MPD location

Re: MPD location

PostBy: coaledsweat On: Mon Mar 10, 2014 7:37 am

It just pulls heat from the appliance into the pipe. Zero gain and depending how close the pipe is to flammables, increased risk. Since the appliance is deeper in the room, I would leave the heat there.
coaledsweat
 
Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

Re: MPD location

PostBy: ddahlgren On: Mon Mar 10, 2014 8:53 am

coaledsweat wrote:It just pulls heat from the appliance into the pipe. Zero gain and depending how close the pipe is to flammables, increased risk. Since the appliance is deeper in the room, I would leave the heat there.


I follow NFPA code very seriously. There is nothing within 3 feet of the stack pipe that can burn and there are no curtains on the windows that sit either side of the stove either. Behind the stove and pipe there is .093 brushed aluminum that goes 2 1/2 feet to either side with a 2 inch spacer behind it so there is plenty of air circulation. In all the years I have been burning wood I have only one fire get away from me for a few minutes and stack went to 1200F on the probe and aluminum was barely warm. To me moving the damper is free heat for same fuel input. Every foot of 6 inch pipe is 1.57 sq. ft. of heat exchanger.
ddahlgren
 
Hand Fed Coal Stove: Crane 404

Re: MPD location

PostBy: BPatrick On: Mon Mar 10, 2014 9:55 am

DDahlgren,

We're talking about 2 very different things with 2 different purposes. First, I think that with single wall pipe you would radiate heat but keeping the heat closer to the stove may give you the advantage of a more complete burn. The pipe is going to dissipate heat quicker than the stove and the stove is going to work harder to maintain the same temp. The stack would seem to pull the heat away from the stove up to the pipe until it is restricted. Very interesting point but I don't know if there is more benefit here.

Second, the base burner/heater comparison isn't apples to apples. Base heaters have the base heater/by pass built into the stove and then either an inner barrel or a series of inner exhaust pipes to insulate and surround the fire pot with hotter exhaust gases. This slows down the speed of the gases exiting the stove and it allows the stove, not the pipe, to extract as much of the heat as possible before it travels up the chimney. This is why you can hold your hand on a stove pipe before or after a mpd on a base heater/burner. Placing a mpd far away from your stove does not do the same thing as it is not the same design. After owning direct draft antique stoves and now base heaters, I'm a big fan of keeping as much heat in the stove as possible and allowing it to keep the inner fire hot which allows a more complete burn. Even if you kept your mpd closer to your stove, your still going to have some heat loss up the chimney and the pipe is going to be radiating/loosing heat regardless. In my opinion you should try 2 mpd's. One close to the stove and the second 18"-24" above as it will allow the first one to keep the heat in the stove and the second to slow down the gases and heat so as much of the heat radiates into the room as possible without having the exhaust temps cool down too much and you have C0 coming back into the house. Having one completely closed and the other 2/3 to 3/4 closed will draw as much heat as possible without letting gases backup into the house. If your draft isn't really good then you don't want to do this.
BPatrick
 
Baseburners & Antiques: 2 Crawford 40 Baseheaters
Coal Size/Type: Stove Coal
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Re: MPD location

PostBy: Sunny Boy On: Mon Mar 10, 2014 10:31 am

It doesn't affect the kitchen ranges and their flues take heat away from the firebox, not surround it. Their flues are close to the length of and have a greater surface area than the base heaters.

And, what about the Glenwood non-base heater back pipe option stoves that come with pipe, or without ? Their extra flue length is all outside the stove body. The pipe adds several feet of pipe length at least before the MPD. I don't remember hearing reports of it affecting how the stoves burn either way.

I doubt you'd be able to remove so much heat with a few extra feet of pipe that you'd see much difference in how well the stove burns, but you very likely will see a difference in room temp.

If you use an IR gun, you will notice a rise in pipe surface temp just after the MPD when it's half to fully closed, that is higher than before the MPD. That's because the hottest gasses in the middle of the flue are being forced outward to go around the MPD, plus there is some turbulence/mixing just downstream of the MPD. That increases the pipe surface temp in that area. However, measuring pipe surface temps about a foot before the MPD and comparing them to about two feet downstream of the MPD, you'll see a difference.

Paul
Last edited by Sunny Boy on Mon Mar 10, 2014 10:36 am, edited 4 times in total.
Sunny Boy
 
Hand Fed Coal Boiler: Anthracite Industrial, domestic hot water heater
Baseburners & Antiques: Glenwood range 208, # 6 base heater, 2 Modern Oak 118.
Coal Size/Type: Nuts !
Other Heating: Oil &electric plenum furnace

Re: MPD location

PostBy: ddahlgren On: Mon Mar 10, 2014 10:35 am

I could be all wet but all the math for heat transfer I have seen no mention of thickness of the steel so 1/4 inch or 22 ga. it is all black mild steel. It would be no different than adding 4 inches to the height of the stove.
ddahlgren
 
Hand Fed Coal Stove: Crane 404

Re: MPD location

PostBy: coaledsweat On: Mon Mar 10, 2014 10:40 am

ddahlgren wrote:
coaledsweat wrote:To me moving the damper is free heat for same fuel input. Every foot of 6 inch pipe is 1.57 sq. ft. of heat exchanger.

It isn't going to make any more heat, you are just redistributing it to the pipe. In doing so you reduce the appliance temperature and with that reducing the convection rate.
coaledsweat
 
Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

Re: MPD location

PostBy: Lightning On: Mon Mar 10, 2014 11:48 am

I'll offer something to chew on lol. Y'all are assuming that flue gases are piling up against the manual pipe damper. This idea was brought up on the "Does a manual pipe damper hold heat in a stove" thread.. Personally I think it will make little if any difference where the manual damper is as to how much heat the flue pipe radiates.
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut Size / White Ash

Re: MPD location

PostBy: Lightning On: Mon Mar 10, 2014 11:50 am

Some careful measurements would be needed before and after the damper to assume any gain...
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut Size / White Ash

Re: MPD location

PostBy: ddahlgren On: Mon Mar 10, 2014 12:01 pm

[/quote]
It isn't going to make any more heat, you are just redistributing it to the pipe. In doing so you reduce the appliance temperature and with that reducing the convection rate.[/quote]

Using that theory it would not help to add 4 inches in height to the stove to add more surface area. Or a small stove with a large fire is the best way to go. Oh well it will not hurt to play with it as a damper is only a few bucks, sometimes you get pleasantly surprised and sometimes you get humiliated but nothing ventured nothing gained.
ddahlgren
 
Hand Fed Coal Stove: Crane 404

Re: MPD location

PostBy: freetown fred On: Mon Mar 10, 2014 12:42 pm

You don't have that done yet?
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Hand Fed Coal Stove: HITZER 50-93
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Re: MPD location

PostBy: Lightning On: Mon Mar 10, 2014 12:57 pm

freetown fred wrote:You don't have that done yet?


Me? No, it works against the air in = air out rule :lol:
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut Size / White Ash

Re: MPD location

PostBy: Lightning On: Mon Mar 10, 2014 1:07 pm

What I mean by that is... The way I see it, the same amount of flue gases will travel the flue pipe from start to finish so I don't know how there would be a gain of heat by moving the damper further downstream... :)
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut Size / White Ash

Re: MPD location

PostBy: freetown fred On: Mon Mar 10, 2014 1:10 pm

There won't be, but ya know we're a curious bunch ;)
freetown fred
 
Hand Fed Coal Stove: HITZER 50-93
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Re: MPD location

PostBy: ddahlgren On: Mon Mar 10, 2014 2:02 pm

Lightning wrote:What I mean by that is... The way I see it, the same amount of flue gases will travel the flue pipe from start to finish so I don't know how there would be a gain of heat by moving the damper further downstream... :)


If that were the case just take the damper out as it would not change the temperature of the stove or change the draft. If the draft goes down when you close the damper then the net flow changed. You are contradicting what happens in real life. All I am proposing is the point that the temperature and net outflow goes down to capture more heat. All I am expecting is maybe 1000 btu gain but the way last winter went that might be a small but welcome gain, plus it is free.

Just ran the calcs.. 3 ft. of 6 inch pipe is 4.7 sq. ft. of area.
@250F =1400 btu
@500F = 6175 btu
So a 4775 potential gain and no extra coal burned and some head room to get that much more heat on a really cold day.
ddahlgren
 
Hand Fed Coal Stove: Crane 404

Re: MPD location

PostBy: Sunny Boy On: Mon Mar 10, 2014 2:30 pm

Lee, It's not an assumption. :D

When you close the MPD and see the mano reading drop (pressure increase), that's proof that the flue gasses are "piling up against the MPD". They get compressed as they slow down to squeeze through a smaller opening (restriction). ;)

The same thing happens with the choke on a carburetor, but to accomplish something else. There it's piling up air outside the carb to cause more pressure drop on the down stream side of the choke butterfly valve (a carb's MPD). The extra pressure drop is to pull extra fuel into the slow moving intake stream of a slow spinning motor (starter motor driven speed only), thus richening the air/fuel mixture for cold starting a motor. ;)

Paul
Sunny Boy
 
Hand Fed Coal Boiler: Anthracite Industrial, domestic hot water heater
Baseburners & Antiques: Glenwood range 208, # 6 base heater, 2 Modern Oak 118.
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