Misting Systems for Air Conditioning Units
During the oil crisis in the late 1970’s, many devices appeared on the market if installed on your automobile would save gasoline. This device would save 20%. That device would save 10%. Back then the price of gasoline had gone sky high and was in short supply. My father-in-law said to me one day if he installed all those devices on his car, he would have gasoline pouring out of his gas tank. Many people today are numb from all of the advertising and tend to discount anything that may save money. “If it’s too good to be true, it probably is.”
Misting systems I think should be looked at as a economical and practical method to reduce wear and tear and also reduce power requirements. “This time I’ll clean my glasses for a better look.” Hunsecker “Sweet Smell of Success.”
Home air conditioning units consist of condenser (3), evaporator (1), expansion valve, and compressor (4). Freon inside the A/C system operates the same as a steam cycle. A steam cycle has a condenser, boiler, turbine, and feed pump. Both a steam cycle condenser and the freon condenser change a vapor into a liquid. The boiler and evaporator add heat to the liquid which changes the liquid to a gas. The turbine and expansion valve allow expansion to a lower pressure. The feed pump and compressor provide for the circulation and raise the fluid from a low pressure to a high pressure.
All freon systems have a high side and a low side. When I say a high side and low side, I mean high pressure and low pressure. The compressor (4) is the means for creating a high and low side in the freon system. The compressor takes the low pressure vapor of the evaporator (1) and compresses the vapor to a higher pressure. The compressor (4) discharges the higher pressure vapor into the condenser (3). A large part of the power requirements are in compressing the lower pressure vapor to a high pressure vapor. The fans (5) and blowers (2) consume say about 20%. The blowers (2) circulate air inside the house and the fans (5) are located on the condenser (3). So, to reduce power consumption the compressor (3) is the place to start.
The expansion valve – not shown on drawing – is located close to the evaporator (1) and separates the high side from the low side of the freon stream. The expansion valve is soldered into the Freon tubing. The purpose of the expansion valve is to allow the high pressure liquid to boil off slowly into low pressure vapor side of the evaporator. When a liquid boils, heat must be supplied. To boil water, the fire from the stove must be turned on. The heat to the evaporator comes from the blower (2) forcing air across the coils of the evaporator (1). When the freon boils, it cools.
Evaporator and condensers are heat exchangers. They exchange heat from one environment to another environment without any mixing between the environments. The tubes and fins of the evaporator and condenser are the means of transferring heat. The freon stays inside the tubes and the air stays on the outside of the tubes. Heat exchangers have many tubes and tiny fins. The fins encapsulate the tubes. The fins increase the surface area so that heat can be more efficiently moved from one environment to another. Heat always flows from a high temperature to a low temperature. For heat to flow a temperature difference between the outside on the tube/fins and the inside of the tube/freon must be at least 10 degree F. At the evaporator (1), the heat flows from the high temperature house air into the freon. At the condenser (3), The heat flows from the high temperature of the freon in the tubes to the outside air. The air is moved through the evaporator by the blowers (2) that circulate air inside the house. The air is pulled through the condenser by the outside fan (5) usually located on top of the condenser.
There is always a temperature pressure relationship between a liquid and its vapor. As long as the vapor is in contact with the liquid – saturation – a definite pressure corresponds to a definite temperature. Water boils at approximately 212 degrees F at atmospheric pressure. In a pressure cooker the pressure increases and so the temperature increases thereby tenderizing the round steak. Freon works on the same principle.
Heat is absorbed by the evaporator (1) and ejected into the atmosphere by the condenser (3). The heat is moved from the house to the atmosphere. As long as there is a temperature difference of approximately 10 degrees or more between the inside and the outside of the tube the heat will flow. At the condenser, the compressor (4) compresses the freon gas to a certain design pressure which corresponds to definite temperature. When the pressure exceeds the design, the compressor may shut down due to high pressure or a recirculation line will open and the gas will recirculate from the high side to the low side of the compressor. In either case, the freon has reached a maximum temperature/pressure. Assume the maximum temperature of the Freon inside the tubes is about 110 degrees F and the outside air is 105 degrees F. The differential temperature is not high enough to cause heat to flow. Heat flow stops and cooling in the house stops. For heat to be absorbed in the evaporator (1), heat must be continually ejected by the condenser (3).
Two options exist to work around high ouside air temperature. Increase the A/C unit size. Size meaning a more beefy compressor and the accompanying beefy evaporators, condensers, and expansion valves. This is all caused by the increased pressure requirements to increase the temperature so that heat will flow when air temperatures increase. This also will cause a beefy cost increase. The other option is to lower the outside air temperature.
When water evaporates, it cools. The idea behind misting is to create a fine cloud or fog of water around the condenser and as the fog evaporates the air temperature will drop. The air temperature does not need to drop much – 10 degree F difference between the air on outside of the tube and the freon on the inside of the tube – to cause the heat to flow. The larger the difference the better.
The advantage of misting is that it lowers the outside air temperature. The critics of misting point out that water contains scale which will coat the fins of the condenser. As the fog is pulled into the condenser, evaporation occurs and the scale collects on the fins. The fins are there to increase the area of the tube which increases the exchange of heat. Scale is a poor conductor of heat impedes the transfer of heat. So scale collecting on the tubes defeats the purpose of misting. A couple of ways can be used to work around this predicament. First, a water softener can be installed and the demineralized water used in the fogging. I haven’t done the numbers but I bet the water softener will pay for itself in power savings. Second, using the condensed water from the evaporator. This water is free from scale as it condenses from the humidity of the house. As the air flows over the evaporator, the cooling causes the air temperature to drop below the dew point and condensation occurs. As an example, after a trip in the hupmobile on a hot August day, a pool of water will collect under a stopped car. This pool is the water that condenses in the evaporator located inside the car. This water is free from scale.
Saving Power and Wear and Tear on the Compressor
Fogging the condenser reduces the outside air temperature which will reduce the high pressure on the compressor and lowers the freon temperature. To move the same heat, an increase in outside air temperature must be met with an increase in pressure of the freon which increases the temperature of the freon. Mechanically compressors resemble a piston and cylinder on a automobile engine. As the piston closes the gap between the top of the cylinder and piston head, the gas is compressed. Whether the pistons are driven by a crank or a wobble plate, the increase in pressure means more energy must be expended. So anything that will reduce the pressure for the same heat load will reduce the power. Also, for high pressures there’s more internal wear, tear, and stress on the internal parts. The bearing that support and absorb the thrust from compressing the gas will last longer from lower pressures.
The refrigeration cycle starts at the high pressure end of the compressor (4) which compresses the freon gas. The condenser (3) transfers the heat from the high temperature freon gas to the outside air. A change of state from freon gas to freon liquid occurs. The high pressure liquid flows to the evaporator (1) where the expansion valve regulates the liquid and allows it to expand into the low side of the evaporator (1) The thermostat (7) sets the expansion valve to allow more or less freon to flow inside the tubes. This causes the house air to cool and also changes the state of the freon from liquid to gas where it treks on back to the low side of the compressor (4). The repeats many thousands of times a day.
Advantages and disadvantages of Misting
1. Demineralized water must be used. Some houses in order to reduce scale build up in their heating boilers are already equipped with a water softener. This is an out of pocket expense but may be offset by power saving. If the A/C design includes lowering the evaporator temperature and increasing condensed water from the evaporator, it may be possible to operate a misting system without a water softener. The OEM would need to include this in his design.
2. Proper spray nozzles must be used. I bought a fan to use in my garage that included a mister. It appeared the money went into the fan and the manufacturer forgot about the nozzles. The fan worked wonderful but the nozzles dripped more water than they fogged. Water was all over the garage in 10 minutes. A top of the line nozzle is a must. Even a boost pump may be necessary to get a high class nozzle to perform. Use metal nozzles not plastic. As the saying goes “there is a lot of wear and tear on a hens rear.” A good quality nozzle is expensive but I think in the long run will be well worth the money.
3. Dripping water from a nozzle is wasted fog. Much fog can be generated from a small volume of water. Dripping is waste and is going to cause the water bill to go up. After all the purpose of misting is to save money.
4. Don’t expect enthusiasm from the air condition new installation and maintenance crowd. Their purpose is to sell more A/C units not find a way to extend the life cycle. I had one technician tell if a misting unit were installed, they would charge the system with less freon because the outside air was cooler. Don’t believe such tripe. However, one of the technicians said they do install misting on commercial systems. It’s good for commercial but not good for residential.