Somewhat Bizarre Question – Smokehouse Air Flow computation
I think that this might be the place for an answer to a question I have. I’m putting together a spreadsheet to compute the number of BTUs needed for a smokehouse. There are a number of formulas that I’ve nailed down, but the sticking point is the number of air changes per hour. This has a huge effect on how big a heat source you need to maintain a given temperature.
I general a smokehouse has an upper exit smoke duct and may have a lower input air duct and/or a circulation fan.
To get things started, here is what they do for a commercial smokehouse: measure the total area of the duct outlet (square feet), and multiply by the sum of the velocities of the supply airflow in feet-per-minute (fpm) from each supply duct. Then you factor in the cubic feet for the smokehouse.
However, this doesn’t answer the question for the simple home smokehouse. One that may not have a fan nor have an input air duct. In this case, I’m assuming, that heat convection is the main driver of air changes per hour.
Any thoughts would be appreciated.
]If you are curious, here is what I have so far http://dl.dropbox.com/u/23689083/SmokehouseComputations%232.xls
TIA,
/Ed
Replies
If you were to actually build the thing you could estimate air flow by observing the speed of the smoke exiting the stack (hopefully just one) and guestimating it's speed in feet per second near the exhaust pipe. (Some sort of chaff tossed into the flow will help with this, and if you did a video and then examined it frame by frame you could be fairly accurate.) Then figure the area of the exhaust pipe and multiply by feet per second to get cubic feet per second (don't forget to convert the area from square inches to square feet). Once you have cubic feet per hour you can divide by the number of cubic feet in the building to get ACH.
Without a fan, the airflow is going to be affected by other variable forces: atmospheric pressure (you know how lazy smoke can be during a high pressure day); the actual heat output of the smoker's heater itself will be a factor (the greater the heat, the greater the bouyancy of the exhaust); and the outside temperature (the greater the temperature difference between the inside and outside, the greater the bouyancy of the heated air.)
All that said, I would still recommend NOT installing a fan, and not overthinking this. Instead, monitor the inside temperature and adjust by use of a damper on either the intake or exhaust.
To start with, I don't think you have the commercial method described correctly. Seems more appropriate to add all the suppply VOLUME flows , divide by the volume of the smokehouse, and you will have the number of airchanges (per minute, if that is the units used in supply air volume.
But let's back up. Why do you need air changes? Seems like the smoke concentration would be higher if airchanges were minimized, and smoke flavor would go into the meat faster. If you are heating with a wood fire, perhaps a small air change would be necessary to maintain the fire. I like the response of putting a damper on the outlet, and going from there.
The air changes can be estimated but a lot of assumptions need to be made. Size of openings high and low. Outside temperature vs inside temp. Height between inlet opening and outlet opening. I did the arithmetic a few years ago when I needed to estimate the amount of air that would escape from a hot industrial dryer when doors were opened, it is amazing how much air can escape from a relatively small opening.
I sent a reply several days ago, but it hasn't been posted so I'll try again.
I should explain that I'm trying to determine how large a smokehouse I can build before I build it. The general approach of DIY smokehouse builders is to use a high BTU gas burner and forget about the details. BTW - they are typically huge. However, I want to use a 110 volt electric element and build a smaller smokehouse.. These elements max out at about 1500 watts.
A few of you questioned how to smoke meat. In general the process is start the process with the air vent(s) open and a lot of smoke. The vents are open to let the moisture from the meat escape. Then the vents are closed somewhat to let the heat build to raise the meat to the desired temp.
A small fan (e.g., computer fan) is sometimes used to distribute the smoke internally to ensure an even distribution of the heat.
xLnMeHKsuz, you are correct about the commercial method being described incorrectly. I was too brief. Your analysis is correct.
rdesigns, you have mentioned many of the variables that the spreadsheet already has. I don't have anything about the bouyancy of the air. This area is where I desired the help. I'm guessing that convection is one of the key factors in determining the air changes per hour, but the formula and variables are an unknown to me. I was thinking that an HVAC designer might have the answers.
It is bizarre that your post from the 3rd only showed up today. Yet another quirk of this wonderful software.
To calculate the airflow you'd have to know the temperature/density of the air (and moisture/smoke will make a difference) as compared to outside air, and you'd have to know the resistance created by your ducts in and out and whatever else might block airflow. The duct resistance could probably be gotten from HVAC tables, but guestimating the "whatever else" would probably be highly error-prone.
But you can probably do a back-of-the-envelope that'll get you within a factor of 4 or so. From there you could throttle the ducts or double up on the heaters (or the reverse) to balance things.
(Don't forget to include simple through-the-wall losses in your heat loss calcs.)
Guys,
Thanks for you comments so far. Let me clear some things up. What I am trying to do is to estimate BEFORE I build a smokehouse how large it can be. I'm wanting to use a 110v electric heating element and they max out at about 1500w. The DIY smokehouse crowd tends to use massive BTU gas burners and not worry about the details.
Not to get too technical on how to smoke, but there are several stages to it. Here is a general description of one typical way to smoke. In the first stage there is a lot of airflow as there is a lot of water (humidity) coming off the meat, usually that is where the most smoke is applied as well. This is usually termed "dampers wide open". Then as the meat dries a bit the damper(s) are closed down, perhaps to 1/4 open, and the meat is brought to its desired final temp.
Sometimes people use a small low speed computer fan to get an even distribution of the smoke within the house. I'd guess that the commercial guys use fans for the same purpose.
xLnMeHKsuz, you're correct about the air change formula. I got too simplistic in describing it. The spreadsheet has most of what you and rdesigns mentioned, but determining the air changes where I'm stuck. I know convection is a big part of it, but I don't know all the variables.
An HVAC contractor may not have much helpful info.
Your installation must consider not only the amount of air moved, but also the density of the smoke along with intermal temperature vs outdoor temperature, insulation values and all the other things you're juggling.
To me, the answer would be to make it all adjustable or variable rather than to aim for precision--unless you can find more specific guidelines elsewhere, which might be difficult. You might "pirate" info taken from the specifications of a manufacturer of commercial smokers that supplies restaurants that specialize in smoked meats.
If you must limit your heating capacity to one 1500W element, you will end up with a very small smokehouse indeed--more a smoke box than a smokehouse.