Technical Information

Air conditioners cool homes and businesses by absorbing heat inside the buildings and releasing or rejecting the heat to the outdoors. Most in use today reject the heat through air-cooled condensers. These units include the typical residential split-system air conditioner with the condensing unit outdoors and packaged rooftop units that are often used on commercial buildings.

The cold refrigerant within the air conditioning unit absorbs heat from indoors through the evaporator. The refrigerant evaporates or boils, capturing a great deal of energy as it absorbs heat and cools the airflow within the home or building. The warm refrigerant vapor is then returned to the compressor where the pressure is raised significantly, causing a commensurate increase in refrigerant temperature. The high pressure, hot vapor is then passed to the condenser coils. The refrigerant vapor temperature is maintained about 30 F higher than the ambient or outdoor temperature. Outdoor air is forced over the coils to reject or remove the heat that was absorbed back in the evaporator coil, plus the energy that was added through compression. In the heat rejection process, the vapor condenses into a high pressure liquid refrigerant. The warm liquid refrigerant which is now 5 to 10 F hotter than the outdoor temperature is then carried to an expansion valve near the evaporator that rapidly drops the refrigerant pressure with a commensurate drop in temperature. The cold refrigerant at about 50 to 55 F then enters the evaporator, absorbs energy and boils, and the cycle repeats.

The energy that is rejected or removed from the condenser is equal to the energy that is absorbed as heat in the evaporator plus the energy that is added by the compressor in raising the pressure of the refrigerant. Or put another way, the cooling capacity of the evaporator, and indeed the amount of compressor work needed, is directly related to the heat or energy that can be rejected through the condenser coil. It is therefore beneficial that the heat transfer through the condenser be maintained at the highest possible level. The heat transfer rate from the condenser coil is modeled by the well-known convection rate equation:

Q = h * A * ( Trefg – Tair ) (1) Where: Q = heat transfer rate [Btu/hr or Watts] h = convection heat transfer coefficient [Btu/(hr*ft2*F) or W/(m2*C)] A = coil surface area [ft2 or m2] Trefg = refrigerant temperature [F or C] Tair = air temperature [F or C]

The optimal air conditioning performance is achieved by maintaining a high heat transfer rate, or heat rejection, Q, through the condenser.

One way to increase Q is to increase Trefg. This can only be achieved by increasing the compressor output pressure. However, the compressor work or energy input must be increased to increase head pressure and Trefg, which ultimately costs more work energy than cooling gained. The air conditioner control system does increase pressure in response to higher outdoor temperatures (Tair) and in response to blockage of the coil or decreased convection coefficient as described below.

There are two practical approaches for maximizing the air conditioner’s efficiency by maintaining the highest possible heat rejection, Q. First, one can insure that the effective coil area, A, and the nominal convection coefficient, h, are not reduced or compromised. It is also possible to increase h by increasing the air velocity across the coil. This is only practical by changing the condenser fan(s) or increasing its speed. Increasing the speed will increase the brake horsepower of the fan motor. Second, one can reduce the air temperature, Tair, entering the condenser coil.

Both approaches, maintaining A and h and decreasing Tair , are benefits provided by Mist Ecology’s evaporative precoolers and are discussed below.

The coil surface area, A, is set by the air conditioning unit manufacturer and is generally slightly oversized for the design load. While this sets the maximum coil area, A, it should be noted that the effective surface area can be reduced by debris and contaminants blocking a portion of the coil. If there is significant blockage, the air conditioning unit responds by making the compressor work harder to raise the pressure of the refrigerant higher, effectively increasing the refrigerant temperature, Trefg. If the contaminant is grease from a kitchen exhaust fan, which are oftentimes located on rooftops near the air conditioning units, the blockage may not simply decrease the effective coil surface area, but the convection coefficient, h, may also be reduced due to the greasy film that will coat the coil fins.

Most home and business owners do not realize how important it is to keep the condenser coil clean of dirt, debris such as cottonwood or grass, and other contaminants.

Keeping a residential condenser coil clean may require removing a protective wireguard and then pressure washing and brushing the fins of the coil. A commercial unit generally has a larger coil than those used for residential purposes and can be in a greasy or dirty and less accessible environment. Cleaning is very often neglected by less conscientious maintenance crews. Unfortunately, there have not been any devices used for capturing kitchen grease and other aerosol contaminants, such as paint, that would be effective in protecting the condenser coil surfaces from being coated with these materials.

Mist Ecology conducted a study on a 15-ton Lennox rooftop commercial air conditioning unit on a restaurant. The air conditioning unit initially appeared to perform at less than the rated cooling capacity. Although the restaurant had only been in operation about ten weeks, all units had a readily apparent layer of grease on the condenser coils. This was also true of a 10-ton and a 20-ton unit that were also present on the rooftop. The roof temperature was 103 F on the afternoon the day data was taken. Based on the power consumption, which was about 16% higher and the cooling capacity which was about 7% lower than expected, the 15-ton unit was operating as though the roof temperature was 122 F.

The control system was reacting to the reduced A * h by increasing the compressor pressure and Trefg. Degreasing and washing off the coil would increase the A * h. This is time-consuming and difficult work on a hot roof. Further, the effect of such degreasing and washing operations would only be temporary.

Mist Ecology’s precoolers provide a more effective means to capture grease and other debris, increasing the cooling capacity and decreasing the electrical consumption of air conditioners.

The second method to improve the heat rejection, Q, is to reduce the entering air temperature to the condenser coil, Tair. One effective means is to adiabatically cool the ambient airflow by misting or evaporating a nominal amount of water into the airflow. Evaporative precoolers work well and not only increase the cooling capacity, but also decrease power consumption, improving the operating efficiency by as much as 30 to 40%.

Use of the Mist Ecology ACSpritzerTM (residential system) has shown that the design has very little water runoff, and achieves about 75% saturation effectiveness. Saturation effectiveness is defined as the percentage of increase in relative humidity, r.h., divided by the maximum possible relative humidity increase or:

Saturation Effectiveness = (final r.h. – initial r.h.) / (100% r.h. – initial r.h.) (2)

The Mist Ecology UltraMisterTM (commercial precooler) is designed for the harsher environment and windy conditions on a commercial rooftop. It has an outer set of filters which prefilter the hot incoming condenser airflow, capturing dust, debris, grease, etc. The cleaner air is then in the plenum or mist chamber where the water misting and evaporation occur, cooling the condenser airflow. And then, finally, the cooler airflow passes through the inner filter which captures any remaining unevaporated mist.

The pressure drop or loss through a coil is typically between 50 and 100 Pa (0.10" and 0.20" H2O). Selecting an appropriate filter involves a trade-off between the low pressure drop of an inexpensive 1" fiberglass filter and the high resistance of a thicker, denser filter. A 1" fiberglass filter will catch almost all of the relatively large grease droplets and most debris. It will typically have a pressure loss of 15 to 25 Pa (0.03" to 0.05") at a face velocity of 1.00 to 1.25 m/s (200 to 250 fpm). In order to minimize the pressure loss and maximize the airflow, the filter area may exceed the coil area.

Mist Ecology’s experience is that the total airflow will be reduced from 1 to 4%. Recalling the convection rate equation:

Q = h * A * ( Trefg – Tair ) (1)

The slight decrease in total condenser coil airflow is far outweighed by the benefits of reducing the air temperature, Tair , particularly and also maximizing h * A.

Mist Ecology’s UltraMisterTM is designed for ease of maintenance and to give years of reliable energy savings and cooling benefits.

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