Finding The Latent Heat of Condensation of Your Air Conditioner

                    - with Darrell Udelhoven
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).

At an evaporator condensing temperature of 55ºF the latent heat transferred to the cooling coil's boiling refrigerant is close to 1,062 BTU per pound.
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

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.

Air Temperature Drop through evaporator coil

Air Temperature Drop Through Evaporator Coil (1987 Period)
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.

A modern 2-ton 13-seer system would produce around .70 of a ton or 8,400-Btu/hr, however at 70% Relative Humidity its capacity would increase to around 1.1 ton or 13,200-Btu/hr or over half of the 2-tons would be used for the latent heat-load. "That is around a 36% increase in latent capacity" with some reduction in sensible capacity, --due to a higher humidity!

Conmdenser's Return Air Latenmt Load Graph

Page 618, Refrigeration & Air-Conditioning (ARI) Second Edition, C 1987
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.


 I assume no responsibility for how anyone uses the information on my web pages
All HVAC/R work should always be done by a licensed Contractor! This information is only placed on these pages for your understanding & communication with contractors & techs.

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 - Federal Refrigerant Licensed - Retired Licensed Contractor
Darrell's Refrigeration Heating and Air Conditioning- Retired
Darrell Udelhoven 

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Posted: 08/30/07