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TEMPERATURE CONTROLLED BLOWER MODULATION FOR AIR CONDITIONING HUMIDITY CONTROL.

By Richard Peters PE, CM

The purpose of this article is to explain some of the common problems encountered with humidity control in modern high-speed air conditioning and how controlling blower speed from a temperature signal can eliminate these common problems as well as increase the comfort and livability of a home.

FACTS AND RULES OF THUMB THAT YOU CAN TAKE TO THE BANK

1. If water stands in a drain pan, the pan is either poorly designed or improperly installed.
2. What is normally called “low load conditions” refers to load from temperature gain and is not necessarily the total load necessary for efficient operation and comfort.
3. Humidity can be a significant load that is not addressed.
4. Every 10% drop in relative humidity is the comfort equivalent of a 3° drop in temperature. I.e. drop the humidity by 10% and you can enjoy the same level of comfort at a three-degree higher thermostat setting. This saves energy.
5. Water adheres to the A/C coil better as the coil gets colder.
6. When the A/C compressor turns off and the blower’s speed remains high, the coil gets warm very quickly and doesn’t grip the water as tightly.
7. The faster that air moves over an A/C coil, the higher the coil’s bypass factor, the warmer the coil, the warmer the delivered air temperature and the less adhesion to water.
8. A warm coil (either right after the compressor shuts off or under high air velocity) doesn’t hold water tight enough to give it time to flow down the entire height of the coil to the drain pan before being blown from the coil by the high-speed air movement through the coil.
9. If (with the compressor running) the air velocity over the coil is less than 500 feet per minute: An A/C coil, under normal operating conditions, will hold water long enough for it to flow down the coil and into the drain pan.
10. The method of determining the air velocity (called face velocity) over the coil is to divide the CFM air delivery from the blower by the square foot face area of the coil. When velocities reach over 600 feet per minute, then water will strip from the coil and down the ducts even if the coil is quite cold.
11.  PHYSICAL CONSIDERATIONS THAT REQUIRE DISCUSSION.
1. The space between the coil plates or fins (Usually 14 fins per inch) is designed to allow air to move freely through the coil and still allow enough air to impact the coil to reduce the temperature of the air enough for effective cooling.
2. The thickness of the coil. Usually allows 3 or 4 rows of tubing.
3. Water that is flowing down the coil takes up space. There is a condition where the amount of water in the coil can build up because it can’t get down to the drain pan fast enough, this excess water takes up free space and reduces the amount of free area for air to flow at an efficient (500 ft. per minute) velocity through the coil. It has the same effect as reducing the size of the coil. In this case, the air velocity through the remaining, or available, free area increases to over 600 feet per minute. At that point water is stripped from the coil.
12. Coil bypass factor is the percentage of air that goes through a coil without being affected by the coil. Cooling is done only to the air that actually touches the coil’s fins.
13. Typical bypass factors for a three-row coil with 14 fins per inch are:

The delivered air temperature is the result of mixing very cold air cooled by the coil fins and the room temperature air that goes through the coil space without being cooled.

 A COMMON ATTEMPT AT SOLVING THE HUMIDITY PROBLEM

 Blower speed timed programs:  i.e. run the fan for a number of minutes (maybe 7 minutes) on low (about ½ speed).  During this time the coil is super cold and super efficient and collects large amounts of water. During certain high humidity conditions, the A/C coil loads up with water faster than it will drain. After the low speed segment has timed-out, the blower ramps to high speed. This does two things: First it warms the coil, which reduces the water’s adhesion to the coil. Second, high velocity through the remaining free space breaks the surface tension of the water so the droplets break apart and they blow off the overloaded coil and down the ducts. This is why many contractors in high humidity areas do not use this feature.

 ANOTHER COMMON ATTEMPT AT SOLVING THIS PROBLEM

In high humidity areas, it is also common to either run the fan on intermittent or turn off the fan for a period of time after the compressor shuts down in an attempt to give the coil time to drain. This is not a solution to the recognized problem of blowing water off the coil and re-evaporation. It is avoiding the problem rather than solving the problem. It also makes the home less comfortable, less quiet and the wide variety of indoor air quality products less efficient.

DRAIN PAN PROBLEMS

 The common statement concerning coil drain pans is: “When you run the blower while the compressor is off, you’ll re-evaporate the water standing in the drain pan.” Somehow this statement has been used to the point that it has become folklore. The proper design and tilt of the drain pan is all that is required for it’s proper function. First, the word drain in the name of the device should provide a clue to its function. Water should not be standing in a drain pan. Second, to evaporate a couple of ounces of water at temperatures we are dealing with is not instantaneous. Third, even if a few ounces of water were evaporated into the thousands of cubic feet of air that is inside a home, it would have about a zero consequence.

 TEMPERATURE CONTROLLED BLOWER SPEED SOLVES THE PROBLEM

MORE FACTS THAT YOU CAN TAKE TO THE BANK

1. Blower speed can be controlled effectively and efficiently by changing the voltage to any high quality shaded pole or PSC blower motor. It has been done since the late 1950’s or early 1960’s.
2. When blower speeds are controlled through quality motors, the fan laws are in full effect.
1. When you reduce the speed of the blower by 50%, you reduce the CFM by 50%
2. When you reduce the speed of the blower by 50%, you reduce the static pressure to one fourth.
3. When you reduce the speed of the blower by 50%, you reduce the power required to one eighth of that required to run it at full speed. (i.e. You can run a good blower motor for eight hours at half speed for the same amount of money that it would take to run it full speed for one hour)
3. Typical speed/temperature relationships for blower speeds controlled by temperature. Using a typical, average designd for a three-ton system. (Using rounded numbers for simplicity)
1. 36,000 BTUH
2. 400 CFM per ton or 1,200 CFM at full speed
3. Minimum speed is set at about 35 or 40% of full speed or about 450 CFM at about 450 rpm blower speed when the delivered air temperature coming off the coil is above 68° F.
4. Maximum blower speed about 1070 RPM when the delivered air temperature coming off the coil is about 50° F.
5. The fan is set to run continuously.
6. For every degree of temperature reduction from 68° F delivered air the fan motor’s speed increases about 34 RPM. Or for every 34^th of a degree temperature drop, the fan speed increases one RPM. This is smooth and seamless.

 TYPICAL AIR CONDITIONING CYCLE USING TEMPERATURE CONTROLLED BLOWER SPEED MODULATION 

1. Room temperature about 75°.
2. Indoor humidity high. (maybe over 70%)
3. Blower running continuous at about 450 RPM and delivering about 450 CFM.
4. Compressor comes on.
5. Air is moving over the coil slowly.
6. The bypass factor is about 10%, which means the coil is super efficient and very cold.
7. Because the coil is very cold and super efficient, it cuts into the latent load and begins rapidly collecting water.
8. Because the coil is so cold, the air coming across the coil also cools down rapidly and reduces the delivered air temperature.
9. Because the air temperature is reduced, the blower’s speed is automatically and instantly increased.
10. Here are some interesting points: The coil has established its grip on the latent load and is collecting water to the coil’s maximum design capability to capture water.
11. The compressor doesn’t know the difference between latent and sensible loads. It only sees a load.
12.  Because the coil is capturing water to it’s maximum ability, the remaining capacity of the compressor is then devoted to the sensible load. The remaining compressor capacity may not be enough to drive delivered air temperature down to the 50° temperature required for full speed blower operation. The delivered air temperature might be 54° degrees and between 900 and 1,000 CFM. (That is why when Florida, gulf coast and contractors in other high humidity areas first install temperature controlled blower modulating controls, they think that the blower is going too slow. However, when they look at their gauges, they see the unit is working its lungs out because it is concentrating on the latent load. A day or two later, the house has dried out. The delivered air temperature is colder and the blower is running faster.)
13.  Compressor Shuts off:
14.  The coil and delivered air begin to warm a little. The blower’s speed immediately and smoothly drops to about 50% of the way between the highest speed that it achieved at its coldest point and the 450 CFM minimum speed. Lets say about 675 CFM.
15.  675 CFM isn’t going to blow water off a 1,200 CFM coil. The coils is going to drain.
16. Interesting point #2: There is a good amount of cooling capacity in pressure equalization. The high-pressure liquid refrigerant moves into the evaporator and does some cooling. It usually takes about 5 to 10 minutes for the pressure in a system, with good valves, to equalize.
17.  During this time, the blower’s speed is smoothly dropping towards the minimum speed. The coil is cool, the coil is draining, and there is no re-evaporation.

 SUMMARY

Temperature controlled blower speed:

1. Eliminates the problem of blowing water off A/C coils.
2. Results in maximum humidity removal by automatically adjusting for latent loads.
3. Allows comfortable and quiet continuous blower operation that permits all accessories attached to an HVAC system to operate at full-time at full efficiency.
4. Increases efficiency.

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