing substantial loads for short periods of time. Here, the amount of water in the system becomes important. A closely coupled system with minimal water volume will have rapidly varying temperatures while a large water volume will smooth out the temperature spikes. Other systems requiring intermittent process water flow are inconsistent with the cooling tower’s desire for a constant flow while in operation. Still others have process water temperatures that are too high for direct introduction into a conventional cooling tower. The hot well/ cold well is a simple solution to the above challenges.

- The load introduces heat to the system at the rate of:

Gpm2 x (Z - Y)degF x 500 = Btu/Hr

- The cooling tower is selected to cool Gpm1 from X to W and rejects the same amount of heat... Gpm1 (X-W)degF x 500 Btu/Hr = GPM2 (Z-Y)degF x 500

- The weir separating the hot and cold wells enhances system performance by reducing mixing thereby reserving colder water for the process and the warmest water for the cooling tower. The weir can be moved to increase the cold water system volume for cyclical applications.

- P1 is sized so that the cooling tower sees the highest temperature consistent with its materials of construction (the higher the temperature the easier the tower duty but excessive temperature can damage the tower.)

- P1 and P2 should be intentionally sized at flow rates differing by at least 10% and the make-up valve placed in the well fed by the pump having the least flow.

- Water will pass over the weir at a rate equal to the difference in the pump flow rates.

- Excessive differences in flow between P1 and P2 should be avoided as they cause increased process water temperatures or reduced cooling tower temperatures- neither of which is desirable.