But another advantage to an evaporator is dehumidification. When air passes through an evaporator, its temperature is reduced below the dewpoint temperature, causing moisture to condense out of the air. That moisture is the humidity in an occupied space.
Most commercial applications rely on the evaporator in the mechanical cooling system to handle most of the dehumidification in the building. That takes on special meaning, considering ASHRAE Standard 62-99, which calls for more outside air to be brought into commercial buildings. That air can be humid, increasing the latent load on the evaporator.
Paying attention to the evaporator design, installation, and maintenance should keep the occupied space at the proper humidity level.
“An oversized coil can actually help system efficiencies, but it will generate high suction temperatures. This leads to higher fin-surface temperatures, which will result in less dehumidification,” says Jeff Maxwell, product and applications engineer, Heatcraft, Grenada, MS.
On the other hand, an undersized coil will result in lower system efficiencies. That’s because the TXV will try to close off the flow of refrigerant to achieve the desired superheat. This will cause lower suction temperatures.
This, combined with either a dirty coil due to low maintenance, poor filtration, and/or insufficient refrigerant charge, can result in a coil freeze-up.
However, the type of fin chosen can also affect air pressure drop and capacity. The more enhancements are made to the fins, the higher the air pressure drop is going to be, but the more capacity the evaporator will have. That’s because the heat transfer coefficient is going to be higher, as there’s more turbulence in the air.
The flat fin has the lowest air pressure drop, but it also has the lowest capacity. The sine fin, which has the most enhancements, would be more conducive to pulling water out of the air. But while it has more total capacity, the air pressure drop will go up. It’s also possible to add rows and fins, which also add capacity.
“Everything is done in efforts to increase how much total capacity your coil can handle. If you opt to go with a higher fin count per inch or more rows to get more capacity out of your coil, then naturally you’re going to increase your latent capacity as well,” says Maxwell. “But the sensible to latent capacity ratio will increase, which may result in an insufficient amount of dehumidification.”
However, adding rows and increasing fin density will increase the air pressure drop. “It’s a trade-off,” notes Maxwell: “air pressure drop versus capacity. With chilled water coils you can look at it as the same thing — increasing your fluid flow rate or increasing your tube-side enhancements with a rifled tube or a cross-hatch tube increases your fluid pressure drop, but it can increase your capacity as well.”
“I get calls all the time from customers complaining that their evaporator is not getting the capacity it was supposed to, or the leaving air conditions aren’t what they should be,” says Maxwell. “Then we have to sit down and virtually troubleshoot their system. It could be a hundred different things, but it’s often a dirty coil.”
Maxwell says that the magic question to ask in this instance is, “Has the system ever worked properly?” If it did work correctly at the beginning but performance has degraded over time, then it’s likely that the evaporator has not been cleaned properly. Of course if the system never worked correctly, it becomes necessary to look at other issues, such as how it was originally installed.
“If the coil wasn’t designed properly, if it’s undersized or oversized, or if it isn’t maintained, all these things can contribute to humidity problems inside,” notes Maxwell.
So if a space has humidity problems, a good first place to look is at the evaporator.
“The Comfort-R mode is obtained by varying the airflow during the cooling cycle,” says Tim Storm, furnace product leader, Trane Unitary Products Group, Tyler, TX. “The first 60 seconds of cooling operation is 50% of rated cooling airflow to chill the evaporator coil. The next 7.5 minutes are at 80% of rated airflow for increased moisture removal and comfort. After 8.5 minutes, the cooling airflow goes to 100% airflow for maximum capacity.”
As might be expected, the Comfort-R mode results in more condensate. Trane notes that it uses sloped drain pans on all residential units, which reduces standing water in the drain pan and allows positive drainage of the increased volumes of condensate.
The Comfort-R cooling mode is available in Trane’s residential gas furnaces, air handlers, and package units with variable-speed indoor motors.
Publication date: 11/19/2001