FINDING
The LATENT
HEAT OF CONDENSATION
The amount of heat energy in BTU's that must be removed to change the state of one ounce or one pound of a vapor to one ounce or one pound of liquid at the same temperature. To find the LATENT BTUH heat absorption transfer split, of your A/C system, --measure the amount of condensate produced in a given time span and pro-rate it to an hour, or to BTU's per hour|(BTUH). ======== Andy Schoen 8-30-07 Quote: I'm looking at my ASHRAE 1997 Fundamemdals Handbook, and it lists latent heat of saturated water as follows: Temp Btu/lb 40°F 1079 45°F 1068 50°F 1065 55°F 1062 http://hvac-talk.com/vbb/showthread.php?p=1604921&posted=1#post1604921
Latent heat of condensation: "multiply pounds of condensate per hour" times the latent heat of condensation, at say 50ºF which is 1065-btu/lb. At
a condensing temperature of 50ºF the latent heat transferred to
cooling coil is close to 1,065 BTU per pound. Most use 1065-btu/lb,
which is okay for our ball-park purposes, so we will use it.
For example, use an alarm timer set for 15 minutes, if you collect 18-ounces in 15 minutes, that's 72-oz/hour or 4.5-lbs an hour. (4.5-lbs X's 1065-btu/lb 4,792.5-BTUH) of latent heat transfer per hour. Or you can convert with ounces: 1065-btu/lb / 16-oz = 66.5625-btu/oz. | ounces X 60-btu/oz = BTUH of latent heat absorption transfer performed by the evaporator coil. 72-ounces *X's 66.5625= 4,792.5-BTUH of latent heat transfer per hour. You can use a kitchen dietetic scale in ounces to weigh the condensate. Using an 18,000-btu/hr system: 18,000-bth/hr - let's say it is 4,792.5-btu/hr latent heat absorbed and 13,207.5-BTUH of Sensible heat transfer, at its Rated Tonnage Capacity.Therefore, 1.5-ton 18,000-btu/hr example would be operating at 26.6% latent, 73.3% Sensible. If you see any errors let me know! ================ What is helpful in regulating %RH is that: as the percent of Relative Humidity rises "the ratio of latent" to sensible, goes way up! A 28,538-Btu/hr system: That would be 10,559 of sensible and 17,979 latent or, only 37% sensible and around 63% latent. This would have to be at a very high Relative Humidity. The
usual latent/sensible split on a 2-ton 13-seer A/C unit at 70% RH is
around 55% latent and 45% sensible, or 13200 latent and 10,800
sensible. At the higher humidity levels a reduction of 100-cfm/ton or,
300-cfm/ton Vs 400-cfm/ton doesn't make much difference in latent
capacity! With proper Btu/hr sizing producing sufficiently long
run-times, reducing airflow may be counterproductive! An
18,000 condensing unit
- 12,600-BTUH sensible equals 5,400-BTUH of latent
heat
transfer, a 70/30 ratio at 80DB 67WB, as humidity increases latent
increases. If you
see any
errors
let me know! When the humidity is
higher,
the evaporator
increases
its ratio of latent heat absorption.
Indoor temperature and humidity load variations graph. Refrigeration & Air-Conditioning (ARI) Second Edition, Page 624, © 1987 Just a
'rough'
brief
demonstration of how the latent heat capacity of the DX coil "increases
with the increase in room relative humidity. Those lower SEER units had higher condenser splits than 12-SEER and higher units. Sorry, I defiled the graph, 90-db outdoor, 80-db indoors with 67 wet bulb or, 50% RH represents the condenser splits shown above graph. Graph: 80-DB & 80-WB line-intersect is 100% Relative Humidity. DISCLAIMER: This
information is for the edification of contractors and techs. I
am NOT liable for your screw-ups, you are liable for what you do! -
Darrell Udelhoven Darrell's Refrigeration Heating and Air Conditioning- Retired Darrell Udelhoven
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