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Thanks everyone for the replies/info you guys are great. I am looking hard at all the options, one friend recommended three 100 ft. coils of 3/4 copper in parallel. This would be pretty straight forward but 1200+ dollars it is hard to handle on a fixed income but must do something. Thanks again and i'll be watching for further posts.
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That will give you about 59 ft.^2 of heat transfer surface.
Assuming you would immerse those coils at in the tank, and because the film coefficient on the tank side of the tube will be due to natural convection (not due to pumping as in the HX), the "U" value I wrote of yesterday will drop from My assumed 250 BTU/(hr.*ft.^2*deg. F.) to something like 20 - 40 BTU/(hr.*ft.^2* deg. F.) if you're lucky. Reason: The overall "U" is a largely a function of the film coefficients (heat transfer rate) of the fluid on each side of the tube. The film coefficient on the tank side of the 300 ft. of tubes will be due to natural convection. Natural convection film coefficients are usually about an order of magnitude or 2 below forced (pumped) film coefficients.
With a "U" of, say, 30 BTU/(hr.*ft.^2*deg. F.), and all other fluid conditions kept the same as my nonsense from yesterday, a SWAG at the required length of 3/4" copper:
Required area = A ~ =Q/(U*temp. diff) = 57600/((30)*(8.5)) = 226 ft.^2
I ft. of 3/4" tubing has an area of .1963 ft.^2. --->>> 226 ft.^2/.1963 (ft.^2/lin. ft.) = 1,151 lin. ft. of 3/4" tubing, probably split into somewhere between 8 and 10 parallel circuits to keep velocity and thus pressure drop to some reasonable level and avoid replacing what would become an underpowered pump. .
If what you had was fit for purpose, I'd still look around for a replacement in kind.
One other thought and THIS IS NOT A RECOMMENDATION, but, IF - A big IF- you circulate the tank water directly through the solar array the system gets simpler and the HX replacement becomes unnecessary.
HOWEVER, other considerations must be addressed, mostly the problem of freezing weather and consequent destruction of the collectors.
There are ways to handle (avoid) the freeze problem, but probably none of them are as effective, as safe, as reliable, or as workable as the use of a secondary loop with non freezing fluid as you had.
I only offer the comment of a direct system as one possibility since you wrote you are looking at all the options. However, there's lots of reasons why I'd not recommend it, especially for an existing system - for starters, too many redesign considerations for me, and that means more to check and more to miss.
Take what you want of the above. Scrap the rest. -
You can purchase a brazed plate HX for under $300, this would require another pump to circulate water from your tank though the HX. Just remember that the BTU rating on the HX isfor 180 degrees, adjust your sizing to your expected average solar water temp for your use.
This would still most likely be your most cost effective option for this job.
K.I.S.SComment
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I spent several years working for a co. well known as the origin of plate and frame HX. They have goods/bads.
Re-gasketing tends to be expensive because the prod. is somewhat proprietary so they have you by the short & curleys.
Also, Plate & Frame can be more prone to leakage than other types of heat transfer products in non steady state applications. Reason: In one sense, they can be thought of as a stack of large gasketed joints. They do better with one or a narrow rage of temps. for operating conditions. Changing temps. can cause things to loosen up some. Solar applications often have variable operating conditions and usually shut down at least once a day --- > increased possibility of leakage. Any leak is not good. Glycol leakage is never good, but a small consideration is that glycol is toxic to (but apparently liked by) a lot of critters.
Also, some attention needs to be paid to the temp. limits of the gasket material. If that mat. is buna or some lower quality material, collector stagnation conditions, which must be assumed will be reached at some times for any # of reasons, can cause temp. excursions beyond the temp. rating of the gaskets. Stress, deterioration and leakage or other failure must be anticipated, considered and dealt with.
As for rating, each new or different application requires a re-rating due to different operating conditions. I don't know of any specific temp. or set of conditions that are a mantra for rating a HX. Indeed, the very term "rating" means a performance estimate based on different conditions or a different/new application for an existing HX. That "rating" scheme being somewhat the converse of a "design" situation where a set of design conditions and a desired output are used to develop a somewhat unique design based on what is usually a somewhat narrow range of operating conditions.
As a general statement, most of the heat exchanger products I've seen that are used for residential and small commercial solar thermal applications are poorly made products done with little thought or knowledge to what would be considered sound engineering and treated mostly as a nuisance, or a necessary evil and usually little more than an afterthought.Comment
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