Mass flow into Mass

[SkyWalker]

hi guys trying to roughly work out what KW a heater battery is doing using kw=massxSHCxTD/3600 but i have a "mass flow" of air and not a mass? how can i convert this to a mass?


8836m3/h is the air flow i have over the said battery.

BTW, am i correct in dividing by 3600?

what im trying to do is find out how many kw i am getting from a heater battery at a particular time, so im thinking maybe i dont divide by a time? (3600), my delta T over said battery heater is 8.2k, and i will take SHC of air at 1

Cheers tech heads! =)

[frank]

For air at 20C, I get that to about 25kW

You are correct to divide by 3600 to convert your mass flow to m3/s (1W = 1J/S)

For air at other temperatures, your density will change which will affect the end result number, but not by much.

If you had 8836m3 of air you would have a Mass. As you have got 8836m3/hour, you have a mass 'flow'

[SkyWalker]

Hi Frank thanks for quick reply, how did you change mass flow into a mass (kg/s) cheers, i find it interesting on site when you have a quick snapshot of what your actually getting compared to name plate ect. I would like to know the calculation for my self

[frank]

Q = m c p dt

Duty = mass flow (m3/s) x sensible heat capacity (for air at 20C 1.22kj/kg) x density (@ 20C 1.02kg/m3) x (T1 - T2)

[SkyWalker]

thanks for the reply frank this will come in handy so much!!!!!a bit different to the product load calc i was using.

when you say air at 20c is this 20c air on i presume? my case is actually 7c so i would use SHC and Density for 7c?

also dont suppose you know of a good table to cross reference difference air temps and get density and shc?

is Specific heat capacity different to sensible heat capacity? if so this this calc would be for sensible which would be fine for heating but if calculating a cooling coil would i also need to do a latent output and add the two? sensible + latent = total kw?

heating is only sensible anyways?

[Magoo]

To be really accurate use the total heat formula, using enthalpy change, cross check power input with an amp meter, if you know the power factor calcs are accurate.

[SkyWalker]

Found this for density and SHC http://www.engineeringtoolbox.com/ai...ies-d_156.html

And this for latent heat
SI-Units
The latent heat flow can be expressed in SI-units (metric) as

Ql = hwe ρ q Δx / 3600

where

Ql = latent heat flow (kW)

hwe = 2465.56 - latent heat of vaporization of water (kJ/kg)

ρ = 1.202 - air density at standard conditions (kg/m3)

q = air flow (m3/hr)

Δx = humidity ratio difference (kg water/kg dry air)

I'm abit unsure on the humidity ratio could someone explain this for me? Keep it screwdrivers and spanners

[Magoo]

Don't forget fan Kws added to total heat formula. Work /or kws is added to total heat for the system. You punch the air through a HX and fan work/ heat will add to gains in total heat. Depending if forced supply or induced flow and where you take DB and WB readings. Total statics loses across fan/plus coil, and velocity readings and discharge static readings versus the supplier fan curve.

[frank]

I'm abit unsure on the humidity ratio could someone explain this for me? Keep it screwdrivers and spanners

I think it would be easier if you were to look up 'Relative Humidity' on Wikipedia, rather than trying to explain it here. Here is an extract to start you off:

Relative humidity which is also termed as humidity ratio is classified in three terms

1. on the basis of pressure (partial pressure of vapor in air / pressure of saturated air)
2. on the basis of mass (mass of vapour present in air / mass of dry air)
3. on the basis of density (density of the vapour present in air / density of dry ai

[SkyWalker]

hi frank, I understand relative humidity I was just unsure what the equation required

let me give an example to see if im using this equation correctly, density of 20c air = 1.202, if I had 20c DB and 17c WB air on to my cooling coil i would plot these values on a psychometric chart and find the kg/kg, in this case it would be 0.011 kg water / kg of air for the air on. 1000 m3/h
So........

QL = 2465.56 x 1.202 x 1000 x 0.011 / 3600 = 9.06kw of latent heat ..........correct? :/

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But it does say difference so I'm unsure if it's air on kg/kg minus air off kg/kg?? If it's this then the equation above is incorrect
and it should look like this ,lets say the air off was 15c DB and 10c WB (0.0058 kg/kg) so we would do 0.011 - 0.0058 = 0.0052 kg/kg difference

So........

QL = 2465.56 x 1.202 x 1000 x 0.0052 / 3600 = 4.28kw of latent heat ..........correct? :/

[SkyWalker]

does anyone know which is correct above? if either? i think its the second?

[frank]

Sorry for the late response Skywalker but haven't had much time these past few days.

When you use the Pshycrometric chart, the kg/kg of moisture on the R/H side relate to the content of moisture at a particular point relative to the WB/DB point.

This table is used when you plot 2 different point on the chart, i.e. how much moisture is removed/added (cooling/humidifying). You draw a horizontal line on the chart relative to your 2 points and the moisture value is (h1 - h2)

This is a little different to the density of air at a particular temperature see here : http://www.engineeringtoolbox.com/ai...ume-d_195.html

The formula I refered to was for density of air per m3

[SkyWalker]

Hi Frank thanks for the reply, so am I correct in thinking my second example is correct?

[kenbarls]

Q = m c p dt

Duty = mass flow (m3/s) x sensible heat capacity (for air at 20C 1.22kj/kg) x density (@ 20C 1.02kg/m3) x (T1 - T2)

I think I'm gonna have to drink more beer to understand this equation.

[SkyWalker]

just by luck i have been working more with pschometric charts at college and found a really acurate way of finding total kw, latent kw and sensible kw.

this is.....kiloWatts (kj/s) = Mass Flow Air(kg/s) x Difference in enthalpy (kj/kg)

Mass flow = m3/s x density of air = (kg/s)

this is a really good equation and im told (by my lecturer) very accurate.

i realise you know this already but it may be of use to newbies to stuff like this. (a bit like me )

Really handy when applied on site though if your unsure what output your getting from a heater or a cooling coil

[frank]

Sorry Skywalker, I've been a little preoccupied just lately and havent had the time to pop back on the site to reply.

Looks like you have found the answer though.

Good to hear that you are learning lots.