Disappearing Latent Capacity - Short #95

HVAC School - For Techs, By Techs

Bryan Orr

Education, Business, Self-improvement, Careers

4.9 • 1K Ratings

🗓️ 26 May 2020

⏱️ 16 minutes

🔗️ Recording | iTunes | RSS

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Summary

In this short podcast, Bryan explains why latent capacity is prone to disappearing. He also explains what actually happens when the latent capacity drops.

When you measure enthalpy split across the coil, you'll learn that the equipment design makes it perform to AHRI design conditions. Those design conditions are 95-degree outdoor temperature and 80-degree indoor temperature at 50% indoor relative humidity. So, the A/C system must remove a lot of moisture. However, we don't usually run A/C units for 80-degree indoor temperatures; we usually aim for a 75-degree indoor temperature. When we have 80 degrees, the sensible AND latent heat loads are higher. Things get tricky when we encounter disappearing latent capacity, which is when you remove less moisture.

If we have equipment with a sensible heat ratio (SHR) of 0.75 at design conditions, we'll likely have a higher SHR with our typical conditions. When the dew point is lower, water condenses on the evaporator coil at a lower temperature; water holds up the surface temperature of the evaporator coil and optimizes heat removal, suction pressure, and compression ratio. When heat transfers to the water on the coil, the sensible heat in the air decreases via a latent process.

When we don't have moisture on the coil, all of the heat going from the air into the refrigerant is making it in via conduction through the metal coil walls. Unless the coil gets below the dew point, it won't remove any moisture; we can still remove sensible heat, but you don't have the advantage of the moisture "holding up" the surface temperature.

In very dry climates, we increase the airflow because we don't want to remove moisture from the air, but we still want heat to be available to the evaporator coil. However, we have to be careful about the bypass factor.

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Transcript

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0:00.0	Well, hi there, this is the HVAC school podcast. I'm Brian. This is the
0:06.5	podcast that helps you remember some things you might have forgotten along the
0:08.6	way as well as helps you remember some things. You forgot to know in the first place.
0:12.1	This is a short episode and we're going to be talking
0:15.5	about one of the weird things about latent system capacity and how that latent capacity just kind of disappears on you at a certain point.
0:25.0	But before we do that, I want to thank our sponsors and our sponsors are Navac and Navac Global
0:28.4	com. If you haven't taken a look at the NEFLI flaring tool, battery-powered flaring tool, if you make a lot of flares, it's probably the easiest
0:35.9	flaring tool out there and it makes great flares.
0:38.1	It works excellently.
0:40.0	Check out the NEFLI at truetech tools.com or at your local supply house. If you do use true tech tools use the offer code get schooled for a great discount at checkout.
0:50.2	Also field piece and field piece.com everything field Piece has been putting out with the JobLink Probes, JobLink Kit,
0:57.0	whether it's the monometer or the indexichrometer or the rapid rail clamps,
1:02.0	they're all excellent tools as well as the
1:04.3	S-Manifold the SM-3-80 V manifold that is my current favorite manifold out there if
1:11.4	you're still in the manifolds, but most of us
1:13.8	are into probes nowadays, but check it all out by going to fieldpiece.com.
1:17.7	Also refrigeration technologies at refrigateech.com, they now are making hand
1:21.8	sanitizer as well as disinfectant.
1:25.0	You can find all of their products at your local supply house or by going to
1:29.4	refriggedec.com. Also check out their new venom packs. Really neat new packaging, highly concentrated formula with a much smaller pouch and spout easier to fill, easier to store on the van. That's the venom packs from refrigeration technologies.
1:44.8	Also speed clean and speed clean.com if you haven't tried their mini split bib kit yet,
1:49.7	that's a mouthful say that 10 times fast mini split bib kit. It's an excellent tool or solution, I
	...

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