- #1
buytree
- 34
- 1
How to calculate the minimum thickness of a duct which hold vacuum inside at 150 deg c? And what's the basic to select the stiffer size and distance between each stiffener.
Thanks.
Thanks.
Note that there is no additive affect of external pressure on the duct. If the duct is under external pressure only, then 304.1.3 applies. If it is subject to internal pressure 304.1.2 applies. If it is sometimes under internal pressure and sometimes under external pressure then each case can be considered separately. You simply need to ensure the duct meets the applicable paragraph for each case. There is no need to make the wall thicker than is required by either of the individual cases. Hope that's clear.buytree said:I was thinking I have to add the effect of the external pressure on the duct (ASME B31.3 Sec 304.1.3), I am working on it.
Good question, which figure to use for your material? Use CS-2 since if you check ASME B31.3 in the materials section you’ll find A 515 Grade 60 (presume you mean 60, not grade B) has a yield of 32 ksi.buytree said:3. I used the ASME Sec VIII Div 1 mentioned in ASME B 31.3 Sec 304.1.3 to find the thickness due to external pressure. I calculated the factor A and B, but I have a lot of confusion when finding factor B. My duct working temperature is 120 deg c and carbon steel A 515 grade B material. Which chart/table should I use to find factor B?
Correct. If you go through the steps in UG-28 and get to step 8 and find your actual pressure is less than the allowable pressure (Pa) then you’re pipe wall thickness is sufficient.4. According to step 8 in Sec VIII Div 1, the pressure obtained is much higher than the actual pressure I used as internal pressure. I think I am safe. Comment pl?
5. When comparing the minimum thickness formula for internal pressure in ASME B31.3 and the ASME Sec VIII. Div 1 there is a slight variation in formula right?
UG-29 covers stiffeners.6. How can I determine the stiffener required for the particulaer duct?
If you go to step 7, it provides an equation for the allowable pressure. Putting the values in with A = 0.00006 gives a pressure of around 8.6 psi which is clearly not sufficient for what you need. You need a value of 14.69.... For values of A falling to the left of the material/temperature line, see Step 7.
Q_Goest said:If your pipe is under external pressure, UG-27 does not apply and you can ignore it.
It looks like you're doing the external pressure analysis correctly but I'm not sure why you think the final thickness needs to be around 1". You went through the calculations correctly for a thickness of 0.31 with the exception that you shouldn't try to extrapolate factor B from Figure CS-2. The minimum value of A that can be used for Figure CS-2 (assuming 300 F) is around 0.00018. When that happens, then per UG-28, step 4, it says:
If you go to step 7, it provides an equation for the allowable pressure. Putting the values in with A = 0.00006 gives a pressure of around 8.6 psi which is clearly not sufficient for what you need. You need a value of 14.69.
So now you need to iterate. Increase thickness a bit. Note that this step by step method results in a highly nonlinear result for Pa. Try for example, a thickness of 1/2". In that case, you get:
L/Do = 27
Do/t = 87
Now go back to Figure G and determine Factor A. To me, it looks like A should be around .00015. Looking at Figure CS-2, we find that factor A is still to the left of the lines on the graph so again we go to Step 7 and use that equation. Now you should find Pa = 33 psi which is greater than the value you need (14.69) so 1/2" thick plate is sufficient and it could even be made thinner.
Try iterating a few more times and see what value of thickness results in a value of Pa = 14.7 psi.
The minimum thickness required for a duct depends on several factors such as the material used, the size and shape of the duct, and the temperature and pressure of the air flowing through it. Generally, the minimum thickness should be at least 0.025 inches for galvanized steel and 0.016 inches for aluminum.
The minimum thickness of a duct is calculated using the maximum pressure drop allowed, the material's yield strength, and the desired safety factor. The formula for calculating minimum thickness is: t = (P x D x SF)/(2 x Y), where t is the minimum thickness in inches, P is the maximum pressure drop in inches of water, D is the duct diameter in inches, SF is the safety factor, and Y is the material's yield strength.
No, the minimum thickness of a duct should never be decreased to save costs. The minimum thickness is calculated to ensure the structural integrity and safety of the duct. Decreasing the thickness can lead to structural failure and compromise the efficiency and safety of the duct system.
Yes, there are codes and standards set by organizations such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) that provide guidelines for determining the minimum thickness of a duct. These codes and standards should be followed to ensure the duct meets safety and performance requirements.
Insulation can affect the minimum thickness of a duct by increasing the external diameter. This increase in diameter can decrease the available space for the duct, leading to an increase in air resistance and pressure drop. Therefore, the minimum thickness should be recalculated to ensure the duct meets the required specifications with the added insulation.