Sizing and Selecting Evaporative Condensers

January 11, 2001
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Forced-draft axial unit.


Second of Two Parts

How industrial refrigeration equipment operates can affect condenser selection. Year-round operation of evaporative condensers requires that the equipment be designed for trouble-free operation.

A remote sump configuration protects the basin water from freezing. Using a remote sump also allows the equipment to be run in fan-only operation without going through the process of draining the sump. However, the amount of water in suspension associated with each condenser can affect the size of the sump and, therefore, the cost of the project.

Cold-weather operation of evaporative condensers tends to allow long periods of pump-only operation. During pump-only operation, water exiting the spray headers will induce airflow downward through the unit. This movement of air and subsequent pressurization of the condenser will force water vapor out through the air intakes, thereby bathing the fan wheels of forced draft centrifugal fan condensers. This water vapor will be prone to form ice in cold weather, impeding fan operation — a scenario to be avoided in all cases.

Regardless of the season, maintaining good water quality will impact the equipment operation and useful life. Sunlight can promote the growth of bacteria.

Equipment that reduces the amount of light entering the condenser will reduce water treatment cost and, therefore, annual operating expenses. Induced-draft models are built with basins open to the atmosphere. Steps must be taken in the louver design to reduce the amount of light entering the basin, as well as keeping debris out and water in.



Ambient Conditions, Maintenance, and Materials

Ambient conditions affect the selection of an evaporative condenser. The design wetbulb temperature has a significant impact on the size of the evaporative condenser.

The location of the equipment will determine the design wetbulb temperature for the condenser selection. ASHRAE and other organizations (such as the National Weather Service) publish data including design wetbulb temperatures for each municipality.

In addition, the water quality, maintenance history, and environment will impact the condenser selection as well as the materials of construction required for each application. For example, if the condenser is installed in a location where it will be difficult to replace or is subjected to aggressive water chemistry, then it should be built using materials such as stainless steel for longer product life and resistance to corrosion.

The materials of construction also affect product selection. The major manufacturers build evaporative condensers from hot dip galvanized steel. The heat exchanger coil, the heart of the evaporative condenser, is built from carbon steel and the entire assembly is hot dip galvanized after fabrication.

The rest of the condenser is typically galvanized steel — either hot dip galvanized after fabrication or mill galvanized. Specific components, such as the water distribution system or air inlet louvers, are most often made from PVC.

Alternate materials of construction, such as stainless steel, are available at additional cost. Specific areas of the condenser may be built from different materials; for example, the basin only may be constructed from stainless steel.

Each option will add to the cost differently for different types of condensers. Each manufacturer will provide an increase in cost for alternate materials of construction, and this cost will then determine the condenser selection.



Operating Requirments

System operation can influence the condenser selection. If the condenser load fluctuates, then the condenser should have multiple fan motors and be able to cycle off the individual fan motors to match off-peak loads.

Alternately, the lower the condensing pressure, the less compressor horsepower is required to maintain a given heat load. Therefore, operating the condenser at full capacity during off-peak loads or during periods of low wetbulb temperatures reduces the system head pressure, which can be beneficial to the total system efficiency.

In fact, compressor horsepower can be reduced 1% to 2% for every 1°F drop in condensing temperature. The wetbulb and pressure requirements for expansion valves are limiting factors in reducing the condensing temperature.

Another method in reducing head pressure and subsequently reduce operating cost is to increase the capacity of the evaporative condenser. Once a given plan area is established for an evaporative condenser, the impact of increasing the capacity within the same plan area is minimal. The additional capacity can be used to reduced the head pressure or for future additional system load.

Future system loads or expansion must also be considered when sizing an evaporative condenser. A short-term benefit of a condenser sized for the future load is reduced operating costs due to lower head pressure.



Sizing and Selection

In sizing evaporative condensers, the first step is to determine the heat load. Often expressed in “tons of refrigeration,” it represents the heat requirement of the refrigeration system.

A ton is a unit of refrigeration capacity represented by the amount of heat required to freeze 2,000 lb of ice in a 24-hr period. This is equal to a heat rate of 12,000 Btu/hour (Btuh).

In order to use the same “tons” as evaporator tons, condenser manufacturers typically use 14,700 Btuh/ton to approximate the heat introduced to the refrigerant by the compressor. It is more accurate and less confusing to discuss heat load solely in evaporator tons or in Btuh.

Additional terms used in condenser selections are “nominal tons” and “corrected tons.” The term nominal tons should be given to the actual heat load for a refrigeration system. During the selection process, the nominal tons are adjusted to reflect the system operating conditions of suction temperature, condensing temperature, and wetbulb temperature, and relate these conditions to the base conditions on which the condenser is rated.

When making a condenser selection, the refrigeration system dictates several important requirements of the condenser. The evaporator load determines the amount of heat to be rejected to the atmosphere by the condenser. The compressor will determine the condensing temperature. The ambient wetbulb temperature also affects this temperature.

The condensing temperature can never be lower than the wetbulb temperature. The relationship between the condensing temperature and wetbulb temperature is the major driving force in condenser size. The closer the condensing temperature is to the wetbulb, the larger the evaporative condenser. Conversely, the higher the condensing temperature, the smaller the condenser, but with the penalty of increased energy consumption due to higher compressor horsepower.



Selection Methods

Most manufacturers use two selection methods: the heat-of-rejection method and the evaporator tons method.

As discussed earlier, condensers are rated for a given set of conditions. For ammonia, the base rating is for 20° suction temperature, 96.3° condensing temperature and 78° wetbulb temperature for ammonia (R-717).

The heat-of-rejection method uses the actual heat load on the condenser calculated by adding the evaporator load in Btuh and the compressor heat in Btuh. The heat load is multiplied by a factor load based on the relationship of condensing temperature and entering air wetbulb to calculate the corrected heat load. The corrected heat load is then used to determine the unit selection.

The evaporator tons method multiplies the evaporator load times factors for suction temperature and for the relationship between the condensing temperature and entering air wetbulb to calculate the corrected tons. The corrected tons are then used to determine unit selection.

The evaporator tons method relies on an estimation of compressor heat based on a factor tied to the system suction temperature using open-type reciprocating compressors. The actual compressor heat will vary with the type of compressor and manufacturer.



Conclusion

Identifying the most important factors such as cost, physical size, operating costs, sound ratings, and equipment features that pertain to the specific project will indicate the type of condenser as well as provide the best equipment selection.

The choices for each customer may vary based on energy efficiency as well as capacity in a given plan area.

Making the right choices will ensure that the right product is used for the right application.

Kollasch is product manager of evaporative condensers for Evapco. For more information, contact the company at P. O. Box 1300, Westminster, MD; 410-756-2600.

Sidebar: Installation Tips

Evaporative condensers require an abundance of fresh air to accomplish their designed function. The availability of that air and the efficiency of its use are prime concerns when installing an evaporative condenser.

A condenser, which uses less air, will likely have an advantage in a well enclosure. If there are existing units on site or future expansions planned, then the location of the condenser and the availability of fresh air becomes critical. Even prevailing winds and piping can affect condenser performance by affecting the airflow.

The key to a good installation is to have unaffected airflow to the air intake of the condenser and no restriction on the discharge. The condenser may require special support to provide free access of air to the unit.

In the event that the airflow is somehow affected by, for example, the building next to the unit or by the prevailing winds, then the discharge air can be directed back toward the air intake and recirculation will occur.

Recirculation of the discharge air can reduce condenser performance significantly. A 2°F increase in the wetbulb temperature will reduce performance by 16%. A 5° to 6° elevation in wetbulb can reduce capacity by 50%. Minimum clearance recommendations should be followed to provide sufficient fresh air for proper operation. Clearances around condensers are different for each type of condenser.

Forced-draft models require the most clearance in front of the fans while air enters on all four sides on most induced-draft models creating clearance requirements for airflow on each side. The site determines how much space is available and the size of the condenser will then determine the equipment selection.

For example, if a condenser were to be installed near the property line, then an induced-draft unit would be preferable, or a forced-draft unit rotated so the fans were facing away from the neighbors.

If the condenser were next to a taller office or apartment building, a forced-draft unit would be preferred to again direct the sound away from the neighbor. If the condenser were to be installed in a well enclosure, then the system engineer would choose between installing a forced-draft unit against one of the walls, or placing an induced-draft model in the center of the well.

Publication date: 01/15/2001

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