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Author Topic: BTu Question for the Science Guy Types - Please Use Small words  (Read 6675 times)
Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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« Reply #15 on: December 11, 2009, 08:53:13 PM »

That's great Nick, but.....

How do I know if I'm overdriving or underdriving a given heat exchanger in a given application without having a formula that tells me that if I have a heat source with X Btu output connected to a heat exchanger with Y Btu transfer with N cfm of air moving across it, R gpm coolant moving through it and A surface area, I can expect Z results?

My comment to use small words was a bit tongue in cheek, but I think many of us would like to know that a particular system will or will not work. It shouldn't be rocket surgery to find a generic, workable formula that will give a general idea of usability.

Dallas,

When I measure a bus conversion in the real world, I take into concideration Heat Loss more then anything esle because our busses

are nothing like residential homes where we can calculate with the Manual J method.  Sooo, Try this. Bring your bus to 70 degrees

 inside with outside ambiant temp of 30 to 35 degrees. Then turn the heat off and calculate how long it takes to drop 10 degrees inside.

If the 10 degree drop is within 15 mins, you now have to look outside your bus from inside from your drivers window area. If you see

snowflake shaped crystels on the windshield, you can concider yourself one cold SOB!! and need a bigger heater!

LOl sorry, I couldn't help myself..... I know, I'm mean! So, where did you think this was going anyway!....... Grin

Me!
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bryanhes
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« Reply #16 on: December 11, 2009, 10:17:43 PM »

Nick,

Smart@&%  Cheesy Cheesy Grin. And here we thought you were getting ready to teach us something, LOL! Was funny though  Grin

I have been reading through this post as I will be trying to figure out how many heat exchangers I would need to run as well in mine. It does make sense though to have the source such as a manifold that feeds all exchangers but what would you do about restricting return
flow?

I just played a little and put this together based on how I interpreted the posts (nothing fancy, its to late for that  Grin). Is this what you were meaning? I would also think with it set up like this it would have to flow back to the supply with little restriction?

Bryan
« Last Edit: December 11, 2009, 10:25:06 PM by bryanhes » Logged
Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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« Reply #17 on: December 12, 2009, 05:26:59 AM »

Hi Bryan,

Your engine should also be a "Zone" or at least valve it so it can be taken out of the loop for stationary use.

Your returns should be piped to a manifold for equal flow back to the webasto.

Nick-
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« Reply #18 on: December 12, 2009, 06:11:17 AM »


Dallas, the formula for a HX rate is       Qrate= mrate * Cpof liquid * (Tin - Tout)


Qrate will be in BTU/hr


Mrate is how much mix you have going through the HX  it is usually in gpm and you need to convert to pounds/hr    you can measure this by taking the return line loose at the return header and time the flow of it going into a bucket. or if you can find a flowmeter even better.

you can weigh a gallon and assume 1 lb = 1lbm for ease of calc and close enough for bus

For Cp(Specific heat capacity) of antifreese, units  BTU/lbm-F degrees
http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html  page down to table on Specific heat capacity

You can use you handy HF IR thermometer to check the Temperature in and Temperture out.

This will get you a close estimate to what your getting out in your setup. Smiley

Good luck and let us know what you come up with. Smiley

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« Reply #19 on: December 12, 2009, 06:40:36 AM »

kyle4501, you meant 30K BTU of cooling minimum, I think...

You doubled your double.

Tim Horton's?

happy coaching!
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« Reply #20 on: December 12, 2009, 09:15:09 AM »

Nick,

Is this what you are talking about? I changed the diagram to include an additional manifold and two shut off valves.

Bryan
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« Reply #21 on: December 12, 2009, 09:33:20 AM »

I like that.

How about some isolation valves for each circuit?

If some thing leaks or needs to be replaced...

happy coaching!
buswarrior
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« Reply #22 on: December 12, 2009, 09:48:20 AM »

BW,

I will put that in my next revision  Grin Heading to OKC for X-mas party.

Bryan
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Tim Strommen
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« Reply #23 on: December 12, 2009, 11:09:22 AM »

I can recommend some books to read if you want to learn a bit more about this:


-T
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Fremont, CA
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DD 6V92TA (MUI, 275HP) - Allison HT740
Conversion Progress: 10% (9-years invested, 30 to go Smiley)
Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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« Reply #24 on: December 12, 2009, 12:05:11 PM »

Nick,

Is this what you are talking about? I changed the diagram to include an additional manifold and two shut off valves.

Bryan


Very good bryan!

Now, try and add either a tank with a fill to add coolant to your system or just a fill port that will hold presure in the return
line before your webasto. This fill or tank will need to be above the heat exchangers to eliminate air in the system. Another
member here on the BBS made a real neat fill port that he constructed behind his kitchen sink. A panel lifted up and there
it was.... Real clever!

Nick-
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Bill B /bus
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« Reply #25 on: December 12, 2009, 08:20:07 PM »

"Most can cool with two roof airs." True. However only 27K or 30K BTU's. Not 60K.
Roof air is 13.5K or 15K BTU's.
Webasto spec is heat output. Not based on fuel input. And also new output is 45K BTU's. Fuel pressure was increased about three or four years ago.
Side note: Webasto doesn't spray the fuel. It produces an extremely fine fog.
For heating systems in  your coach Sure Marine assumes 175F fluid and a flow rate and 12+VDC to fan motors to get the rated output.
I have noticed that the heaters take about 5-6 mins to really get to full output. This correlates nicely with the Webasto coming up to operating temperature.
Nick, excellent explanation on the sizing of a house heating system. We went through the exercise about 6 years ago to change out a 1967 heating boiler. Considerable downsizing of the boiler. Running a slightly smaller nozzle than the old one had a end of life. And we're using a lot less fuel oil.
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« Reply #26 on: December 13, 2009, 12:26:54 AM »

If I run a cube heater rated at 1,500watts in my coach it will raise the inside temp 30 degrees above the outside.  I assumed that it would do that no matter how hot or cold it was outside.  It seems to work out as a theory and I am comfy down to (74-30)  44 degrees,  The watts translates to BTU and I assume that I could accurately determine my furnace capacity requirements.  My propane furnace is rated at 48KBTU and it will keep the interior sauna warm down to the -8 degrees I have been in.

Dallas,

If your second heat exchanger has 90% of the fin area of the one that has a BTU label then the unknown most probably will have a 90% of the BTU rating.  Logical?  Find a yardstick!

John
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« Reply #27 on: December 13, 2009, 03:29:45 AM »

Bryan your latest revision is more or less how our system is plumbed.  The problem with a parallel system is that you need to maintain a sufficient pressure differential between the two manifolds to ensure equal flow through all the heat exchangers.  Otherwise the flow will be preferentially to the lowest resistance circuit.  In our case the ProHeat pump simply isn't adequate to maintain that pressure differential.  I have added a slave pump in the highest resistance loop which is a work around that does pretty well but the right solution would be to have enough excess pump capacity to maintain the pressure drop between the two manifolds.  Some engineer could calculate how much that pressure drop needs to be to sustain maximum flow through all your heat exchangers.  You could probably accomplish the same goal with throttling valves in the low resistance circuits (which would also restrict your heat output) and in theory you could also do it with flow dividers but I don't know whether they exist for this application. 
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JackConrad
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« Reply #28 on: December 13, 2009, 04:46:07 AM »

This is kinda related and just FYI. When we installed our ProHeat, we wired all the heat exchanged 12 volt circulating fans to an aquastat that is installed in the hot water line from the ProHeat. The fans do not come on until the water temperature is 120 degrees. When the ProHeat shuts down the fans continue running until the water temperature is 120. This prevents that initial blast of cold air when the ProHeat first comes on. Jack
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« Reply #29 on: December 13, 2009, 06:26:54 AM »

JohnEd, you got that 30 degree rise with your cube heater under the outside temp and wind conditions at that time. If it is colder outside, it will take more heat/more cube heaters to do the same job.

Some law of physics, rats!

Wind is a huge player in air infiltration. Exchanging air via the cracks is the biggest devil for HVAC.

The interesting part of this discussion, it might determine that many of us have a lot more heating capacity than we need, depending on how and where we use the coach.

FWIW, 40K BTU is roughly equal to 8 of those 1500 watt cube heaters.

Now, before someone goes and fine tunes themselves a little too close, remember, same as the engines in our vehicles, excess capacity is what gives us better acceleration...

So, lots of furnace capacity will warm the coach up from cold faster, the same way lots of AC will cool down a hot coach faster.

But, excess power typically uses more energy to cruise, once the chosen "speed" has been reached, than if a power plant rated closer to the energy needs of cruising is chosen.

Another vote for multiple small devices instead of a single large one?
Same as the cylinder shut off in all the big V8's now for efficiency, and the greatly sought after redundancy if one of them breaks.

Great fun!

happy coaching!
buswarrior

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