Discussion of 1.08 CFM Delta T

[john1]

All,

I was hoping to have a discussion / direction on applying q=1.08 CFM delta T. I have a pump room that is 10' high, 13.3' wide and 14.8' long. Equipment is providing heat gain to the space of 52,480 Btu/hr and I am using a wall exhaust fan and intake louver. I have traditional designed around a 15 to 18 degree rise (use 18 for here). This calculates out to be about 2,700 CFM (81 ACH). This is about 20ft/s and the outside air is 99 deg. The equation tells me that I will be exhausting 117 degree air which I am having trouble (today) believing that air will actually heat up that much moving it through such a small space and that quickly. An air turn over rate of 15 seems more reasonable, but equation wise that is 500 CFM and a 97 degree wise.

The goal is to limit the temperature rise in the room. Is "q=1.08CFM delta T" being applied/interpreted correctly in this situation? Am I outside the limits of what that equation is meant to provide?

thanks

j

 

[MintJulep]

If you're asking us to check your algebra, then yes

1.08 x 2700 x (117 - 99) = 52488.

However,

2700 cfm / 133 ft^2 = 20.3 ft/min not 20 ft/sec

Not very fast.

 

[john1]

Not really needing an algebra check (other than not being able to type the correct units). Agreed that 20 ft/sec isn't that fast, but I guess that depends on what you base fast on. Still for this situation, an "amount" of air will move through the building in less than a second. Once the "system" reaches equilibrium, will that room reach 117 degrees based on the exiting air temperature? The btu/hr generated to the space are localized around the equipment. The btu/hr are not distrubuted equally about a cross sectional area of the building. So is the equation correctly predicting a temperature rise for the entire exiting air of the room or is q=1.08CFM Delta T over simplifying the actual conditions occuring in the room?

j

 

[MintJulep]

Consider that 50 ft/min is a rule of thumb limit to avoid "drafty" complaints in office buildings. 20 ft/min is pretty near undetectable by feel.

117 is an accurate prediction of the average discharge air temperature if we are happy ignoring radiation and heat loss by conduction through the building walls.

If you need another comparison, consider that 52,000 BTU/hr is roughly half the heating capacity needed for a modest 3 bedroom house in the Northeast US, and you're dumping it all into a bedroom.

 

[john1]

AaaRRggghhh....My apologies MintJulep (sometimes you have to club me in the head 2, 3, 4.. times). I got 20 feet per second stuck in my head when doing the calculation instead of feet per minute. Ok, everybody on the boards give one more shake of the head.

 

[AbbyNormal]

exhaust air will be a little hotter, 1.08 os based on 70F air

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[Zesti]

(You guys really need to change to metric...)

The actual amount of heat generated by the equipment might not be as high as you think.

There is often some exageration (..) and maybe not all equipment is working at 100% all the time etc..

What you do need to consider is the maximum allowed temperature for the equipment. This might give you the maximum temperature difference between entering and leaving air temperatures.

I would think that air speeds are not an issue because presumeably nobody will be staying in the pump room.

 

[willard3]

Why change to metric? A computer doesn't give a damn if it divides by 10 or 12 or any other number.

 

[imok2]

Gennerally, electrical/pump equipment rooms require only ventilation to keep equipment from overheating. Most electrical/pump rooms are designed for 95*F to 105*F. If the OSA temperature is used to ventilate the electrical/pump room the room will be 10*F to 15*F above outside summer design temperatures.So if you use 2 CFM /sq foot that would be about 296 CFM

 

[Zesti]

Off Topic:

@willard3

Well, maybe not change to metric as such but what seems confusing to me (and it was AbbyNormal's remark about the 1.08 being based on 70F that sparked my remark) is that it seems that often when using Imperial units a lot of "factors" enter the equations. These factors then include physical constants and factors to account for conversions of units all rolled into one.

When using:

Power [Watts] = flow [m3/s] x density [kg/m3] x specific heat [J/kg.K] x Temperature diff.[K]

It is clear to anyone what is going on and everything fits neatly together without conversion factors.

Or how about:

Theoretical Fan/Pump power [Watts] = flow [m3/s] x Pressurediff. [Pa=N/m2]

You can't deny the neatness of that one...

Anyway, just my opinion.

PS: I have heard that people in the US do learn metric units in schools but tend to forget about all that when they enter the professional world. Any truth in that?

 

[MintJulep]

I think you will find that most US engineers under 50 or so are perfectly conversant in both SI and foot-pound systems.

 

[mauricestoker]

Too late in life to change from 40's, tall boys, regulars and ponies to think about drinking liters and half liters.

 

[ChasBean1]

I don't like US units, but we’re forced to live by them. Now, unfortunately, I think by them. Give me cfm and gpm any day, and I have a complete feel for it. Give me SI; I have to convert to have a feel for it.

But anyone who has experience with both knows that SI is much more sensible. Everything is better.

Example - pressure of a fluid column of 100’ (30.5 m) of air: P = rho * g * h

English: .075 lbm/ft3 * 32.2 ft/s2 * 100 ft / 144 in2/ft2 / 32.2 ft-lbm/lbf s2. Multiply by the gravity then divide by the gravitational constant? What’s This For? Then maybe since it's air, convert PSI to in. w.c., which would be this answer times 29.9 (a mercury atmosphere) divided by 14.7 (a mercury to psia equivalent), * 13.6 which is a ratio for mercury density compared to water density. So you get about 1.44: w.c.

SI: 1.2 Kg/m2 * 9.8 m/s2 * 30.5 m * 1 N•s2/Kg•m = 359 Pa

SI is so much better in all calcs, all results, common sense factor is a 10 for SI and about a 3.5 for US units. I want the conversion to happen and I want to think in SI (this would take about a year or two for the thought conversion process). But we should be forced to change.

Maybe Obama will help?

 

[AbbyNormal]

SI will be a farce until they make a metric sheet of drywall

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[IRstuff]

The only reason units are even remotely relevant is simply because there are only about 2 calculation tools that can truly handle units, Mathcad, and SMath, and the latter's ability is not fully implemented.

In Mathcad, which I use daily, both equations are equally valid, and equally easy to enter.

And if that were the way your calculators worked, the unit representations would be completely transparent and irrelevant. In Mathcad:

0.075*(lb/ft^3)*g*100ft = 359.1019Pa; this is EXACTLY the default output from Mathcad. No fuss, no muss

Unfortunately, Mathcad is expensive, and high maintenance, but unit conversions are trivial and transparent in it. Hopefully, before my children retire, there will be an inexpensive calculator/calculation tool that does the same thing for less than $20. Some current calculators are almost there now.

TTFN

FAQ731-376

 

[AbbyNormal]

SI is simplier math for sure.

When I was 12, Canada officially went metric.

People adapted easy enough to speed limits, the temperature scale, litres and kilograms in their day to day lives.

But when it came to construction the only drawings done, were those for government jobs (as of 1998 when I left Canada anyways)

The reason I say metric is a farce is because it is all a 'soft' conversion, wall studs listed as being on 400 or 410 mm centres, trusses on 610 mm centres. The sheet rock is still four feet wide, the centres are 16 and 24 inches its not `1.2 metres wide'.

I never made or bought a prefabricated section of 300 mm diamter round duct or spiral, it was 12 inch.

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[EnergyProfessional]

actually many things are metric, but called IP. The 3.5" disk actually is 90 mm exactly, which is a bit different than 3.5"

Coming from Europe (in WI now) of course I favor metric systems. The thing that drives me nuts is the so many units for the same thing:
Pressure in psi, psf, inch of water,feet of water, inch of mercury etc. and not a convenient conversion. Most people that use those units don't really know what they mean anyway.
whether you like SI or not, but at least it only has one unit for pressure (with it's 10-fold derivations in kilo, mega etc, but a monkey can convert that by floating points)

Don't even get me started on the 120 volt system here. I'd love to have a 230 volt system to not need the 208 V etc.

 

[MintJulep]

but at least it only has one unit for pressure

Really?

Pascal
Bar
Kg/square meter
Kg/ square centimeter
dyne/square centimeter
Torr
microns of mercury
mm of mercury
cm of water
meters of water
Atmosphere
Barye

 

[EnergyProfessional]

Everyone uses Pa in SI. Maybe sometimes people convert the end result to atmospheres to give "non engineers" a point of reference.

kg/m² is not pressure. kg is mass. And mass as weight force depends on gravity. N/m² is Pascal. kg/m² is zero Pa in space, so this unit is pointless.

Bar is just 1e5 Pa, not a different unit. Just coinciding with approximate atmospheric pressure. I only know torr, microns etc. from history books (and from the US). Really, no one uses that in Europe. Maybe some old folks in the UK use them in conversation, but they also drive on the wrong side.

 

[mauricestoker]

I used metric through college, and metric for eight years working overseas. Personally, neither was easier than the other. Math is math. The sticking point is conversion for available manufactured equipment and supplies. If your vendors/suppliers are metric, you specfiy in metric. Not sure I've ever noticed a difference, other than drinking beer.