Insulation Thickness Calculation

[PS]

Hello Engineers,

I need to calculate the insulation thickness needed for CHW pipes and ducts. The operating range of temperature in pipes is 6 to 12 deg C and of ducts is air at 14 deg C. No return air ducts used in the system. The return is through ceiling plenum. It is mentioned in the specifications to use nitrile rubber for insulation with a conductivity of 0.035 W/mK.

Would anyone please guide me in selecting appropriate thickness of insulation? Is there any formula to calculate its thickness?

I also read a few articles. They suggest two methods: (i) To prevent condensation; and (ii) based on energy efficiency (from ASHRAE Std 90.1). It was mentioned that the insulation's outer surface temperature should be greater than the dew point temperature in first method. And what is the allowed heat gain in pipes considered in the second method? I'm confused which one to use. Would you please let me know about the selection?

Thanks in advance for the support.

 

[willard3]

You will have to consult ASHRAE volumes: it's there.

 

[PEDARRIN2]

I like to compare my calculations with what I would get with these resources:

https://www.wbdg.org/guides-specifications/mechanical-insulation-design-guide/simple-calculators

https://insulation.org/training-tools/3e-plus/

 

[lilliput1]

You have to follow building energy codes which references ASHRAE 90.1 for required thickness of insulation.

 

[PS]

Thanks all, esp @PEDARRIN2. Those links were really awesome!!

And regarding ASHRAE 90.1, it is clearly mentioned that the calculation of thickness is based on energy efficiency. So, I tried calculating the required thickness of insulation that is sufficient to avoid condensation. I followed this procedure:

1. Assumed the allowable heat transfer for different pipe sizes from 5 to 25 W/m pipe length ("Q" denotes heat flow).

Allowable heat gain:
15 mm - 5 W/m
25 mm 6 W/m
32 mm 7 W/m
40 mm 7 W/m
50 mm 8 W/m
65 mm 9 W/m
80 mm 10 W/m
100 mm 12 W/m
125 mm 13 W/m
150 mm 15 W/m
200 mm 18 W/m
250 mm 22 W/m
300 mm 25 W/m

I have assumed these values from:

https://www.wbdg.org/guides-specifications/mechanical-insulation-design-guide/simple-calculators

and ASHRAE 90.1 standard. This makes sure that the final temperature near AHU is NOT increased even by .1 deg C.

2. Find the suitable outer surface temperature for the insulation ("T2" denotes the outer skin temp of insulation).
This is done by finding the dew point temperature of the space and then adding 2 deg C. That is "T2 = DPT+2 deg C"

3. Find the suitable insulation and its thermal conductivity from data books. For this example I have take nitrile rubber with thermal conductivity k=0.0385 W/mK.

4. Then use the one-dimensional steady state heat transfer equation to find the thickness of insulation.
Q= [(T2-T1)/{ln(r2/r1)/(2*pi*k*L)}] and thickness of insulation (t) = r2-r1,
where,
T1 is the pipe surface temperature = the operating fluid temperature = 6 deg C
T2 is the insulation outer skin temp to be maintained, say 24 deg C (DPT= 22 deg C, DBT=35 deg C and RH is 50%)
r2 is the outer radius of insulation from the center of pipe = need to be found??
r1 is the outer radius of pipe from its center, say r1 = 0.055m for the nominal diameter 100mm
k is the thermal conductivity of the insulating material = 0.0385 W/mK
L is the length of pipe = 1 m assumed.

Upon solving the heat transfer equation for 100mm pipe, I get the following values:
r2=75mm
and thickness t = 75-55 = 20 mm.
allowance 5 mm
Therefore 25 mm thick nitrile rubber preferred for insulation.

Conclusion:
I assumed the heat gain to be minimal so I can save energy and also I have taken care so that the insualtion will NOT condense water. Is my approach right?

Your kind responses are highly appreciated.

Thanks!

 

[quark]

Insulation thickness required to avoid condensation is more than that required to minimise temperature rise. The choice will be the thicker one, in any case.

 

[chicopee]

If you are a purist, you should also research "the economic thickness of insulation" which is detailed in any fundamental of heat transfer textbook, instead of relying solely on data published by ASHRAE or by insulation manufacturers. Researching the proposed topic will give you a better understanding of heat transfer through insulation.

 

[urgross]

Try this site:

https://insulationinstitute.org/tools-resources/free-3e-plus/

It's free and mandatory for USA union thermal insulators