Heat transfer rates thru indirect coils

Karl_Northwind

Has anyone out there any curves or charts that can assist me in calculating the heat transfer thru an indirect tank's HX? 



I'd like to figure out how many BTUS i can reasonably get thru the top coil in a HTP superstore contender 119 SB, with say a 10-15 deg delta T from one side of the coil to the other.



thoughts? Rules of thumb I haven't heard?



thanks

Karl

Paul Pollets

Recovery ratings

Most manufacturers have a rating sheet for the tank(s) showing recovery rates based on BTU input and degree rise from incoming cold water to outgoing hot water. The 119 Superstore will give you "first hr. rating" at 308 gphr at 140 degrees with an input to the coil of 180 degrees. Incoming cold water to the tank is 55 degrees with a 90 degree rise (according to their specs).  If you call the rep, they may have further charts that show detailed temperature variations vs. input for the products. 

CMadatMe

Transfer Rate

Your question is vague. Let's assume you supply/return piping from the indirect is 1" copper. On a 10 degree delta-t you can get 45,000 btu's through the pipe.



 To heat 120 gallons of water to 120 degrees with an incoming cold water temp of 50 degrees you need 70.000 btu's. On a 20 degree delta-t you can deliver 90,000 btu's through 1" copper.. Do you need more btu's? Then try a 30 degree delta-t. You can get 135,000 through the pipe.  Of course all of this is relevant to your circulator sizing.

BobbyG

pulling btu's from the top coil

I think he wants to use the top coil to "gain" heat for a heating zone?

Have the bottom coil heat the tank from some means (like wood-fired boiler) and have domestic hot water in the tank, but then use the top coil for a heating zone?  Is that what your asking about? 

If it is, that is a very good question.  I have only "envisioned" a smaller zone like 30-40,000 btu but don't have any hard and fast numbers in writing to tell me this is correct.  I haven't ever had to look into it yet either.  But I am curious.

Karl_Northwind

Transfer rates.

you got it Bobby G



I have built a couple of systems that way.  I send the solar fluid thru the bottom larger coil, havepre-heated water in the tank, and if the tank is hot enough, and the space is calling for heat, I run the floor fluid thru the top coil.  I have a little buffer between the solar and my heat delivery system. 



I've only done this with really small systems like some tubes under a bathroom floor, under 100 SF.  



it works fine.  What I'm looking for is at what point will it not work satisfactorily.  that's where actual engineering comes in. 



WOW.   30-40Kbtu is a small zone?  I forget that you guys are all big time heating guys.  the houses we tend to work on have loads less than 40KBTU in central wisconsin.

my house in the design stage right now has a load of 22Kbtu for 1600 SF, and that is the topic of my next post.



Keep the info coming.



thanks,

karl

hot_rod

20,000

BTU/hr is a rule of thumb that HTP once told me. Unless you have a DHW recirc pump that upper coil is in still water.



When youheat a solar or indirect tank from the bottom the warmed water rises. I wonder if the upper coil causes the cool water to be replenished by warmer water from ???



Heat exchange is greatly improved with two pumped flows. An external plate HX with a small recirc pulling from the top 1/3 of the tank is another option.



I have my Phoenix setup with a Taco HX Radiant Mix HX and it moves 30,000 without a problem.



hr

Karl_Northwind

HTP COIL

actually 20,000 btu would be fine for the types of applications I am using this top coil for.



the big question is what delta T the coil wall is under.  I'd be OK with a 20 deg delta T. 

I'm disabling the coil when the tank top temp drops below 110.  we're just dumping a little heat into the floor from there.



thanks

Karl

hot_rod

the delta T

will depend on the flow rate and the return and tank temperature. IF you could maintain the temperature around the coil always at 120 for example, and you could calculate the flow rate you, and you knew the HX efficiency, you could get close.



But the tank temperature will always be changing, as will the return as the floor warms, so the delta T is always moving.



I think this is the goal of the Delta T circulator the Taco has introduced. It watches and maintaining a selected delta T by varying the speed of the circ.



hr