Fan sizing for heat removal

[mattud]

I am a non-HVAC mechanical engineer that needs help. I have an industrial machine application where I need to determine the required fan size. I have determined the required CFM based on heat removal only (no losses in the duct). My duct has an area contraction, a 90 deg elbow and another area contraction due to packaging limitations. The total length of the run is less than 5 ft. Do I simply calculate my friction pressure at each contraction and the elbow and add them together, or is there more to it? Any information would be helpful.

 

[DouginMB]

What are the airflow and duct sizes? If the duct is sized to allow an air velocity of 1000-1800 fpm, you won't need to calculate a static pressure for selection purposes. 5 feet of duct is not very long and should only result in a static pressure drop of 0.25" (at most). Of course, this depends on the duct dimensions and how the air terminates (wall cap,louver,roof cap).

 

[62hog]

mhayas,

The CFM will not change due to your ductwork. What will change, though, is the static pressure you need to specify your fan for. The static calculations will need to be as you have said. I would also add a safety factor of your choosing.

Just to be sure you have sized your CFM properly, did you use a formula similar the the following:

CFM = BTU /(1.08 * (Delta Deg F))

Hope this helps.

 

[lilliput1]

Are you cooling with outdoor air or conditioned air? Is the air 100% OA so you have a way to bring it in (supply air ventilation fan or outdoor air louver) and exhaust it out? If you are only using the ehaust air to draw in OA for cooling through the intake louver, make sure you use 500 FPM intake velocity through louver free area & allow about 0.25" loss for intake to be added on to the exhaust losses. Now placement of the exhaust is critical. You may have to duct the exhaust fan so it picks up exhaust over the hot spots. Include heat from light, heat from people, heat from all the equipment including transformers in the room. For quick estimate assume the equivalent outdoor air temperature + solar load is 130°F & do heat loss by conduction through the building skin. For cooling with OA usual maximum indoor design is 104°F (maximum rated
operating ambient for motors, transformers). CFM = Btuh/(1.1 x TD) Where TD = 104-outdoor air design temperature.

 

[pardal]

mhayas:

Would you please state the CFM, pipes, elbow, and transition duct size?????



Pardal

 

[mattud]

Thank you for your input.

Yes, 62hog, that is similar to the formula I am using.

I have determined I need 500CFM to cool a rollerscrew that is generating heat. The fan is pulling in air from the plant through a filter. The plant air could easily reach 110F in the summer. The duct is as follows - 38.21 in[sup]2[/sup], contracts to 19.10 in[sup]2[/sup], 90 deg elbow, contracts to 12.46 in[sup]2[/sup], exits to rollerscrew. My initial calculation shows a static pressure drop of .56". Does this sound reasonable???

 

[ChasBean1]

DP is too low. Size the fan for 500 cfm at 2.5 in w.c. and please report back how it does.

 

[lilliput1]

You CFM may be too low. What is the load & the temperature you want to cool it down to. If the plant air gets to 110°F you should switch to using outside air. You may have to design a recirculation system so in winter the discharge is routed back to mix with the OA. You can put controls for the desired mixed air temperature that the controls will try to attain until it goes to 100% OA. Main thing is to prevent freezing. You should not forget pressure drop through equipment in calculating fan total pressure. The filter loss when dirty would be at least double the clean filter pressure drop. If you give me your email I can give you a sample of a Excel spreadsheet based duct static pressure calc program. Note however that you have to indicate pressure drop of equipment. The program do list typical equipment & typical pressure drops.