How do I calculate the overall vapour flow?

In summary, the conversation discusses the use of various values from the textbook "Building for a Cold Climate" in calculating the permeance and vapor flow rate through different materials. The steps involved include determining the permeance of a 10mm gypsum board using ratios, finding the vapor pressure through relative humidity and saturated vapor pressure, and using the vapor flow rate equation to calculate the change in pressure. The speaker is unsure of how to incorporate the "50mm cavity" and area values, but assumes an area of 1 square meter based on the textbook example. Overall, the conversation focuses on the diffusional flow rate of water vapor through different materials.
  • #1
Homework Statement
A cavity wall consists of a 100mm brick exterior with 50mm cavity and 10mm gypsum board. The inside temperature is 20°C and RH of 40%. The outside temperature is -10°C with RH of 85%. Calculate overall vapour flow over one day.
Relevant Equations
R = 1/M, RH = Pw/Pws x 100%, Qv=A(µ/l)(pw,1 - pw,2)
Step 1:
Values are from textbook 'Building for a Cold Climate'
Mbrick = 46 ng/s*Pa*m^2
Mgypsum = 2870 ng/s*Pa*m^2 (for 9.5mm)
took the above value and used ratios to determine permeance for 10mm Gypsum board (2870/9.5 = X/10)
Mgypsum (new value) = 3021.05 ng/s*Pa*m^2

Step 2:
Values are from textbook 'Building for a Cold Climate' (pressure over ice)
T in = 20°C --> Pws = 2.337 kPa = 2337 Pa
T out = -10°C --> Pws = 259.7 Pa

Step 3:
Resistance --> R = 1/M
R brick (common) = 1/46 = 0.0217
R gypsum = 1/3021 = 0.000331

Step 4:
Used relative humidity (and saturated vapour pressure) to find vapour pressure
RH = Pw/Pws x 100%
(1) 20°C Temp (in), 40% RH --> 0.40 = Pw/2337
0.40 x 2337 = Pw
Pw (in) = 934.8 Pa
(2) -10°C Temp (out), 85% RH --> 0.85 = Pw/259.7
0.85 x 259.7 = Pw
Pw (out) = 220.75 Pa

Step 5:
I understand that normally I would add all M-values (permeances) to get total permeance and then use the vapour flow (Qv) rate equation:
Qv = A(µ/l)(pw,1 - pw,2) or Qv = A*M*(pw,1 - pw,2)
With the values that I have - and am sure of - the only part of the calculation I've figured out was the change in pressure, where I subtract the high vapour pressure with the low vapour pressure:
pw,1 - pw,2 = Δp
934.8 - 220.75 = 714.05 Pa

I'm confused as to how the cavity part comes in ("50mm cavity") and the value for area since the givens only consist of thicknesses. I know that the textbook example uses 1sqm so I feel that I should assume the A-value as well. Are there any more assumptions I am missing? Are the steps that I've done so far correct?
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  • #2
**Book is called Building Science for a Cold Climate
  • #3
Your data describes the diffusional flow rate of water vapor through each material, as a function of the partial pressure of water vapor, correct?

Related to How do I calculate the overall vapour flow?

1. What is vapour flow and why is it important to calculate?

Vapour flow refers to the movement of gaseous particles in a given space. It is important to calculate because it can help determine the efficiency of processes such as distillation, evaporation, and condensation. It can also provide valuable information about the composition and behavior of gases in a system.

2. How do I measure the overall vapour flow?

The overall vapour flow can be measured by using a flow meter or by calculating it using the ideal gas law. The flow meter can provide direct and accurate measurements, while the ideal gas law involves measuring variables such as temperature, pressure, and volume to calculate the flow.

3. What factors can affect the overall vapour flow?

The overall vapour flow can be affected by a variety of factors such as temperature, pressure, composition of gases, and the physical properties of the system. Changes in these factors can lead to fluctuations in the vapour flow.

4. How do I calculate the overall vapour flow in a closed system?

To calculate the overall vapour flow in a closed system, you will need to measure the volume of the system, the pressure, and the temperature. You can then use the ideal gas law (PV = nRT) to calculate the number of moles of gas present, which can then be used to determine the overall vapour flow.

5. Are there any limitations to calculating the overall vapour flow?

Yes, there are limitations to calculating the overall vapour flow. The ideal gas law assumes that the gases in the system behave as ideal gases, which may not always be the case. Additionally, the presence of non-ideal conditions such as high pressures or low temperatures can also affect the accuracy of the calculations.

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