Harry,
1) i'm not going to debate with you, especially since you are rude...2) you are wrong about this... try and take this as an opportunity to learn something...
3)think about what i and trad are saying... re BTU and temperature,which holds more heat in BTU, a small red hot needle at 4000 F or
50 gallons of water at 100F?which air flow is pulling more BTU out of an exchanger, 100 CFM at
100F or 1,000CFM at 75F?have a nice day
Mark
The only reason you claim it's neither here nor there is because you can't explain it with your rules of physics which don't pertain to this universe. A car radiator is a simple and perfectly valid example of a heat exchanger. The heat from the coolant is transfered to the air moving through the radiator. With the same amount of coolant flow, the more air you move through, the more heat that is removed. That's one reason many cars today are equipped with an auxialliary fan for the radiator. If it's very hot out, you're stuck in barely moving traffic, etc, and the coolant temp rises above the normal range, a switch turns on the fan to move MORE air through the radiator removing MORE heat with it. Doesn't get any simpler than that.
If it worked the way you claim, we'd instead be restricting the air so that the air coming out was as hot as possible. You're confusing temperature with heat. A car radiator with a high volume of air moving through it will transfer MORE heat to the air. The temp of the air coming out will be lower, but there is MORE air and the increase in mass of air more than offsets the fact that it's a lower temp.
Another example would be a simple hot water radiator in a house. With no fan you could hold your hand by it and you'd feel some hot air rising. Now place a fan there blowing lots of air through it. The air will not be as warm. Does that mean that LESS heat is being transferred? Of course not. You're getting MORE heat, but the air is at a lower temp because more mass is being heated. Capiche?
Says who? It depends on what you're using the air for. If I'm using the heat for a chemical process and want it to be 120F, then I want it to be 120F, not 200F.
If the application requires it to be at a certain minimum temperature, eg heating water in a water heater, then yes you would need it to be at a certain minimum temp.
In the stated application, the goal is to transfer the heat to air moving through a furnace to heat a home. As long as the water moving through the heat exchanger is above the temp of the water, you get maximum heat transfer with the fastest airflow, all else being held equal. I've explained this several time now. Let' say the water from the solar collector is at 130F.Start at the limit of zero airflow. The air around the heat exchanger is also 130F because no air is moving. The water temp equals the air temp, no heat is being transferred. Start increasing the airflow and now you will have air coming out that is slightly below 130F and water going back to the collector that is slightly below 130F. Some heat is being transferred. Ramp up the airflow and you get more heat transferred. Will the air temp out of the heat exchanger be less? Sure. Does that mean less heat is being transferred? No, because a lot more air mass is being heated. Capiche?
I've answered any questions you've put forth. Yet I get no reply to mine. I'll state my simple question a third time:
I give you a quiz. Our friend here builds his proposed system. He puts an auto radiator in the return side of his furnace duct work and connects it to a solar collector. The solar collector heats water to 130F and it gets pumped to the radiator. The incoming air in the return duct is 65F. He has a variable speed blower so that he can vary the airflow through the radiator.
Which airflow results in the water leaving the radiator at the lowest temp and consequently the most heat transferred?
A - 1 CFM airflow B - 2 CFM C - 10 CFM D 100 CFM E - 1000 CFM F - 2000 CFM, the max the blower capacity?
Why won't you answer this very simple question?
No, actually we are trying to extracrt as much heat from the water as possible - which can be more air with low net temperature gain - or less air with high net temperature gain. The more air/lower temp actually gets more HEAT than the low air high temperature. If you are sitting in the airstream it FEALS like you are getting more heat with the low flow/high temp option, but in reality, you will heat the entire room or home FASTER withthe more air/lower temp option. In this case, the difference would likely be marginal - but it would be there- and would be quantifiable.
And in SOME engines, even today, if you remove the restriction of the (open) thermostat the water flow increases velocity to the point it does not remove enough heat from the block/heads and you get localized overheating of the engine.
No I'm not disagreeing. IF using a "bypass" system the dampers would take the "bypass" out of the system when there is no appreciable heat to be gained from the system, allowing the solar panels to get the water hotter so the delta T would be higher, raising the efficiency - and therefore extracting more heat from the system..
Same thing could be accomplished by a set of water control valves tho keep the heat in the reservoir untill the temperature was high enough to do an efficient job of heating the interior of the house. Water valving is more complex, and the difference in efficiency re: switching water vs switching air, would be small enough to not make the complexity worth while. Shutting off the air flow through the secondary heat exchanger MIGHT increase the efficiency enough to make it worth while. It would also eliminate the problem of extracting heat FROM the return air, into the water, when the solar input was too low to provide a net heat gain.
Can't do that without water controls in a non-bypass system.
I'm NOT saying this method would necessarily be BETTER for the OP, but it definitely bears looking into for the reasons given.
Recap:
No additional restriction in the main ductwork. Smaller heat echanger could possibly be employed Less complicated water controls Possibly higher total efficiency.
Add to this, the installation might be simpler - just 2 cut - ins to the return duct (or even just one if the "bypass" augments the cold air return from heated airspace) - and for an "experimental system" it makes implementation a lot easier and more easily reversible. A simple "zone damper" like a Famco POPC or Suncourt 8 inch motorized unit , at about $70 would do the job., but a modulating unit like the Alan AZRDMOD series would provide variable control if desired, for more like $200.
Nope. That's why I call it a "bypass" Instead of restricting total airflow, I am advocating he SUPPLEMENT the airflow
Add a duct blower , like a Fantech FR225 - about 450-500 cfm for 8 inch "bypass" duct.
It would require having the circulating fan on "constant" which I do all year, just as a matter of course, for filtering and comfort purposes. So, at least for me - not a drawback.
As my 28/40KBTU furnace only runs 8 hours a day on the COLDEST days here in Ontario, and averages closer to 6 hours on low fire - I only need a total of 168000 btu heat input to keep the house comfortable..
If I have 6 hours of useable sunlight, a solar assist system could conceivably provide well over 10% of my heating needs throughout the winter, and a lot more heat, and a much higher percentage, during the "knee seasons" when more sun is available and less heat required.
The payback would still be SLOW - my total annual gas bill for heat and hot water is about $700 per year - - - but the house would be more comfortable in the early fall before I turn on the furnace, and late spring, when I shut it off.
The OP isn't the only one who's considered it - but I'd need to provide the solar panels too - which he already has.
NOT necessarily - with a higher TEMPERATURE primary fluid, the Delta T at the secondary heat echanger will be higher, so the efficiency of the exchanger will be higher, and more heat will be transferred to the secondary fluid.
No, but if the efficiency of the heat transfer can be increased by 50% by having a higher delta T at the secondary heat exchanger, it is POSSIBLE that slowing the flow rate through the primary to increase the primary fluid temperature MAY provide more actual HEAT to the house. It would depend on the efficiencies of both heat exchangers.
It also depends if we are talking a closed series loop, where the waterflow in both exchangers would be equal, or if we are talking separate loops, where the GPM flow through each exchanger was optimized. I'm ASSUMING we are talking a hybrid system, with a frost-resistant primary fluid, and THREE heat exchangers in the equation - solar to primary fluid - primary fluid to heat storage, (water) and heat storage to secondary fluid (air) where the echange fluid flows can all be separately controlled.
NOT a simple question with a simple answer that is always right.
Simple question, with simple answer that is only a SMALL part of a more complex question.
Answer F will extract the most heat from the water IF there is more heat available than can be extracted at 100cfm.. If 100cfm will extract enough heat to get the water exit temperature to within a few degrees of the duct inlet temperature, 500,000 cfm will not extract any more heat - and the return temperature of the water will not be below the 65 degree F air inlet temperature.
If the heat echanger was 100% efficient you would get 65 BTU per gallon (imperial) out of the heat echanger in one minute at 1GPM or 60 GPH primary flow rate. So to answer your "simple" question, we need to know the flow rate of the "primary fluid", as well as it's temperature, to know how much "heat" is available. There is only 1 btu of heat per degee F of temperature change in 10 lbs of water. 10 times the water flow gives
10 times the POTENTIAL heat.
That's good. I guess that leaves harry all to himself.
You have the same situation with the solar collectors as you do with the radiator. Slowing the water flow or air flow isn't going to get you more heat or improve the efficiency.
What reservoir? It would have to be one hell of a reservoir to make any difference in heating the house. I think it's simple. Either on a decent sunny day you get enough energy out of the solar panels to make it worthwhile to fire the' blower up or not. Once it starts it's probably gonna run most of the time, even on days where you just need a little heat. On colder days the furnace will have to come on too.
=A0Water
Smaller than what? How are you going to heat a house with a smaller heat exchanger? For it to work, I'd say he's going to need as large a one as he can fit practically, or pay for, etc.
Yeah, we;ve been down that path before. And you know my opinion, which is circulating air all the time in most cases is an energy losing proposition. Look at the typical duct work. In my house for example, I have long runs that go 50ft through an unfinished basement, then up two stories through outside walls. The last thing I want to do is be moving air through them where it can lose energy 24/7.
How big does he need ? A standard tube and shell heat echanger can be good for 50,000 to 60,000 BTU per square foot.. A smaller exchanger with higher air velocity can be every bit as good as a large one with low air velocity - and being "off line" or "parallel" has NO EFFECT on the duct resistance.
Well, around HERE, nobody in their right mind rund heating ducts in outside walls. And with the cost of housing, unfinished, unheated basements are an almost unheard-of "luxury".
Hardly call your house "typical" here. In MY house, themain "trunk" hot air duct runs through the laundry-room /wife's office, exposed on the bottom to the room, and the top to the main floor above - with accoustic tile ceiling. The heat ducts to the upper floor runs through the center load-bearing wall, and the ductwork to the main floor registers run between the floor joists between the main floor decking and the accoustic tile ceiling of the rec room / my office and the laundry room/ wife's office.
The one stupid thing they DID do is running the water lines to the upstairs bathroom up the outer wall from the main floor bath.
My house is VERY TYPICAL of housing in my area - circa 1970 - and most are built even more efficient today. Most houses built here since the mid to late 40s are built in a similar fashion
One of the issues he is trying to figure out is how much radiator he will need to efficently couple the heat fron the solar panels to the interior of the house. I think he knows the one he is going to install in the fireplace is going to be way too small. I dont doubt it will heat the den the fireplace is in. I should be able to estimate this once he gets his test system up and going by measuring change in temp of the room and change in temp of the water flowing through the radiator.
Jimmie
No, it just shows that this house is far more advanced than Traders
- which runs the heat pipes up the outside wall and has an unheated basement.
How is your Euro-shack any better than mine?
We have places like that here too. But they are not "typical" here - or there. Ond on the whole, American, and particularly Canadian homes are more energy efficient than the average British dwelling, by a fair amount.
What is the normal R value in the walls and roof of the average new british house? What is required by "code".
How much gas does it take to heat a 1300 sq foot home in the coldest part of Britain (which is still, on the whole, not as cold as most of Canada)???
They build differently in the USA than we do up here in the "great white north". 6 inch walls with strayed foam insulation are not out of the ordinary here any more. The house my father built 30 years ago was heated by 1/2 cord of wood per winter- and it was a "conventional" urban split level of over 2000 square feet.
I heat my "snow belt" Ontario home for $700 worth of natural gas a year - which also provides all my domestic hot water.
Well, you're wrong again, this time on two points. First, in the example I gave you, the metal cubes were replaced 5 times. That is a long way from infinite.
Second, in the heat exchanger in the furnace duct, we sure do have infinite cubes. It's called airflow and as long as the system is running, new colder air is constantly moving across the heat exchanger as long as the system is running. Just like one cold metal cube replaces the one that has warmed 1 minute in the simple example. Imagine little blocks of air being changed every minute. Then every 10 secs, then every sec, etc. There you have the furnace heat exchanger.
Now the problem here is that again, you never answer my questions. Do you or do you not agree that by replacing those metal cubes that have warmed slightly with new cold
0F cubes the 500F cube is cooled faster than just leaving one cube there for the full 5 mins?Here's the experiment again:
Let's say you have 10 identical masses in the form of metal cubes. One of those cubes is heated to 500F. The others are all at 0F. I do the following two cases:
A - I take one of the 0F cubes and place it on top of the 500F cube and leave it there for 5 mins.
B - I take one of the 0F cubes and place it on top of the 500F cube for 1 min. Then I replace it with another 0F cube for 1 min. I do that for the same 5 min period.
Which results in the 500F cube being cooled off the most? Which results in the most heat transferred?
Do you agree or disagree that the answer is B? A simple yes or no at this point please. This is a question an elementary school kid could answer based on experience. And one that a high school science student could answer based on science.
And if you agree the answer is B, which we all know it is, then what is happening is exactly the same as in the heat exchanger in the furnace duct. The only difference is that we're using air instead of metal cubes to cool the heat exchanger. We don't have infinite air, nor is that required. The MORE air we put throught the heat exchanger, the MORE heat that is transferred. If we put enough air through it we've extracted all the heat.
OMG, you are so totally confused here it's unbelievable. A car radiator has air flowing through it just as the heat exchanger in the furnace ducts does. Both are infinite in the sense that the airflow is continous as long as the systems are operating. Exactly like it would be in the metal cube example if I kept replacing those metal cubes every 1 min for the duration of the experiment. I actually can't believe you just made this even more bizarre claim. This from the guy that is always slamming the US over being behind in technology? You really are lost in the wilderness here.
I can't understand any of this? First, a radiator from a car would work under exactly the same principles whether in a car or a furnace duct. The physics don't change. Second, there is no need to use an unspecified car radiator. CL provided a data sheet to a heat exchanger manufactured exactly for the purpose of being a water to air exchanger for this kind of application. We've given you the link 3 or 4 times now. The datasheet clearly shows that as you increase airflow the BTUs of heat transferred increase. You can see that the increase slows as a decaying exponential, which is EXACTLY what I told you 10 posts ago. But you won't even address that datasheet because there is no way around it. You just trim it from every post.
Here it is one more time:
The spec sheet is right here:
Airflow CFM BTUS
500 28 600 29 700 34 800 36 900 37 1018 38Your problems with science here are many. One big one is that you don't understand the difference between heat and temperature. Another is that you think the physics of heat transfer change whether the heat tranfer is used to warm a house or to cool a car because in the car it's "waste" heat. And you can't grasp the concept of infinity very well either.
Yes, it's for 6 GPM of water flowing through the heat exchanger. And it shows water at temps of 140F, 160F, and 180F. And HOW MUCH HEAT IS TRANSFERRED VERSUS AIRFLOWS OF 500, 600, 700, 800, 1000 CFM
As the airflow goes up, so does the BTUs of heat transferred. Do you agree that is what it shows or not?
And yes, if you made another chart that showed say 3GPM per minute of water flow instead of 6GPM, the water would be coming out cooler. You would then have LESS heat transferred. And you'd have a similar table showing that as airflow increases from 500 to 1000CFM, the amount of heat transferred increases with it.
That table for 6GPM isn't some unique point where special rules of physics apply. It shows how the heat exchanger works and is an operating point within the scope of this discusssion, where if the OP built his system he could get 30K BTUS of heat out of it.
Look at the other heat exchangers that are larger and show greater flow rates. Find me one that does not show that as you increase airflow, the BTUs of heat transferred go up.
Now I agree that is one missing piece of information that should be spec'd on the data sheet. Without it we don't know the specific air entry temp that resulted in those BTU numbers. However, it doesn't matter. Whatever the air entry numbers, clearly:
The spec sheet does state:
" These air to water units are ideal for heating water in residential and public buildings. Compact and lightweight, these units are designed to maximize heat transfer by utilizing a series of 3/8" copper tubes with a high density of aluminum fins. These fins are spaced in such a way that the fin density is an impressive 12 fins per inch. This unique design allows for heating loads of 50,000 to 60,000 Btu per square foot (please see performance chart below)."
And with water entering at 140F to 180F it's obviously being used in that chart to heat air. At that point, it does not matter what the incoming air temp is. Let's assume for the moment that the particular chart used 65F air. The chart shows the BTUs of heat tranferred versus increasing airflow. As you increase airflow, the BTUs go up. Now, if the incoming air were cooler, say 40F, you'd transfer more BTUs of heat because the temp differential between the hot and cold sides of the heat exchanger is greater.
If the incoming air is 100F, you'd transfer less BTUs because the temp differential is smaller. You would then have a very similar chart that starts out with a given BTU of heat transfer at 500CFM and INCREASES as you increase the airflow to 600, 700, 1000 CFM. Capiche?
Sure does. Do you agree that this chart also clearly shows the BTUs of heat transferred INCREASED steadily with the airflow? That MORE heat is transferred in that table as the airflow increases? That it is that way for the table on every size of heat exchanger? Yes or no?
No shit Sherlock. But remarkably, the application the heat exchanger is targeted for is exactly what the OP was looking for. And the water flow rate and the airflow rate are within the range that a system to heat a home via solar would be. In other words, that chart shows exactly what the OP could use. It's not some curve from some bizarre or unique heat exchanger.
I can't understand why you can't understand the simple concept that a heat exchanger, be it this one, or a car radiator has a CONTINUOUS MASS moving through it. We don't have a single hand, we have say 100 people with hands at 98F, that each place their hand on the warm stove for 30 secs. That transfers MORE heat than one person leaving their hand there because new hands at 98F are constantly replacing hands that have warmed slightly. The greater the temp delta, the greater the heat transfer. You stated that yourself. You wind up with 100 people with slightly warmer hands instead of one person with a hot hand. Does that mean less heat is transferred? Of course not because the mass of those 100 hands more than makes up for the fact that they are only slightly warm. Capiche?
Same thing applies to a home radiator on a hot water or steam system. The more air you move past it, the lower the air temp will be, but the more heat you will extract from the radiator and the cooler the exit water temp will be.
Same thing applies to a car radiator. And it matter not a wit that it's waste heat being transferred to the atmosphere, so please don't start that again.
There you go again. It's been stated to you many times now by Mark, CL, and I that the object of the excercise was to heat the house. Which means getting the most heat out of the heat exchanger, not the hottest air. You are confusing TEMPERATURE with HEAT. Try looking up the definition.
As one more example, I could go down and turn the blower down on my furnace to 100CFM instead of
1300CFM. Assuming it would not kick off due to the high limit safety switch, I would then be getting very hot air out of it. Does that mean I'm getting MORE heat and it's going to heat my house faster, etc? No. I'd be getting LESS heat even though the small amount of air coming out is a lot hotter. Capiche?900 btu/cu ft which is 31,783 BTU/cu meter - more or less.
There is the whole aspect of diminishing returns that both Trader and Harry are forgetting about - PERHAPS Harry more than Trader.
The difference in efficiency is real, but how great? = particularly since we are working with a closed loop system, and the heat not recovered stays in the system, raising the input temp the next time 'round, which will increase the efficiency. It's not like we are wasting any heat (or at least not huge amounts) by not recovering it on the first pass.
And passing more air through the exchanger does increase the BTUs extraced - but by how much, particularly when lowering the "dwell time" does reduce the efficiency somewhat.
At what point does the loss of efficiency from the one side over-balance the loss of theoretical heat transfer from the other.
My gut feeling is the gains/losses from either side of the arguement are close enough that there will be a fairly broad band of overlap where the difference in heat transferred will be RELATIVELY insignificant in the grand scheme of things. You get to the point where "close enough IS close enough.".
Both theories are valid - but when the rubber hits the road - or in this case Fluid 1 hits fluid 2 - how much difference will each make???
The only way to know is by setting up a system and testing. Not DIFFICULT to do as a test before final install with a variable speed blower and heat exchanger, and a flow control valve to contol both water and air flow. Don't need to measure air temperature change/heat absorption because if the heat leaves the water, it went to the air. You need to be able to measure inlet and outlet water temps and water flow, as well as air flow. That's all.
I'd definitely be doing this before making any mods to the heating/air handling system in the house.
And PERHAPS a thermosyphon circulation system would be self optimizing???? Just a thought.
Try sticking to the example, instead of changing it. We're not talking about changing the burner rate. We're talking about changing the AIRFLOW rate of the furnace with a given burner rate. Whatever the burner rate, you get more heat out of that example of a heat exchanger the MORE air that flows through it. Yes, that air will be at a lower temperature, but given the increase in air mass, it still picks up MORE heat going through the furnace. Capiche?
And where do you address the whole rest of my post? As usual, you don't. You just try to steer the discussion somewhere else.
Give me a break here. I haven't forgot about diminishing returns:
A - I can refer you back many posts ago where I said that the more airflow you pass through the heat exchanger the more heat you get out and that the additional heat gained decays exponentially with the increased airflow. And yes, if you put enough air through it, all the heat is extracted and the outgoing water temp and the air temp are equal and no more heat can be extracted. I clearly stated that many times here too.
B - I posted the data from the heat exchanger data sheet you provided which fits the application nicely. It shows that additional heat is still being extracted at over 1000CFM. Yes, the additional gains are diminishing.
C - This started over the issue of placing dampers in the airflow stream. I said you might as well just use max airflow, because there is no downside to doing that from the heat exchange standpoint and you will have heat exchange at a maximum. I think you agree with that, no?
It doesn't increase the efficiency. That water leaving the heat exchanger from the furnace is now HOTTER and will consequently pick up LESS heat going through the solar array, which is just another heat exchanger. Put
65F water into the solar array and you get max heat out of it. Put in 150F water, or whatever the max temp it is capable of achieving, and you get zero heat out of it.You sure are wasting it, per the above.
Totally wrong, per the above.
It doesn't
Your gut is wrong. Sending that water back to the solar array with all the heat extracted from it in the furnace is how you get the maximum heat out of it, which is what the OP wants. If that isn't possible, the next best thing is to get as much heat as possible extracted from it. And you do that by using maximum airflow.
exhaust air thats too cool will make occupants feel cooler espically at higer speed airflows.
a modern high efficency forced air furnace is a excellent example, during cool down the blower remais on and can feel cold if your standing in front of the air vent
Not totally wrong, if when you decrease the air flow you also increase the surface area - so you get the same CFM flow but at a lower velocity.. All goes to the TOTAL design of the system. Can't just change one parameter and expect to totally optimize the system.
I more or less agree with you - but with a solar "loop: if the heat is not all extracted the first time round it just means the fluid will be hotter coming by the next time - which may increase the efficiency so more heat is extracted next time through.. Depends a lot on how much solar input he is getting.
And as far as the dampers - I NEVER advocated putting dampers in the MAIN airflow. My damper suggestion was to get away from the "restricted airflow" straw man somebody put up, putting the heat exchanger "off-line" to avoid the restricted air flow. And even then - yes, they are likely pretty close to superfluous.
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