Latent Heat –The Masked Superhero of Thermodynamics

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What is Latent Heat 

Webster defines latent as present and capable of emerging or developing but is invisible, concealed, or inactive.  Google describes latent to be hidden until circumstances are suitable for development or manifestation.  Either way, something is there, but it is out of sight, hidden… and it’s also waiting for the right circumstances to step out of the shadows and show itself.  Like a masked superhero, with their secret identity, waiting until there is a crisis before they change into tights and save the day.  Latent heat is like that too… it’s a superhero, but always out of sight.  Often only discussed in hushed tones during the summer cooling season or at boring Manual J classes. 

In thermodynamics, latent heat is what is added or removed to change the physical state of a substance… but NOT the temperature.  The latent heat of fusion is when ice melts into water, without a change in temperature.  The latent heat of vaporization is when liquid water becomes steam, and the latent heat of condensation is when that vapor condenses into a liquid.     

What is interesting is it takes a lot of heat to turn ice into water, and even more heat to convert water into steam (see Figure 1 below). 


Two for One 

This means that the thermometer shouldn’t move when ice turns into water, or water becomes steam.  It’s two physical manifestations of the same compound at one temperature.  That’s weird, right?  I think it’s weird.   

Oooooh. Aaaahhhh. Gee, Wes, that is some fascinating but useless information. Thanks, tons.   

Latent Heat: Applied 

On a hot summer afternoon, after you replaced the HVAC system, you may notice that the house begins to cool down, but the thermostat hasn’t moved.  Part of the reason is that you just crawled out of a 130°F attic, and everything feels cool.  The other reason is that when a cooling system starts in a hot and humid house, there is a great deal of heat transfer.  But much of relief you feel is due to the latent heat of condensation.  The humidity in the air condenses and is discharged down the drain.  The thermostat will not reflect this heat transfer, but if you happened to watch the drain lines, you would see the unit is removing thousands of Btus of latent heat. 

Sidebar 1: For every pint of water removed, the air conditioner has removed 972 Btu. For simplicity, you can round up to 1,000(only an engineer will raise a fuss). A condensate drain that can fill a 16 oz soda bottle in 10 minutes is on pace to remove almost 6,000 Btu/ hour. That is heat removal you won’t see on the thermostat, but it’s a comfort that your customers will feel and will thank you for it.

To see this proven, let’s look at the expanded cooling capacity for one of my favorite units, the XYZ model AC-30(Figure 2). To set the stage, on a hot summer afternoon, while you changed the air conditioner, is it possible the indoor conditions could reach 95°F and 35% RH? Yup. At that temperature and relative humidity,the entering wet bulb (EWB) temperature is72°FWB. At that EWB and a 95°F outdoor temperature, this air conditioner has a capacity of 33,680Btu/h. That’s a lot! But only 18,500 Btu/h is sensible capacity (Figure 2, #1). The rest of that, 15,360 Btus, islatent capacity,and it’s working like the caped crusader, unseen, but fantastically productive. As the EWB drops down to 63°FWB(75°F and 50% RH), the total capacity drops slightlyto 28,020 Btu/h,but the latent capacity is greatly reduced (26,560 Btu/h –sensible, 1,460 Btu/h –latent, Figure 2, #2).

Figure 2 Model XYZ AC-30

Typically, after the equipment is replaced, most homeowners will not stand and look at the thermostat, they will thank you profusely for saving their family, they are grateful for your hard work under such miserable conditions.  But if they did stare at the dial, and if they happen to say in a snarky tone, “Hey, its not cooling off very fast, or the thermostat isn’t working.”  You can change into your superhero costume, and then explain the latent heat of condensation while they write the check.
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