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Using Heat Exchangers 01.03.2011



Date: 01/03/2011

Another edition of our SunMaxx Solar webinar series. Surprisingly in Upstate, New York, we’re having a very good solar day. Our collectors as of right now are about 150 degrees. We’re dumping into our forced air furnace, so we’ve reached our set point. Our domestic hot water tank is already maxed out at 135. So it’s one of those rare beautiful solar days in not so sunny Upstate.

I want to spend the next half an hour talking about heat exchangers. Now, there’s a lot of debate now in the industry about the effectiveness of internal coils versus external heat exchangers, the pros and cons of each. And I’d like to make it clear with everyone that over the next 29 minutes now, I really can’t cover too much in too much detail but I do look at this as a good way to begin or supplement an existing learning experience. So, I’d like to welcome you to ask questions as we go through. And certainly, email me any particular questions that you have afterwards and I can turn your attention to some other resources. Okay? So, I do have a bit to cover here so I’ll get started. Please do ask me questions if you have anything you’d like to – you’d like me to cover. Can I just get a confirmation from someone that you can hear me okay? Great. Okay, super. Thank you.

Alright. Now, generally there’s two types of heat exchangers. Those that are outside of the tank and those that are inside the storage tank. Now, some of you may have your preferences. Each serves different purpose. Outside of the tank, we’re generally talking about braised plates or also known as plate and frame exchangers, shell and tube heat exchangers, which tend to be stainless steel or tube within a tube heat exchangers. These require secondary pump. Internal exchangers most often are the smooth copper. They’re coils of smooth copper. Occasionally, we see a corrugated stainless steel and as well as finned copper exchangers. Now, the finned copper maximizes the space required. They have an extremely high heat exchange capacitance.

Okay. Some other miscellaneous exchangers that we can talk about are the Solar Wand. I’ll go into a little bit of detail on the Butler Max Solar Wand and how effective that is at small domestic hot water systems. The Sidearm thermo-siphon, this is an old school method of heat exchange. Typically mounted on the side of a solar tank which utilizes convection and a dropping – a more dense colder fluid drops into the exchanger which causes the heat exchange from the solar loop into the tank and the colder fluid drives the convection cycle which forces the warmer fluid back to the roof to pick up – exactly.

These have been gaining some attention, the Sidearm thermo-siphons. Some manufacturers are beginning to look into these and perfect the old school style. You know, as a manufacturer, SunMaxx has not quite found a particular use for them that outweighs the traditional heat exchangers that we have. But we’re still open to, you know, product updates.

Another one that is actually being used right now as I speak is the water to air exchanger. This is like a car radiator. We have one right now. It’s mounted in our cold air plenum which is taking heat from our solar loop directly the glycol on one side and the air on the other side. So more specifically, our water to air heat exchanger is more appropriately the heat transfer fluid to air exchanger.

Okay. Now, let’s talk about internal coils for a moment. I like internal coils. These to me are a simpler design and any time you can have a simpler design, there’s less room for failure. And not only a simpler design but it’s easy to size up an internal coil, especially when you have custom-built tanks like the non-pressure tanks from STSS. Okay. Most often, this is not always the case, but most often these coils are three-quarter-inch copper. And if I’m using a three-inch copper coil, I can size up a copper coil with the production of my solar loop per square foot equals the heat transferability of one linear foot of coil.

So basically, if I have 100 square feet of collector area, I’m going to use a 100 linear feet of three-quarter-inch copper. That’s going to ensure that I never over produce. The difference in cost is minimal. It allows me for some, if I want to expand my system, if for some reason I want to increase my production with reflectors or anything like that, one foot to one foot is generally the rule of thumb that SunMaxx practices based on our experience with these non-pressure tanks.

Now, Vaughn also makes these internal coils that are finned copper. And the heat exchange rate in finned copper is about ten times that of smooth cooper. So, it requires a coil ten times smaller. And this is really important when you’re trying to maximize your heat exchange in a small tank, for example, a 50-gallon tank. Traditionally, if I use a 50-gallon tank, that size tank is going to limit the size of the coil that I can choose which would in turn limit the size of the collector field. By using finned copper, these are submerged in replaceable coils from Vaughn. The heat exchange capacity is about ten times that of smooth copper.

Now, when I size up internal coils for domestic hot water flow, the most important consideration there is the flow rate on the domestic side. Alright. So, each coil, assuming that it’s a three-quarter-inch copper, it’s going to accommodate about 4 to 5 gallons per minute of flow rate. That flow, 4 to 5 gallons, will allow me for just about a 90% heat exchange effectiveness across the coil assuming that my ground water is coming in at 50 and my solar loop is at 120. So, as I increase that flow rate, you obviously will decrease your heat exchange effectiveness. So, for domestic hot water loops, if my limiting factor is flow rate, I can pipe several in parallel to achieve the same results. So in this little photo you see here, we’ve got – in that total tank, there’s eight coils. So those eight coils at 5 gallons a minute allows me to preheat about 45, 40 to 45 gallons of cold water per minute. So, that’s a pretty high flow rate for larger jobs.

So, for solar loop, sizing coils, one foot of coil per one foot of collector absorber. Okay, and be careful about sizing up your collector field based on gross area. Because if you size that based on gross, for example, on evacuated tube, SunMaxx 30 as an example has a gross area of 52 but an absorber area of only 30 square feet. So, you size these based on an absorber area. And for domestic hot water loop, you size that based on the required flow rate not so much the differential that you’re looking for but based on the flow rate.

Okay. Here’s an example of how we use a non-pressure storage tank with the coils. So for the solar loop, the solar loop are the two loops that are tied in the pumps off from the pump stations. So, we have eight coils for the solar loop. All eight coils are tied in parallel returning back to the collector from the bottom of the tank. So if you notice, the hottest fluid should go in to the top of the coil and the coldest fluid comes out of the bottom and back to the collector. You want to remove as many BTUs from that solar loop as possible so you always go in counter-current flow relative to stratification. So, inside the storage tank, as the heat moves up, the solar loop should move in opposite direction. And then the two coils on the top are for the domestic purposes. So, we have two coils in parallel. Since we have two, you know that each is capable of 4 to 5 gallons a minute so you a total domestic flow of less than 10 gallons a minute.

What’s nice about these non-pressure tanks is they are customizable. So depending on the stratification that I’m looking for or the different quality of BTUs, I can mount these coils anywhere in that tank that I wished to and I can achieve, and I can pull a high quality BTUs out of the tank or I can pull low quality BTUs depending on the level of that tank that I insert the coils. Anybody have any question at this point?

Okay. Here’s an example of the internal coils. I’ll take a look at that, Drew. That’s a good point. Give me a second and I’ll take a look there. Now, the use of internal coils for solar loop, like I said, my preference is internal coils because of their simplicity. You may pay a little bit more especially for the non-pressure storage tank when you’re getting into commercial jobs. Bu they do present a much simpler approach. And in the end, and again, this is my opinion, that the simpler that we make these systems, the less points of failure that can occur. And ultimately, the more professional the installation will seem if there’s just less points that can fail. But again, that’s my opinion.

Okay. So, then continuing to look at this pressurize storage tank with the internal coils. Obviously, the bottom internal coil is use for the solar loop. The top internal coil is tied in to the heating loop. Now, as I mentioned before, your solar loop should always move in counter-current flow to the direction of the heat movement inside the storage tank. So, you want the hottest to transfer with the hottest, coldest to transfer with the coldest.

So, on the top coil, usually the top coils inside of the smaller pressurized storage place are not capable of delivering the maximum amount of heat that our load requires. Let me reiterate that point. Most pressurized storage tanks do not have coils that are sized large enough to handle heat loads for most residential homes. These top coils can supplement a heat load but they can’t replace the heat load.

So, in this particular drawing or schematic, you see a boiler that is supplementing the storage tank. Okay? So, we have our domestic hot water load coming off of the storage tank and that domestic hot water load, the only source of energy is that storage tank. The storage tank however has two sources, the solar loop and the boiler. So, rather than letting this tank delivers the entire BTU load to the house, this tank supplements the boiler and in turn, the boiler supplements the tank. Okay? So, you could take a look at that schematic and very good.

Alright. Now as I mentioned, this internal coil, internal wand exchanger, this is a pretty unique product. It comes from Barry Butler from Butler Sun Solutions and from what I understand he’s got about 4,000 wand hours as he’s calling them. Those are the customers who bought wands and accumulated the hours of use. And so, over 4,000 wand hours and he said, one return and it was under warranty. So, there is a very high degree of success with these wands. The limiting factor obviously is flow rate. Okay? He’s got a three-quarter-inch copper tubing that’s coming down off the solar return down into the bottom of the wand and then back up and out for the solar supply. So, he’s limited to relatively small systems for domestic hot water. I have sold and designed many systems that are using this wand and what I’ve seen, the thing that’s most impressive is the stratification that we’re seeing inside these tanks. It’s very adaptable for most existing hot water tanks.

Let me just read this question here. That’s correct, Adam. Yes. So, one of the things as I said that’s most impressive is the stratification. So as an example, a ream Marathon hot water tank where a 36-inch wand on a normal solar day is seeing about a 50-degree differential from the top of the tank to the bottom of the tank. So, there’s no disruption in the stratification of the tank when you use this wand. In fact, the heat exchange actually starts at the top of the wand, and so the hot is always mix with the hot just like we like it and it moves in counter current to the stratification. So, I was really surprised with the stratification that we see but it really separates the quality that allows you to use just what you need and save some for later I guess.

Alright. Another external heat exchangers, a lot of these stainless steel shell and tube that you see here are used for swimming pools. This is a perfect way to tie in to take a combi-system which is producing significant amount of heat in the winter time and then with a simple three port valve and a stainless steel shell and tube exchanger, we could pull heat off of the solar loop and dump it directly into our hot tub or swimming pool. One of the nice things about these shell and tube heat exchangers is they have two different flow capacities. So, for the solar loop, we’re using, there’s a one-inch port and on the pool loop, it’s up to an inch and a half port which means we can get about 60 to 70 gallons a minute on one side of the heat exchanger and maintain a flow of 5 to 10 gallons a minute on the opposite side of the heat exchanger.

Let me show you a diagram here that illustrates the use of an external heat exchanger. As you notice, all we’ve done with this design is remove the coil and brought it to the outside. One of the nice things about external exchangers is they are serviceable, if need be, we have areas with a lot of mineralization, so you can remove the heat exchanger. Another nice thing is it is easy to oversize an external heat exchanger. So for example, if I have a very high load for my solar loop, a pressurized storage tank will have a fixed-sized coil, and it may not be suitable. So, the only way for me to get a larger coil is to buy a larger tank. Well I may not need a larger tank, so by using internal coils you’re really limited to the size of the heat exchanger, but by using external heat exchangers, I can go from a 10 plate and double the capacity to the 20 plate for an increase in cost of about 20 percent or less. Really in the end it only amounts to less than a $100 difference. So, in my mind, one of the best features of using external heat exchangers is your ability to scale it up at a low cost. So you can oversize heat exchangers, which we recommend over sizing them anyway.

So I’ve got a couple of questions here. External heat exchangers can be dismantled easily if you use unions. Okay, so if you’re not using unions then it becomes a little bit more difficult. So you know with the additional cost, try to use unions wherever possible and valves. Then the question regarding the SunMaxx, the solar wand, that’s really rated for not so much the size of the storage tank as it is the capacity. These can transfer about 9,000 BTUs per hour. Yes, so the wand heat exchangers are capable of about 9,000 BTUs an hour heat exchange, which means they’re going to be good for about 40 to 50 square feet of collector area total. A little bit less with evacuated tubes because of their higher production per hour during peak production days.

And then the use of electric elements, regarding the stratification with electric elements, that is one of the things that most often the electric element ports come in the middle of the table range. And I would like to see electric element ports be closer to the top two-third, and then set these on a timer as to not maintain your solar preheat by way of electric element during any part of the solar day. So there is some manipulation that can happen to help increase the effectiveness of electric heating elements when you’re using internal coils.

Alright, now sizing up heat exchangers, Tim, shell tube heat exchanger can be adequate for radiant floor as long as it’s sized properly. So, all we need to know is the total load on that radiant floor and the flow rate, and then we can size up a shell tube. However, I would probably recommend, rather than a shell tube, I would recommend a braised plate heat for radiant floor heating. You’ll get a better heat exchange through braised plate than we do with shell tube, the only benefit to me is that the shell tube, being stainless steel…

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