Venting dc outside

[Michael W. Clark]

No, it doesn't imply that. The motor does the work of moving the air, that energy is then converted to heat by the friction the air encounters.

Then why doesn't the duct get hot?

[Leo Graywacz]

If you put a thermometer on the duct and run the collector I'm betting it will.

[Michael W. Clark]

If you have a 2.5hp motor, 36,000 ft3 of air space in the shop, and are drawing about 1000 CFM, how much temperature rise can you expect if 100% of the nameplate horsepower goes into heating the air?

Answer:
2.5hp = 6365 Btu/hr
Btu/hr = 1.08 x CFM x DT
DT = Btu/hr / 1.08 / CFM
DT = 6365 / 1.08 / 1000
DT = 5.9 F

If 100% of the motor rated power went into heating the air, you would see a temperature rise of 5.9F on the 1000 CFM of air. Assuming the heat is evenly disippated to the rest of the 36,000 ft3 in the shop, you would get a temperature rise in your shop of 9.8F per hour. This does not seem reasonable.

Once again, this assumes that all of the motor nameplate power goes into heating the air/shop. This physically cannot be true. If you are pulling 1000 CFM through the blower, you are moving 75 lbs per minute, does this not take energy? If you were to move (1) 80lb bag of quickcrete from one side of the shop to the other every minute for an hour, would that not take energy?

IF the motor is about 85% efficient, then about 1120 Btu/hr is emitted to the room ((2.5/.85 - 2.5) x 2546) , raising the temperature of 36,000 ft3 of air 0.1F/hr. This calculation would be in accordance with ASHRAE if you are exhausting to the outside. The fan will add some heat due to mechanical inefficiencies. Of all the systems I have measured, I cannot recall a situation where we had more than a couple of degrees in temperature across the fan.

Regarding friction in the duct causing a temperature rise. It would be interesting to measure the air at the inlet of the hood, and the air temp at the fan inlet. Once again, on the systems I have measured and balanced, I have found little to no temperature variation other than those caused by the processes. In fact, most of the processes I measured were elevated temperatures, and the temperatures always decreased going to the fan because of the heat dissipated through the duct walls from the hot air.

We know there is friction in the duct and that friction causes heat. However, I am not aware of any publications or instances where the heat load resulting from air friction in the duct is taken into account for ventilation system design or HVAC cooling load considerations. This leads me to believe that it is mostly negligible.
Mike

[Leo Graywacz]

OK fine you've convinced me It is impossible that my empirical data that I can feel could be correct. Your math is the only way that works.

I'm telling you. It puts out heat, I can feel it, I can see it on my thermometer. It puts out heat.

[Mark Rakestraw]

We need a physics major to chime in here. I think related to the "conservation of mass principle" is a "conservation of energy principle". So just like mass doesn't disappear (the burning log converts to ash, moisture, various gasses) energy never is "used up" or dissappears, it can only dissipate and/or change form. At some point that moving air stops moving, and we know (?) that an "object in motion tends to stay in motion unless acted upon by an outside force". So if that outside force isn't friction converting the kenetic energy of the moving air to heat energy what stops the air from moving and where has the energy gone? That's all the Jr. high physics I know and I'm now talking above my pay grade.

[Carl Beckett]

Hey, Ive never been one to shy away from a physics dialog - but first I want to ask Paul if he is getting what he needs from this thread?

There are some basic thermodynamic principles some of you are hitting on here - one is to carefully consider the control boundary that you are discussing. Within a particular control boundary, you can have energy in - change in state within the boundary - and energy out. It all has to balance. So a crude limit evaluation is to just take all of the energy in (electrical input to the motor) and convert it to heat. As a first check its worth evaluating how much the shop temp would rise if you had 2hp worth of energy being dumped into it (which I believe is something like 1500Watts... so imagine a 1500W electrical heater in your shop.

As is pointed out THERMAL MASS off the equipment and building, etc etc - is quite substantial and all slow the rate of temperature increase.

As for heating of the air itself - I like this equation: Q=m*Cp*deltaT. It has to do with the temperature rise of a fluid (air in this case) as a function of the specific heat of the fluid and mass flow rate - balanced against a heat flux input. Bottom line (and Im not even going to do the calc) - at 1100CFM you arent going to get a very high temperature rise in the air (at least not due to the heat being added by the motor). And another interesting thing to note is that any blower losses or even pipe losses that did convert to heat, would likely get subsequently 'cooled' by the airflow itself (this 'cooling' would increase temp in the airstream). Note the motor isnt much cooled by the airstream since the air doesnt flow over it.

So this leaves 'losses'. And change of state (work being done). First - I dont think these systems are anywhere near 85% efficient. maybe 15% is closer (induction motors alone are sub 85% and then you have all the bearing/friction losses, and the blower itself as a very inefficient device. Duct losses due to eddies and turbulence and wall friction, yep. So my $.02 is that MOST of the energy does go into the system as a loss, and that MOST losses eventually end up as heat (this is often the case - everything ends up as heat eventually).

Some of you have observed the air coming out of the filter as higher than ambient. And some of you have observed the ambient shop air increasing in temp? They are probably both true TO SOME DEGREE simply by definition. In the practical sense though, its a question of the mechanism and by how much do these increase. If you are really trying to solve this mathematically, you are going to have to start by drawing a diagram of the system and define the boundaries, and then solve for each piece within these boundaries. (electric in, motor loss out, friction loss out, etc etc)

[Matt Meiser]

We really don't need a physicist--there plenty of real world experience right here that proves its true.

[Phil Thien]

Many cyclones I've observed have had quite warm motors. I know the motor is running within the manufacturer's specifications, but they can get warm (hot) enough that it can be uncomfortable to keep your hand on it for any period. Combine that, with the fact that they are TEFC (FAN COOLED) means they have a fan blowing air over them. So I liken them to a small hot water or steam radiator running in your shop, with a fan blowing over them. I would not be surprised at all if people noticed them warming their shops.

[Michael W. Clark]

So if you pick up a weight and move it across the shop, you have done nothing except generate heat? Moving the weight was not "free". Useful work was done and that took energy.

[Leo Graywacz]

So, I'm thinking of venting my dc to the outside, and I'm wondering about a few things. I plan to run a cyclone, either a thien style top hat, or an oneida, with the blower from my 1.5 hp dc after it, so I will be pulling from the cyclone. I would then push the air outside, sans filter. I may put in a wye, so I can choose between the filter, and the outside, so i don't lose too much heated or cooled air.

My question is this. Would I lose a significant amount of cfm's if I put 2 90s in about a 6 foot run? I would likely put the blower over the top hat, or cyclone, to keep those runs as short as possible.

Thanks, as always.

Has anyone answered his question?

I have 2 90's on my collector and it doesn't seem to affect it. The filter had way more static pressure than the 2 90's

[Michael W. Clark]

OK fine you've convinced me It is impossible that my empirical data that I can feel could be correct. Your math is the only way that works.

I'm telling you. It puts out heat, I can feel it, I can see it on my thermometer. It puts out heat.

I understand, and I know the motor puts out some heat but part of your electric bill goes to move the air as well.

Im going off off of my experience as well. I've done a lot of system design/balance/ and testing.

[Leo Graywacz]

I went into my shop one morning and left the heat off. I turned on the collector. It was 45 degrees when I turned it on and about an 1/2 hour later it was 52 degrees. Not much, but it did heat the air.

[Michael W. Clark]

I went into my shop one morning and left the heat off. I turned on the collector. It was 45 degrees when I turned it on and about an 1/2 hour later it was 52 degrees. Not much, but it did heat the air.

Then you have some temperature stratification as David mentioned because you could not have done that with a 2.5 hp electric heater with 36000 ft3 of air.

[Leo Graywacz]

Absolutely have stratification. There are distinct levels that have substantial temperature change. This is very noticeable in the summer. Where it is 75 at the 6' level and 95 at the 15' level. And it changes very abruptly at a certain level. You can feel it as you climb a ladder.

Now, with my air vented outside I suspect the temp changes while running the DC will be unnoticeable.

[Michael W. Clark]

Absolutely have stratification. There are distinct levels that have substantial temperature change. This is very noticeable in the summer. Where it is 75 at the 6' level and 95 at the 15' level. And it changes very abruptly at a certain level. You can feel it as you climb a ladder.

Now, with my air vented outside I suspect the temp changes while running the DC will be unnoticeable.

Definitely agree with the stratification. I have seen that many times in the systems I've worked on. I worked on one in up-state NY where it way 60F outside but 125F in the rafters (yes I measured it because it was hot and the lift basket would almost burn your hands). I suspect you are heating with some of the motor inefficiencies, maybe you have some bearing heat in the fan, but you are also exhausting air from near the floor level which would encourage a circulation in your shop and help bring the temperature up at the floor level.

I don't think the temp changes would necessarily be unnoticeable when venting outside, because you are still going to circulate the air. Its going to depend on where your make-up air is coming in, the location of the drops, how well the air circulates, are you short-circuiting the make-up air, etc.

Just saw your other post, I think the losses for 90s and straight duct were given previously in the thread, buried somewhere in these rabbit trails.