Heat exchanger questions

[cumminstinkerer]

My system is a work in progress and somewhat unconventional in some ways. The pump I am running will run 18gpm with the head that I have, My water to air exchange is rated for 160000 btu @ 180 and 15 gpm, the issue I am having is I'm only get a drop across the exchanger of about 7-10 degrees, yet the heat out the registers is great and the house is staying at set temp, fan speed is on high so I cant really increase air flow any at this point, I don't have the propane furnace even there, I made a box to replace it that houses the HX, Fan and AC coil. I just find my deltaT to be very low for what you are all suggesting as normal. I'm am seeing less than 2 degrees of drop from boiler to HX unless DHW is running, Homemade side arm is between boiler and HX. My total system only hold about 180 gallon. In my previous threads I've discussed some of what I've done to the boiler. I now have my differential set to 8 degrees C on the exterior of the boiler outlet line, this gives me a good swing at the boiler so I can get my burn times stretched out. It all works fairly well still tweaking on the boiler some but just curious if the low DeltaT on the HX is an issue.
Thanks

 

[Bob Rohr]

You should be able to find a performance chart for the coil, from the manufacturer. It generally shows output at various flow rates either as GPM or ∆T, and air flow (CFM) conditions. The coils are designed and test to operated a a range of conditions.

Low ∆T indicates high flow through the coil.

Actually lower ∆T indicates higher average temperature across the coil and higher output, not lower as many confuse that.

At some point the additional pumping power required to get a tight ∆T is not worth the expense. This graph shows how output increases at a slower rate at the top end of the graph.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_12_0.pdf

 

[hondaracer2oo4]

So your concerned that your temp drop across the water to air hx is 7-10 degrees? That is a much better "problem" to have than a delta t of >20 degrees. Like bob said it just means that your water is moving so quick that it isn't dropping much. The only real issue here is that your spent money on a bigger pump than you needed and are using more electricity than needed.

 

[cumminstinkerer]

you guys will laugh but the pump is just a water pump from northern, it probably is way overkill and they only last a couple years but for the $90 bucks they cost what the heck. I figured it was just a flow thing and nothing to worry about, This whole system was kinda a throw together on the cheap, I put all together right after I got divorced because there was no way i could afford the lpg and the mortgage and food. I have made several changes to it since then and continue to adapt it. I've got it to where it even has all out secondary combustion.

 

[Bob Rohr]

Be careful with "way overkill" on pumps. With excessive velocity you will get wear and possible pin holes. usually if you hear the fluid flowing through then pipes you are pumping too fast.

2- 5 feet per second is the guideline for hydronic flow speed. The size of the pipe and the pump moving it will determine the speed (fps)

 

[Bob Rohr]

Heat transfer and fluid velocity of speed can get confusing. There is no such thing as the water flowing too fast for heat to jump off It is technically wrong and a myth spread by some in the pumping industry.

There is a point where excessive velocity causes noise, and the additional pumping power to get those high flow rates and low delta T is seldom worth the effort.

Here are some examples. A 12' piece of typical copper fin tube baseboard supplied with 160F.
This board is rated by the manufacturer to release 600 btu/ foot at 200F

The graph, fig. 3.5 shows the relationship. So increasing the flow from 1 gpm to 4 gpm only increases output from 250- 264 btu/ ft.

From .5 gpm to 1.5 is the sweet spot, above that flow rate performance increases much slower, but output will increase at faster flows.

The reason is the average temperature of the baseboard increases at the higher flow rates, more BTU jump off because the emitter is warmer.

The same applies to a radiant loop.

Here is a 300 foot loop, fig. 3-8a same supply temperature. One runs a 22 delta T @ .65 gpm. The other a 15 delta T @ 1.1gpm. So the smaller, tighter delta T provides a more consistent temperature, higher output from one end of the loop to the other, indicated by the color. The tighter delta will also be more comfortable.

This is true of fin tube, air coils, radiant, cast iron radiators, really any heat emitter.

So the thought, or marketing spin that a heat emitter can or will remain at a fixed temperature drop as the flow changer (delta T pumps) just isn't technically possible.

 

[dogwood]

With the head that your system has, would it be practical for you to swap out your circ with an inexpensive 3-speed pump like the ones Grundfos makes. They're less than $100. You could then observe if different speeds (gpm's) will get you a better temperature drop across the hx. Mine does best at the low setting for our in-plenum water to air hx. The lower speed also appears to have the beneficial side effect of decreasing mixing in storage. What kind of temperature drop would you like to achieve?

 

[cumminstinkerer]

I found the print for the pump yesterday, it is rated at 12gpm at 110ft of head, yes that was not a typo, big pump, but they were dirt cheap, I bought the first one for like $60, it died after four years because the ex "accidentally" drained the water while the pump was running, The replacement only cost me $25 because it was still under warranty. It is a big pump but ive run the smaller ones and compared electric bills and this one makes no difference there. As long as its okay to have such a small DeltaT across the HX I'm not to concerned, and why change something that works well if it wont hurt anything else

 

[Bob Rohr]

Ideally you select the pump to just cover the load, as in how many gpm and the pressure drop of your piping circuit.

The pump should operate in the middle 1/3 of it's curve. that is why the pump manufacturers offer so many choices.

Running a pump "off" it's curve will only be inefficient, but cavitation and pump life may be an issue.

Sort of like adding too much turbo boost to a Cummins.

 

[cumminstinkerer]

I like the analogy there Bob, course when is it enough and too much with boost, just before the head comes flying off lol. I will do some more figuring and see but sure not gonna replace the pump til this one gives me an issue, always good to have a plan in place though.

 

[Bob Rohr]

An off grid friend of mine tried to talk me into running my cummins on old cooking oil, as he was doing. I told him that I didn't think that would end well. Guess what

One day while stopped at a wet intersection his fuel line came off and coated the intersection in cooking oil. After that and a few major engine rebuilds, the barrels of grease he collected has turned rancid in his back yard.

Maybe he can burn it in his Hardy OWF.

 

[cumminstinkerer]

Bob My cummins is a 97, I've got her turning about 425 HP, I'm pretty well maxed out. I tore it down two years ago at 285,000 it was doing some funky oil pressure things and I had laid it over a couple years before that so figured I hurt the bearings. When I opened her up the bearing were still good but the geartrain was a mess, the Dowel pin got me, broke the center outta the cam gear and took out all the rest except the crank gear. I went ahead and went through it, after honing the new rings were only .002 over the minimum ring gap, I'm up over 315,000 now and going strong, never have done the pump or charger. I've got another one I helped build running around the IAMO and Midwest Pulling circuits that's running over 1500HP at 5200 rpm with no water injection and a 2.60 charger. Now I'm working on build a DT466 to go into a hot farm 5488 that might get down your way some when we get her done. What town are ya from?

 

[Bob Rohr]

I'm in SW Missouri, rural farm country. Plenty of pullers and oil heads around these parts.

 

[cumminstinkerer]

lol, I know I have a couple good friends live in Cassville, actually have some sheet metal work I did on a tractor running around down there, Thad Rouscha's 900 called "some assembly required", I did all the metal and puddy work then primed it and sent it sown there, Ronnie Whitham and his son Scott have been friends of mine for years