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[DPRG] Air Conditioning At the Garland Warehouse

Subject: [DPRG] Air Conditioning At the Garland Warehouse
From: DeltaGraph at aol.com DeltaGraph at aol.com
Date: Sat Jun 4 10:42:32 CDT 2005

>I am not an HVAC person, but you need to make sure you have enough flow
>across the AC coils or they will freeze up.  It sometimes happens when
>you let your filters get too dirty.

Kip, I am not an HVAC person either, but I do now that most home AC units 
evaporators operate with temperatures above freezing when sufficiently  charged 
and when expansion orifice is not clogged in system resulting in too low of a 
refeigerant pressure in evap. coils.

Automotive evaporators are usually run below freezing, I would guess to 
conserve evaporator core size and weight and I never thought about it, but your 
dirty filter idea is maybe part of the reason for each system's characteristics.

In the car you have less filtration going on so maybe more success in running 
a super cold evaporator. In a house a slightly sub-freezing evaporator would 
tend to ice if not enough air-flow. So maybe to help combat that problem, like 
if someone forgets to clean filter. The system will not freeze up unless also 
has problems listed above. 

I love speculating about things that I don't know much about, but I have had 
two experiences with evap. coil freeze up home unit, biz unit and car. related 
to both blocked expansion valve (orifice) and insuf. charge. 


>I believe you get the most efficiency with more air moving across the
>coils, so more airflow is better. 

I agree.  Sounds like one of those laws of thermodynamics.  


>I am assuming your duct is round. So 14 inch duct has a cross section of
>154 Sq Inches. A 10 inch Duct has 79 Sq Inches.  So you need at least
>another 10 inch Duct. A 16 inch duct is 201 Sq Inches.  The cheap 8 inch
>ducts are only 50 Sq Inches. The extra duct also helps distribute the AC
>around the room.

Yeah, I jumped on the cross sectional area thinking about resistance in a 
wire...
I found it interesting that the air flow is not quite proportional to cross 
sectional area by my home depot chart.

8"    50 sq in  230 CFM  (4.5 multiplier)
14" 154 sq in 1000 CFM (6.5 multiplier)

CFM rating @ 0.1 static pressure (units? Pascals?)

I am thinking somehow maybe circumference plays into disturbance in air flow 
creating backpressure.

Anyway according to my holy grail slip of paper from home depot you get more 
bang from your buck that adding cross-sectional area alone.

When it gets a little warmer I was thinking we could get club to spring for 
about $60 and get 25' of 12 inch flex duct from home depot. If it does not work 
we can probably somehow get that big slinky back in the box for a return. The 
insulated flex duct they sell these days is really pretty. Like a sliver 
Mylar husk over the insulation with a plastic covered slinky inside. I want to 
find an excuse to run some around in my house that is visible. 

- Ron










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