Increasing Efficiencies Through Fogging

September 18, 2006
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It is truly ironic that air-cooled refrigeration and air conditioning systems perform at their worst, when they are needed the most. The same system that produces 95 tons of cooling when the ambient temperature is 85°F, produces just 82 tons of cooling when the temperature is 115°, which is a 14 percent reduction in capacity.

Over that same temperature range, the system's power draw increases by 31 percent from 97 kW to 127 kW, while its efficiency decreases by 53 percent kW/ton of cooling from 1.01 to 1.55.

To understand the cause of this fall off in performance, it is necessary to remember that in its simplest form, mechanical cooling consists of passing air across a refrigerant-laden coil in order to remove heat from one location and then piping the refrigerant to a second coil over which outdoor condenser air is drawn to dissipate the heat.

HOT WEATHER OVERLOAD

The problem occurs during the dissipation of the heat. On hot summer days, the condenser air is unable to absorb as much heat so cooling capacity is reduced. Furthermore, since the system must operate under higher head pressures as a result of the added heat, power usage increases.

Over the years, attempts have been made to mitigate this problem by drawing more condenser air across the coil and reconfiguring its design. While this has helped considerably, further improvements figure to be incremental.

That being the case, the next logical step to improve the upper ambient performance of a cooling system would be to lower the temperature of the condenser air. After all, if hotter air means reduced cooling capacity and increased energy use, shouldn't cooler air have the opposite effect? In fact, it does. The problem is finding a way to lower the condenser air temperature without expending more energy than stands to be saved. This is commonly accomplished by evaporating moisture into the airstream.

FOG TO THE RESCUE

One method of evaporation is evaporative fogging. It should not be confused with the all too common practice of sprinkling water on undersized condenser coils on hot summer days in order to increase the cooling capacity. At first glance the benefits might seem similar, but an uncontrolled water flow can cause water and sewage costs to rise dramatically, while unfiltered water can leave deposits on the coil that can further inhibit proper operation. Evaporative fogging prevents these unwanted effects.

Fogging systems have been used for decades in such applications as building humidification, cooling generator turbine inlets, outdoor cooling, cold storage humidification, greenhouse cooling and humidification, equipment cooling, dust suppression, odor control, and livestock cooling. The common element among these applications is that equipment and operating costs might be difficult to justify if traditional cooling systems are required.

STEP BY STEP, PIECE BY PIECE

The evaporative fogging process begins by passing water through a reverse osmosis filtration system. The filtered, PH-neutral water then flows through a high-pressure water mist pump, where its pressure is elevated to 1,000-1,200 psi. Finally, the clean, pressurized water travels through high-pressure tubing to fogging nozzles strategically placed around the system to be fogged.

Evaporative fogging is far from a one-size-fits-all technology. Each system component is critical to attaining the desired results. The filtration system should be of the reverse osmosis variety and be specifically configured for the application at hand. This ensures the elimination of any minerals that could be deleterious to the ongoing operation of the system to be fogged.

The water pump should be a positive displacement type and designed for continuous operation.

The high-pressure tubing should be capable of withstanding operating pressures well in excess of 1,200 psi. The fogging nozzles must be capable of generating water droplets on the magnitude of 5 microns or smaller to facilitate their absorption into the airstream.

Size is a critical consideration with each of the components. An undersized pump will fail to achieve the desired pressures. An oversized pump will waste energy. An undersized filtration system will reduce fogging capacity. An overabundance of fogging nozzles will waste water.

A properly configured fogging system will cause flash-evaporation of the water into the air stream. The mixture of water and air reaching the condenser coil will be in a gas phase and at a considerably lower temperature. This will cause cooling capacities to rise and energy usage to fall.

REAPING THE REWARDS

The benefits to be derived on a particular application are dependent on a great many factors. As a general rule, the higher the ambient temperature and lower the relative humidity the greater the improvement in efficiency.

On a typical air-cooled chiller, a reduction in condenser air temperature from 115° to 105° will result in a 14 percent improvement in efficiency. A 20° reduction will save 27 percent. Testing has shown that savings of this magnitude are achievable, although actual savings depend on factors such as saturation level.

One of the earliest practitioners of evaporative fogging is MicroCool, Thousand Palms, Calif. MicroCool has continued to develop its "flash evaporation" fog systems for a wide variety of commercial and industrial applications. Today, the company designs, engineers, and manufactures a wide range of high-pressure water fogging systems, for commercial, industrial, and residential applications.

"The traditional way of pre-cooling condenser air has been to force it across a wetted media," explained Mike Lemche, MicroCool general manager. "For small, single-system projects, the wetted media approach can work quite well. For larger projects, however, fogging may be a superior solution."

Lemche pointed to supermarket refrigeration condensers as an example.

"These are often elevated above the roof so it can be difficult to force the air across the wetted pad. By comparison, it's a relatively simple process to place fogging nozzles around the perimeter of the condenser. You can also achieve a higher degree of saturation with a fogging system and do a better job of lowering the condenser air temperature."

To further simplify the process, MicroCool recently began shipping its C3 system, a single-skid module that combines a high-pressure pump, a reverse osmosis unit, and all the necessary controls.

"The C3 comes pre-engineered for each project and with a single-point wiring and piping connection," commented Lemche. "That greatly reduces field labor and eliminates installation errors."

While the tendency might be to cubbyhole evaporative fogging as a technology for hot, dry climates, the applications are far more wide reaching. With energy costs having risen dramatically over the last few years, areas like New England have become a hotbed for evaporative fogging systems, according to the company.

"Any cooling system that is short on capacity on peak days is a potential candidate," said Lemche.

"Beyond the capacity increase, customers also realize a reduction in peak demand (kW) and a large decrease in energy use (kWh). In areas like New England, where the electric utility companies offer energy conservation rebate incentives for evaporative fogging, customers can realize a payback of two years or less, which is a 50 percent annual return on investment. That's aw-fully hard to beat."

Publication date:09/18/2006

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