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Maximal working temperature of Solar hot water system?

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  • Maximal working temperature of Solar hot water system?

    I was wondering if somebody could clarify to me how is maximal working temperature of solar hot water system defined?

    On a couple of places on the web I found that one of reasons to use this limit is to prevent the water reaching the boiling point (100 in Celsius, 212 in Fahrenheit). So you limit the tank maximal water temperature to 95 degrees (203F). Of course if water is the heat transfer fluid.

    But I also found that some solar hot water systems have maximal working temperature as low as 49 C (120F). What determines the exact shw maximal working temperature?

    Type of heat transfer fluid used? Quality of the pipes, tank? Is there some table which shows this?

    Thank you for the reply.

  • #2
    Originally posted by bernard View Post
    I was wondering if somebody could clarify to me how is maximal working temperature of solar hot water system defined?

    On a couple of places on the web I found that one of reasons to use this limit is to prevent the water reaching the boiling point (100 in Celsius, 212 in Fahrenheit). So you limit the tank maximal water temperature to 95 degrees (203F). Of course if water is the heat transfer fluid.

    But I also found that some solar hot water systems have maximal working temperature as low as 49 C (120F). What determines the exact shw maximal working temperature?

    Type of heat transfer fluid used? Quality of the pipes, tank? Is there some table which shows this?

    Thank you for the reply.
    All of the above and more, depending on who's doing the defining.

    1.) Above all: SAFETY FIRST ! NEVER NEVER EXCEED THE MAX. ALLOWABLE WORKING TEMP. OR MAX. ALLOWABLE WORKING PRESSURE OF ANY EQUIPMENT ! DOUBLE CHECK THAT !

    2.) For domestic hot water (DHW) systems without solar heating devices, at least in the U.S., many working systems are usually code limited by a temp./pressure to about +200 deg. F. and +150 PSI.

    Most solar thermal devices used for heating fluids under pressure such as for DHW applications are rated for the same pressures as DHW common and non solar DHW systems. The temp. ratings may be higher for the solar components. They are usually the same or higher, and better not be lower.

    Other countries I'm familiar with have similar requirements.

    3.) Be careful of definitions. A "Max. allowable working temp." of 120 deg. F., may be referring to some component. FWIW, that seems pretty low for a domestic water heater. Sounds like a pool water heater or some other low temp. application, not for DHW in the usual sense.

    4.) Unless specifically designed for preheating only, most DHW systems using solar devices are capable of, and usually and routinely exceed +120 deg. F. operating temp., often by a fair amount.

    5.) The max. attainable temp. of a solar water heater is a function of the design. Under no flow (stagnation) conditions with no, or malfunctioning, relieving devices, temps. in a flat plate under full sun can easily rise to ~ 150 deg. F. or more above the ambient temp. If the working fluid is H2O and the temp./pressure is high enough, depending on the press./temp. relieving device, the water will boil off, and/or H2O will leak from the relieving device (you hope).

    There are tables, charts, resign methods and a whole subculture of the engineering profession dedicated to the safe design of boilers and pressure vessel systems. I spent most of an engineering career there. I'd suggest you stick with reputable equipment designed for the application. This may not be rocket science, but fooling with temps. and pressures that can do damage is nothing to be trifled with.

    You can limit the max. tank temp. to anything you want, but the solar collector and the sun have their own ideas about how hot they will get.

    Comment


    • #3
      Thank you for the profound reply J.P.M.

      Originally posted by J.P.M. View Post
      Most solar thermal devices used for heating fluids under pressure such as for DHW applications are rated for the same pressures as DHW common and non solar DHW systems. The temp. ratings may be higher for the solar components. They are usually the same or higher, and better not be lower.
      Why they may not be lower (than 200F) if I may ask?

      Comment


      • #4
        Originally posted by bernard View Post
        Thank you for the profound reply J.P.M.



        Why they may not be lower (than 200F) if I may ask?
        You're welcome. Profound is not usually the first word most folks use to describe my mental spoor unless/or used as an adverb (as in profoundly) and followed by a four letter and/or other derogatory words/phrases/epithets.

        On temperature: Most DHW systems in the U.S. are commonly set to heat H2O to about 60 deg. C to 70 deg. C. by thermostatic control. Most code mandated safety relieving devices for DHW systems are designed to operate at something below the "boiling point" of water at sea level atmospheric pressure (= ~ +100 deg. C.). The logic is more involved than that, but that's a decent working tool.

        Most DHW tank systems have other safety measures in the heating portion, usually using CH4, other gas input (propane) or electricity, to limit input so that heat input is cut off before some high temp. is reached. The relieving devices are sort of a "last ditch" safety barrier. Solar devices for water heating sort of dove tail into those type systems' requirements.

        Some of the particulars can be and are a bit complicated, but the system limits can be defined lower than the limits set by design or code. However, those system limits based on pressure, temperature or other considerations can NEVER be higher than the lowest design limit(s) of any single component.

        For particulars, refer to the A.S.M.E. Boiler and Pressure Vessel Code, mostly Sec. II, IV, VI and predominantly VIII.

        Comment


        • #5
          Thank you for the reply once again and the references, will definitively have to check them out.

          Just this please:

          Originally posted by J.P.M. View Post
          On temperature: Most DHW systems in the U.S. are commonly set to heat H2O to about 60 deg. C to 70 deg. C. by thermostatic control..
          The 60-70 deg C temperatures are sanitary fixtures delivery temperatures, not minimal temperatures of the storage tank? Are these two temperatures the same or do they exibit some difference due to heat loss in piping from the tank to the sanitary delivery fixture?

          Comment


          • #6
            Originally posted by bernard View Post
            .....Why they may not be lower (than 200F) if I may ask?
            The COMPONENTS of the solar heater, may somehow become backfed from the domestic system, and you don't want any parts to melt.
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            Comment


            • #7
              Originally posted by Mike90250 View Post
              The COMPONENTS of the solar heater, may somehow become backfed from the domestic system, and you don't want any parts to melt.
              Thank you Mike. Would you be a bit more precise please? How will the components get "backfed"?
              And how will parts get melt if lower than 200 deg F working temperatures of the tank are used?

              Comment


              • #8
                Originally posted by bernard View Post
                Thank you Mike. Would you be a bit more precise please? How will the components get "backfed"?
                And how will parts get melt if lower than 200 deg F working temperatures of the tank are used?
                If you have direct fluid flow between the solar thermal system and the DHW fluid or a heat exchanger between the two systems, it is always possible that a user error or a component failure will cause normally heated DHW to heat parts of the solar PV system to a higher temperature than they would otherwise reach just from thermal input.
                Since a DHW unit may experience a control failure and increase its pressure or temperature above the normal working levels, up to the point that the thermal/pressure safety relief valve opens it makes sense that the solar thermal system components exposed to that same temperature and pressure be rated for those extremes too.

                Not that when a heat exchanger is used the solar system will NOT be exposed to high DHW system pressures. And if the circulating pump shuts down it is unlikely that the most distant components, such as the thermal panels, would be exposed to high temperatures either.

                It is perfectly sensible engineering to make a system with a lower temperature and pressure rating from components with a higher rating, especially if there are no intermediate specification components available.

                In particular, note that PVC pipe is not rated for the temperature and pressure range common to DHW systems. CPVC pipe and Pex pipe, as well as metal piping, can handle these conditions. But even those specialized plastics probably cannot handle 200 degrees at high pressure. So the assumption is made there that the relief valve in the DHW heater will prevent the downstream piping from being exposed to high temperatures, at least not for extended periods.
                SunnyBoy 3000 US, 18 BP Solar 175B panels.

                Comment


                • #9
                  Thank you inetdog.

                  But if I understood you correctly, everything you said:
                  "DHW part failure, solar thermal components exposed to same higher temperature, pressure"
                  is related with higher working temperatures (than 200 def F).

                  My question question was related with a quote from J.P.M.:

                  Originally posted by bernard
                  Originally posted by JPM
                  Most solar thermal devices used for heating fluids under pressure such as for DHW applications are rated for the same pressures as DHW common and non solar DHW systems. The temp. ratings may be higher for the solar components. They are usually the same or higher, and better not be lower (than 200 deg F).
                  Why they may not be lower (than 200F) if I may ask?
                  So now I am confused. Is your answer related higher working temperatures than 200 deg F?

                  I apologize in advance if I misunderstood you.

                  Comment


                  • #10
                    Bernard: I believe it would help me (and perhaps others, but I certainly can't/won't speak for them) if you could give me a little bit of background on why you asked your original question. Maybe I'll be able to back up a few steps and not feel like I'm missing the mark by attempting to answer questions you may not be asking.

                    Thanx in advance.

                    J.P.M.

                    Comment


                    • #11
                      Hi J.M.P.

                      I started using SolOpt shw system design application.
                      One of its inputs is maximal working temperature of shw system.

                      I read a couple of introductory articles on shw system components, and have understood that this working temperature is also significant for triggering the discharge of the surplus heat by importing new cold water into the tank.

                      I tried contacting the author on explanation on how to define this value, but haven't got a reply ever since.

                      The reason for using SolOpt is purely out of curiosity towards SHW systems (and PV systems). I am not an engineer do not intend to make shw system in reality. I am not trying to become a Solar systems expert, but to inform myself a bit further in the present changing world of finding other energy alternative sources.

                      So no problem for asking J.P.M.
                      I mostly understood the replies you gave, just if you could clarify this part please:

                      Originally posted by bernard View Post
                      Originally posted by J.P.M.
                      On temperature: Most DHW systems in the U.S. are commonly set to heat H2O to about 60 deg. C to 70 deg. C. by thermostatic control..
                      The 60-70 deg C temperatures are sanitary fixtures delivery temperatures, not minimal temperatures of the storage tank? Are these two temperatures the same or do they exibit some difference due to heat loss in piping from the tank to the sanitary delivery fixture?

                      Comment


                      • #12
                        Solar domestic hot water systems (SDHW systems) are, as the name states, for the purpose of heating water for domestic use using solar energy.

                        Often, solar thermal collectors and associated piping are added to existing conventional domestic hot water (DHW) systems.

                        Those conventional systems are designed with certain max. (and min.) pressure, temp. and often other restrictions or limits. Those limits are usually part of what are called design conditions and are the conditions under which each component must operate safely. The design conditions are meant to be representative of expected operating conditions plus some "extra" conservativeness to try to account for screw-ups, mfg. tolerances, bonehead moves by users, etc.

                        In the U.S. and using customary, or British system units, most/many DWH systems in homes have storage tanks of between, 30 and 120 gal. with heating elements energized by electricity or fossil fuel. The common operating conditions are usually at a pressure of 40 to 60 P.S.I, and a temp. of between 60 deg. F. and about 160 deg. F. or perhaps higher. Those are often the "expected" conditions to be encountered in everyday use.

                        The tanks in such systems are usually A.S.M.E. pressure vessels with a design pressure of +150 PSIG and a design temp. of 300 deg. F. The DHW system must meet any applicable local codes. Such codes often follow and use ASME requirements including those for safety valves and relieving devices. Commonly those limits for pressure and temp. are 150 PSI and and 200 deg. F. If either or both of those conditions exist, the safety devices will (hopefully) operate and relieve the condition(s).

                        IF a solar heating system is added to a conventional system, safe, good and customary practice, at least as I learned it, would be, unless conditions dictate otherwise, to design such a system using design pressures and temp. that are appropriate for the expected operating conditions of the solar components, and under NO circumstances less than the design conditions of the conventional heating equipment the SDHW is connected to.

                        Often conventional solar flat plate collectors will have design temps. (expected or possible max. (or min. for freezing) temps.) that are higher than conventional DHW systems. The reason: At some point, either by design or accident, a reasonably well designed liquid cooled flat plate will have a pump failure for some reason and the collector(s) will get up around 250-300+ deg. F. Hopefully, other safety devices near the collector will operate under those conditions.

                        A SDHW system can be at a different pressure "separate" from a conventional DHW system. In that case, the collector will be in a separate "loop", and will have thermal contact with the conventional "loop" through a heat exchanger. A heat exchanger is a device, as you may well guess, that transfers heat from one fluid to another, usually without physical contact between the fluids, where the collector "loop" fluid, often a glycol/H2O or other fluid passes through one chamber of the heat exchanger, separated from the potable water flowing through the other chamber by a pressure boundary or wall. Thus, a heat exchanger with 2 chambers can and usually does have 2 design pressures and 2 design temps. Often, both sides carry the more severe of the two conditions, but not always.

                        If I were to guess, I'd say you might benefit from an understanding of how solar thermal works and not get too hung up on definitional terms. For example, there are terms such as "minimum operating temp." and "minimum design temp.", "operating pressure", "design pressure", "maximum operating design pressure", "test pressure" and many more. Those terms all have very specific meaning in pressure vessel design that may or may not be the same as is commonly used and understood.

                        If you do not want to get involved in the minutia as you say you do not want, but seem to be in up to your neck, I'd respectfully suggest you keep the cart before the horse, stick to the basics and don't try to get too involved in what amounts to pressure vessel design and heat transfer. Start with the simple stuff.

                        Take what you want of the above. Scrap the rest.

                        Comment


                        • #13
                          Thank you J.P.M.

                          That was an astonishing reply.

                          Kind regards,
                          Bernard.

                          Comment


                          • #14
                            Originally posted by bernard View Post
                            Thank you J.P.M.

                            That was an astonishing reply.

                            Kind regards,
                            Bernard.
                            You're welcome.

                            Opinions vary.

                            Regards

                            Comment

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