Sensible vs Latent
What is the difference between sensible and latent loads? Working contruction and hearing the jargon I think I have a rough idea but what does it really mean. Thanks C
The simplest explanation is latent load means humidity. Sensible load means temperature on the thermometer. I sympathize, it sounded very strange to me too until I got a chance to work awhile with the concepts.
Originally Posted by contrades
Hope this helps -- Pstu
Latent heat - heat that causes a change of state but not a change in temperature; example: heat needed to change water to steam
Sensible heat - actual temperature, heat you can feel; causes change in temperature but not change of state
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.
Latent heat is the moisture in the air that must be dehumidified out befor any actual cooling takes place.
Moisture (humidity in air) is measured in "grains of moisture per cubic foot of air".
The higher the air temp the more moisture it can hold. Ex:98 degree air hold more moisture than say 70 degree air. This is called realative humidity.
The higher the RH the more water there is to dehumidify and that is considered a "High latent load".
Once the a/c has run long enoug to dehumidify the air, then it can be cooled to a lower temp.
Sensible heat is the heat you feel on your body and measure with a thermometer.
So, on a 98 degree day w/98% RH the a/c will reduce that RH to a much lower number and it will go out the drain as condensate. Thus, the latent load is reduced down to a more comfortable level.
Now we can run the a/c and bring that 98 degree air to a comfortable level.
The laten load is a very large part of the "total load" that must be reduced to achieve comfortable levels of cooling.
Last edited by hvaclover; 04-03-2007 at 10:35 AM.
Another explanation might help clarify the sensible/latent cooling process. Will use cooling 70^F, 50%RH, 50^F dew point, 52 grains moisture as an example with total heat of 25 btus per lb.of air. As the air is cooled, sensible heat is removed until the air reaches the dew point. At this point, the air is 50^F, 100%RH, 50^F dew point, 52 grains moisture. This is roughly 5 sensible btus/lb of air of cooling, no latent heat removed. As the air is cooled below the dew point, both sensible and latent heat are removed. Cooling to 40^F, 100% RH, 40^F dew point, 38 grains of moisture, and 15 btus/lb. of air, removing 2.5 btus sensible/ 2.5 btus latent per lb.
Because the cooling coil is colder than the air being cooled, the cooled air is usually 95%RH instead of the sited 100%RH.
Next is to express the sensible/latent ratio. The example has 7.5 btus/lb sensible and 2.5 btus/lb of latent of air. It's a 75/25 sensible/latent ratio. Understanding this shows the difficulty of removing humidity without over-cooling space. A high eff. dehumidifier removes latent heat as water and returns the heat plus the electricity used. Removing 5 pints/kw returns 8,500 btus of sensible heat. This is a enough heat to raise a basement 3^F. Raising temp 1^F decreases %RH 2%. Thanks for reading. TB
The guy said he works construction Teddy Bear. Why don't you just post a psychometric chart and get your point across?
Originally Posted by teddy bear
Sensible and latent: Add 150 BTU to a pound of 50 degree water, and its temperature increases 150 degrees to a temperature of 200 degrees. This sensible relationship ends at 212degrees.
Unexpected or hidden heat is released or absorbed as it changes is called latent heat. Latent heat is absorbed or released as it changes.