Arkansas HVACR NewsMagazine May 2020

S tate, National, Chapter News Tech News

Jim Bergmann explains it this way. Think of a sponge being like air. When it is fully expanded, it is like the air in the return. When the sponge is fully saturated and can accept no more water it is at 100% RH and when it is completely dry it is at 0% RH. Let’s imagine that the sponge is 50% saturated and full size in the return. When that sponge (air) goes over the evaporator coil it is compressed, because colder air can hold less moisture. Once that air is compressed (cooled) enough it will begin to give up moisture. The point at which it starts to give up moisture is called dew point or 100% relative humidity. Once that air leaves the coil, it still remains in approximately the same temperature state (compressed sponge) as it was when it went over the coil. This means that unless heat is added or removed from that air, it will remain at 100% relative humidity. There are several reasons why the air in the supply will be slightly below 100% in the supply. First is contact factor or bypass factor which are both terms used to demonstrate the efficiency of a coil at “contacting” the air. The greater the surface area of the coil and the longer the contact time of the air on the coil the more efficiently heat will be transferred from the air to the coil. So why is it less than 100% RH in the supply?

RELATIVE HUMIDITY IN THE SUPPLY AIR STREAM Bryan Orr

When you first start checking your supply air with a thermo-hygrometer you may notice that the relative humidity is REALLY HIGH. Often the RH in a supply duct will be

between 85% and 96% relative humidity on a system that is

In order for dehumidification to occur the air must reach dew point, otherwise known as 100% relative humidity

functioning as designed. The reason for this is fairly simple.