Solar Space Heating Storage Tank Size [RE-wrenches]

Dean T. Newberry deant at dcn.org
Mon Mar 8 21:11:42 PST 2004


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Hi Travis,
Heat transfers occur from conduction, convection and radiation.
   In a water tank, conduction and convection are the significant modes 
of transfer. Consider the heating mode. Heat is conducted to the water 
nearest the heat exchange surface. The water layer nearest the heat 
exchanger surface absorbs heat and rises. Cooler water from the bottom 
of the tank.rises toward the heat exchange surface and is itself warmed. 
The process of  heating water rising and cooling water falling is 
convection. The rate of heat transfer from the heat exchanger to the 
water in the tank is governed by thermodynamic and fluid movement  laws 
involved in the process.         Higher temperature difference higher 
heat transfer, thicker fluid slower heat transfer etc. To increase the 
rate of heat transfer, raise the primary temperature, increase the area 
of heat exchanger, thin the fluid, or stir the tank to simulate thinning 
the fluid. Tradeoffs are capital cost, operating cost, maintenance cost, 
all judged against the system lifetime.

For example: 
    Take two warm cans of  beer.
        Place one in a bucket of ice water.
        Place the other in a sheetrock pan of ice water and spin it with 
your finger for one minute.

Drink the beers and report back which one was colder.
Do this at the end of the day.

cul  deant


   
Travis Creswell, Ozark Solar wrote:

>Hello,
>
>Assuming the internal heat exchanger is properly sized I don't understand
>why you would want to stir the tank up.  Wouldn't the system work better by
>taking advantage of stratification?  For example, you would have the hottest
>water near the top ready for use.  If you stirred the tank up wouldn't you
>risk not having hot enough water to be usable for space heating?  Sure there
>might be more BTU's in the tank but your back up heat source would have to
>bring the temp.
>
>A drain back system would eliminate the need for the heat exchanger and, in
>my experience, mixes the tank up.
>
>I used flat plates when an external heat changer was the only option. They
>simply put every other heat exchanger to shame in terms of efficiency.
>
>Regards,
>Travis Creswell
>Ozark Energy Services, Inc.
>
>
>
>
>-----Original Message-----
>From: Dean T. Newberry [mailto:deant at dcn.org] 
>Sent: Sunday, March 07, 2004 9:07 PM
>To: RE-wrenches at topica.com
>Subject: Re: Solar Space Heating Storage Tank Size [RE-wrenches]
>
>Hi y'All
>There are a couple of ways to transfer more energy from a heat exchanger 
>in a tank.
>The first is to move the water in the tank Grunfoss makes a  vertical 
>axis special pump for this. Use a relay to slave it to circulation 
>demand on the loop side of the heat exchanger, you only need to stir the 
>tank when exchanging heat in or out. .
>RBI sells replacement boiler pipes, copper fin tube in various sizes. 
>The copper fin tube is what Heat Transfer Products uses in it's tanks to 
>promote heat exchange.
>I'll see if I can find the formulas for calculating the heat exchange 
>given a delta T, heat exchange area and rate of flow on each side of the 
>heat exchanger.
>Since you are going to pump both sides of the heat exchanger it might be 
>cheaper and easier to control with a flat plate heat exchanger.
>
>cul  deant
>
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-- 

 
* Cooperative Community Energy* 	*Dean T. Newberry*

430 D Street, Davis CA, 95616
Tel: 530 758-6064
Fax: 530 758-3684
Email: deant at dcn.org
Web:  CCEnergy.com       
      Talbott Solar <http://www.dcn.davis.ca.us/go/deant/>

Contr. Lic. # B-667908

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