For different elevations, it's simpler just to find the specific weight (lb/ft3) and use this formula: Multiplier = 0.24 x density x 60min/hr (Cp, isn't a constant, but for an HVAC engineer, it's always pretty close to 0.24) Examples: 0 000' - 1.08 1000 - 1.04 2000 - 1.01 3000 - 0.96 4000 - 0.94 5000 - 0.89
Thanks for calculating these out. You may find interest in the precalculated tables that I made, which you can find here: github.com/mitchpaulus/psychrometric-tables/releases/download/v0.1.0/psychrometric-tables-2018-09-04.pdf
It could be either depending on the conditions - this is a shorthand formula that is meant to be a quick calculation that generally works for wherever the CFM is measured. A more detailed analysis would take into account the complete psychrometric state which accounts for temperature, humidity, and air pressure.
Hello, I was working on problem 23 in the “6 minute solutions edition 2” workbook and tried to utilize this formula to calculate a reheat coil load. Doing so yielded an incorrect answer. I think a video explaining where this can and cannot be used would be very helpful.
Great video! I'm really curious to know, how did you decide to use cp instead of cv for this calculation? I've searched all over and can't find a great answer. Thank you for your help!
This is often a point of confusion. cp is defined as the change in the enthalpy of a substance per unit change in temperature at a constant pressure while cv is defined as the change in internal energy of a substance per unit change in temperature at constant volume. However, just because the phrases "at constant pressure" and "at constant volume" are in the definitions, doesn't mean these values are limited to be used in just those types of processes. They are valid for any substance in any process. With that in mind, in this case, we were looking for a difference in enthalpies. So the appropriate specific heat was cp. And we were looking for a difference in enthalpy since this was an open system in which mass flows in and out. A general rule of thumb that has served me well is that for open system problems, you are likely going to care about differences in enthalpies, which may lead you to use cp, and for closed system problems, you are going to care about differences in internal energy, in which cv is the more appropriate property.
Thank you for taking the time to help me and with such a great explanation! So the flow work term pv in h=u+pv is because now mass is flowing in and out, as in "control volume" vs "control mass". I think I have it, I hope haha.
Of course. In general, you will use this in the analysis of heating/cooling coils along with the conditioned air entering the space. I'll give you two examples: Ex 1. You will often start by estimating the design cooling load of a space. You can do this using knowledge of the outdoor air conditions and what materials the wall, windows, and roof are made of. Say you are using a VAV system for air-conditioning, and you know that you will be supplying air at 55°F, with the room at 75°F. If you calculated the cooling load to be 21,600 BTU/hr, then you could use this formula to determine that you will need approximately 1,000 CFM of air. You can then size your air handling unit and supply air fan appropriately. Ex 2. Say you need to size an electric resistance heater for outdoor air preheating so you do not freeze your water pipes. From guidelines/codes you will know the required amount of outdoor air you need, and you will also know what is the coldest temperature you expect to see, along with your preheat setpoint. If you need 1,000 CFM of outdoor air, and will require it to be heated from 0°F to 45°F, then you will need a heater with a power of around 48,600 BTU/hr or 14.2 kW.
Can you help me (I am an HVAC student) I am wanting to find CFM’s for rooms once I know the Btu/h heating and cooling loads. I have the Manual J8AE spread sheet and the CFM outputs seem high to me and I wanted to see other calculations. I tried the Sensible Heat Formula of CFM = Sensible Btuh / 1.1 * Delta T But when I input those numbers I get higher CFM needed in areas that have lower Btuh heat loss/gain. Here is my example Home is in Laguna Beach California The area labeled 1st Flr Master Bedroom has a heat loss of 9138 Btuh and a Heat Gain of 4807 Btuh Manual J8AE want to provide 299 cfm for heating and less air (244 cfm) for cooling If I do the sensible heat formula CFM = Sensible Btuh / 1.1 x ∆ T , I am using 29 ∆ T for heating and 10 ∆ T for cooling I get 286 cfm for heating and more air (437 cfm) for cooling. This does not make sense to me with needing more cfm with less of a Btuh load. Can you show me where I am thrown off?
For a given heat load, you generally have two factors you can adjust - the flow or the ∆T, and they are inversely proportional. So if you have 9,138 BTU/hr of load, you can have any CFM you want, it will just affect what the ∆T is to make the energy balance work out. However, do note that there are usually other factors that come into play such as temperature stratification when the ∆T is too high or excessive pressure loss if the flow is too high.
@@MitchellPauluswhat if ACH included into process? I have to calculate required air volume for an examination room and when I just go through the heat gain from the equipments (which is 3,14 kw) and having at the hand ACH 12, Dt =7 degree.. what formula should I use to accurately calculate air flow? Btw room area is 40 m2, and height is 3,2 m
why did you use Degree Fahrenheit instead of Degree Rankine in Gas constant Cp of the air which is 0.24 Btu/lbm.R? It kinds of confusing to me. Can you please elaborate it? thanks
sir, how it come to solution to take out the delta Temp. of the other equation to combine with the other one, just because of they have the same structure? in the other hand it must be multiply that to the equation? can you please explain further? thank you.
Good question. The quick answer is that it doesn't matter whether the cp of air is per °F or per °R since a difference of 1°F is the same as 1°R. I used °F simply out of convenience. The value of cp is normally applied to temperature differences, not an individual temperature. So for example, if you wanted the energy to raise the temperature of 1 lbm of air by 1°R, this is exactly the same as energy as it takes to raise the temperature of 1 lbm of air by 1°F. To dive a little deeper, the Fahrenheit scale is considered an "Interval" scale and the Rankine scale is considered a "Ratio" scale (See en.wikipedia.org/wiki/Level_of_measurement#Interval_scale for more explanation). But since they have the same interval between unit values, any operations dealing with differences is going to be equivalent between the two. However, you need to be careful when using multiplication of a single temperature value, for example with formulas dealing with radiation in which you have terms like σ T^4. In those cases, you must be using the absolute temperature.
I'm not sure I completely understand the question, but here's an attempt at an answer. We are considering only a single temperature change. Once you evaluate that temperature change, you can consider it as a single number. At that point, you can factor that value out of equations just like it was any other variable.
You can just pull out density out of your ass to make the equation work.? Wow...Im learning it but, I’ll feel like “thats just the way it is.” Thank you for video.
For different elevations, it's simpler just to find the specific weight (lb/ft3) and use this formula:
Multiplier = 0.24 x density x 60min/hr (Cp, isn't a constant, but for an HVAC engineer, it's always pretty close to 0.24)
Examples:
0
000' - 1.08
1000
- 1.04
2000
- 1.01
3000
- 0.96
4000
- 0.94
5000
- 0.89
Thanks for calculating these out. You may find interest in the precalculated tables that I made, which you can find here: github.com/mitchpaulus/psychrometric-tables/releases/download/v0.1.0/psychrometric-tables-2018-09-04.pdf
Great video! the standard air condition that is assumed, is on coil (Mixed air temperature) or off coil (Supply air temperature)
It could be either depending on the conditions - this is a shorthand formula that is meant to be a quick calculation that generally works for wherever the CFM is measured. A more detailed analysis would take into account the complete psychrometric state which accounts for temperature, humidity, and air pressure.
Ecellent explanation paulus
Hello, I was working on problem 23 in the “6 minute solutions edition 2” workbook and tried to utilize this formula to calculate a reheat coil load. Doing so yielded an incorrect answer. I think a video explaining where this can and cannot be used would be very helpful.
Good Work, subscribed. Thanks
thank you ❤
It takes time to understand
Thanks Mitchell. It helps.
احسنت الشرح ممتاز ،Excellent
If CFM is cub feet per minute how it became BTU/Hr in Q
Great video! I'm really curious to know, how did you decide to use cp instead of cv for this calculation? I've searched all over and can't find a great answer. Thank you for your help!
This is often a point of confusion. cp is defined as the change in the enthalpy of a substance per unit change in temperature at a constant pressure while cv is defined as the change in internal energy of a substance per unit change in temperature at constant volume. However, just because the phrases "at constant pressure" and "at constant volume" are in the definitions, doesn't mean these values are limited to be used in just those types of processes. They are valid for any substance in any process.
With that in mind, in this case, we were looking for a difference in enthalpies. So the appropriate specific heat was cp. And we were looking for a difference in enthalpy since this was an open system in which mass flows in and out.
A general rule of thumb that has served me well is that for open system problems, you are likely going to care about differences in enthalpies, which may lead you to use cp, and for closed system problems, you are going to care about differences in internal energy, in which cv is the more appropriate property.
Thank you for taking the time to help me and with such a great explanation! So the flow work term pv in h=u+pv is because now mass is flowing in and out, as in "control volume" vs "control mass". I think I have it, I hope haha.
Nice work thanks
Dear sir in sensible heat formula
Q=1.08x cfm x dt
Here can dt be in T= degree c or f
Or it should be in degree farhnit
Degrees Fahrenheit
@@MitchellPaulus thank u sir
Dear sir also can y make one video for derivation of sensible and latent heat in metric units such as in KW
how to calculate change in temperatur?
I LOVE YOU MAN
Can you go over where this forumla would be used in designing an hvac system
Of course. In general, you will use this in the analysis of heating/cooling coils along with the conditioned air entering the space. I'll give you two examples:
Ex 1. You will often start by estimating the design cooling load of a space. You can do this using knowledge of the outdoor air conditions and what materials the wall, windows, and roof are made of. Say you are using a VAV system for air-conditioning, and you know that you will be supplying air at 55°F, with the room at 75°F. If you calculated the cooling load to be 21,600 BTU/hr, then you could use this formula to determine that you will need approximately 1,000 CFM of air. You can then size your air handling unit and supply air fan appropriately.
Ex 2. Say you need to size an electric resistance heater for outdoor air preheating so you do not freeze your water pipes. From guidelines/codes you will know the required amount of outdoor air you need, and you will also know what is the coldest temperature you expect to see, along with your preheat setpoint. If you need 1,000 CFM of outdoor air, and will require it to be heated from 0°F to 45°F, then you will need a heater with a power of around 48,600 BTU/hr or 14.2 kW.
Is it possible to compute for cfm requirement with temperature difference
Yes, using algebra, CFM = Q / (1.08 * ΔT) where Q is in BTU/h and ΔT is a temperature difference in °F.
great video! do you use manual D to design ducts?
My personal work is focused more on existing building commissioning, so I don't end up doing much design.
Can you help me (I am an HVAC student)
I am wanting to find CFM’s for rooms once I know the Btu/h heating and cooling loads.
I have the Manual J8AE spread sheet and the CFM outputs seem high to me and I wanted to see other calculations.
I tried the Sensible Heat Formula of CFM = Sensible Btuh / 1.1 * Delta T
But when I input those numbers I get higher CFM needed in areas that have lower Btuh heat loss/gain.
Here is my example
Home is in Laguna Beach California
The area labeled 1st Flr Master Bedroom has a heat loss of 9138 Btuh and a Heat Gain of 4807 Btuh
Manual J8AE want to provide 299 cfm for heating and less air (244 cfm) for cooling
If I do the sensible heat formula CFM = Sensible Btuh / 1.1 x ∆ T , I am using 29 ∆ T for heating and 10 ∆ T for cooling
I get 286 cfm for heating and more air (437 cfm) for cooling.
This does not make sense to me with needing more cfm with less of a Btuh load.
Can you show me where I am thrown off?
For a given heat load, you generally have two factors you can adjust - the flow or the ∆T, and they are inversely proportional. So if you have 9,138 BTU/hr of load, you can have any CFM you want, it will just affect what the ∆T is to make the energy balance work out. However, do note that there are usually other factors that come into play such as temperature stratification when the ∆T is too high or excessive pressure loss if the flow is too high.
@@MitchellPaulus Thank you
@@MitchellPauluswhat if ACH included into process? I have to calculate required air volume for an examination room and when I just go through the heat gain from the equipments (which is 3,14 kw) and having at the hand ACH 12, Dt =7 degree.. what formula should I use to accurately calculate air flow? Btw room area is 40 m2, and height is 3,2 m
how to compute cfm aircon 3tr & 1.5 hp
Cool
We are closing in 🤪
What is BTU, LB, FT^3? Are these dimensions in international system? Then what is distance to the Moon? Ooouh, and the radial speed in MPH..?
why did you use Degree Fahrenheit instead of Degree Rankine in Gas constant Cp of the air which is 0.24 Btu/lbm.R? It kinds of confusing to me. Can you please elaborate it? thanks
sir, how it come to solution to take out the delta Temp. of the other equation to combine with the other one, just because of they have the same structure? in the other hand it must be multiply that to the equation? can you please explain further? thank you.
Good question. The quick answer is that it doesn't matter whether the cp of air is per °F or per °R since a difference of 1°F is the same as 1°R. I used °F simply out of convenience. The value of cp is normally applied to temperature differences, not an individual temperature. So for example, if you wanted the energy to raise the temperature of 1 lbm of air by 1°R, this is exactly the same as energy as it takes to raise the temperature of 1 lbm of air by 1°F.
To dive a little deeper, the Fahrenheit scale is considered an "Interval" scale and the Rankine scale is considered a "Ratio" scale (See en.wikipedia.org/wiki/Level_of_measurement#Interval_scale for more explanation). But since they have the same interval between unit values, any operations dealing with differences is going to be equivalent between the two.
However, you need to be careful when using multiplication of a single temperature value, for example with formulas dealing with radiation in which you have terms like σ T^4. In those cases, you must be using the absolute temperature.
I'm not sure I completely understand the question, but here's an attempt at an answer. We are considering only a single temperature change. Once you evaluate that temperature change, you can consider it as a single number. At that point, you can factor that value out of equations just like it was any other variable.
@@MitchellPaulus 👍
You can just pull out density out of your ass to make the equation work.? Wow...Im learning it but, I’ll feel like “thats just the way it is.” Thank you for video.
Will you be my tutor?
less is more
wrong, sensible heat and total heat, where is the latent heat,steam.sensible heat is thermal, latent heat is humidity.
Kittipon khaongam
Hello can I have your email please Paul ??