Direct-Expansion Water-Source Evaporator Temperature 1

[microhydro]

I'm extracting heat from a 37°F stream flowing at 220 gpm. I plan to use copper coils immersed in the open flow as an evaporator. Conventional wisdom suggests that I keep the evaporator temperature high enough to prevent icing. However, the limiting convection heat transfer coefficient is on the refrigerant side [23.4 BTU/(hr•ft^2•°R)]. The water side of the evaporator is much better[758 BTU/(hr•ft^2•°R)]. So, I'm thinking that I could run the evaporator much colder, say 0°F. My guess is that ice would form until an equilibrium between ice formation and ice melting occurs. Thus I would see much higher heat transfer with the larger delta T (37°F versus 6°F). Does anyone have ideas or experience with this?

 

[MintJulep]

Ice is a good insulator.

 

[coolbydesign]

Food for though


1)For every two degrees drop in SST (saturated suction Temperature) capacity drops required power increases.
(2010 ASHRAE Handbook - Refrigeration Pg 1.2)
2°F drop = Capacity 96.4% Required Energy 104.8%

This table is for line loss, however it applies to evaporator temperature as well. These numbers are for R22 however, numbers are similar for all refrigerants.

2) Ice is a good insulator, and will simply reduce your external U value to match your internal value.

3) Check your U values. Refrigerant in plate, free flowing external water. The last heat exchanger I bought was specified with a U value of 96 btu/sqft/°F/hr.... Material 304 stainless 18gauge
R134a internal
Water external.


Good luck

 

[microhydro]

The problem I'm trying to solve is that to collect 25,000 BTU/hr from the creek, I need 175 sf of HX area, about 1400 ft of 3/8 copper tube. The pressure drop would be about 7 psi. This length seems excessive.

1. Hmm. I see what you are saying. At 0°F my suction pressure is half what it is at 31°F, and thus the evaporator Q is about half.

I tried 20F for the evaporator, and found my system capacity reduced to 20,000 BTU/hr, but the evaporator coil length also reduced to 550 feet, which would work.

2. Then, yes? Ice will form on the evaporator coils in the creek flow and reach a steady state?

3. I used Bo Pierre's correlation for the evaporator refrigerant coefficient. It was about half of the Dittus-Boelter value. The water side coefficient is a Dittus-Boelter value for a 4" tube in tube. I figure it is just a ballpark value.

 

[vpl]

Is this related to the system in your previous posts (thread391-255618 and thread391-271060)?

What's inside the copper coils?

Patricia Lougheed

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[coolbydesign]

""1. Hmm. I see what you are saying. At 0°F my suction pressure is half what it is at 31°F, and thus the evaporator Q is about half.""

No actually, the COMPRESSOR capacity will be half at the lower SST, the evaporator capacity increases with the lower temperature.

""I tried 20F for the evaporator, and found my system capacity reduced to 20,000 BTU/hr, but the evaporator coil length also reduced to 550 feet, which would work. ""

2 Check your calculations... Q=U A LMTD
If the SST increases from 0 to 20 degrees. your LMTD will drop by about 1/2 and for the same capacity your AREA should double... not decrease.

If the area of your tube correlates to 1400 lineal feet of 3/8 tube.
Then you have many parallel circuiting choices. Ie
1 circuit 1400 ft
2 circuit 700 ft
10 circuit 140 ft
20 circuit 70 ft
40 circuits 35 ft.
There is always a circuiting arrangement that will work.
Use a TX valve with distributor and it should work just fine.
1/2 inch tube can also work 0.131 Sq ft/ lineal ft. this would reduce the length somewhat and give additional circuiting options.

As for pressure drop, remember this is an evaporator, with two phase flow, you need two phase correlations to calculate pressure drop.