Expansion joints for fan application

[Engineer6512]

I am working on an industrial fan application. These are 50 hp, 1800 rpm, 10,000 cfm fans operating at a pressure of 57" WG.

The fans are mounted using vibration isolators on structural steel. There are flanged connections to both the suction and discharge ductwork. Maximum vibration is 68 mils.

In the fan manufacturers documentation they are saying that expansion joints are required on fan inlet and outlet. The problem is that I have extremely limited space as it is. I am planning to simply use approx. 1/2" rubber gasket between the flanges on both inlet and outlet.

Just wondering what some of your thoughts are on going this route instead of one of the commerically availble expansion joints.

Thanks.

 

[CESSNA1]

BEAZLEY: Check ducting manufactureres - there are flanges and bolt on expansion joint systems that have an 2"-6" between them separated by an airtight fabric. They do not add any eccentric loading to the fan or ductwork and are far better than a solid connection like you propose.

regards
Dave

http://www.wsec1.com

 

[rmw]

The reason the fan mfg wants expansion joints is to keep ductwork loads off of the fan housing. Loads imposed by expanding/contracting ductwork can tweak fan housings and cause rubbing and vibration problems.

Whatever you end up doing, store bought expansion joints or shop built, (both work) they should accomplish the purpose of isolating the fan from the duct loads.

No situation that I have ever seen with enough duct expansion that required EJ's would have been solved with a 1/2" piece of rubber gasket between the flanges (which is what I interpreted you to have said, as opposed to a stand off type EJ made out of 1/2" think rubber.)

You did not give a temperature, but 57" wg is not necessarily a plain vanilla EJ. Be careful. If it is air at ambient temps, don't forget internal EJ's if your duct is large enough to get into.

rmw

 

[Tmoose]

And there will be thrust rods or tie rods across the joints, and those tie rods will need slightly engineered springs of their own.

Sample spec, with some basis in practicality -

Fans and Air Handling Units:

1. Such units shall have electrical flexible connections not less than 36" long and the flexible duct connections with a free length of not less than 8".

 

[rmw]

In my limited experience, I haven't seen seen any tie rods used on fabric type duct work expansion joints, (except for shipping purposes, later removed) while they are commonly used for metallic corrugated pipe work expansion joints. I don't recommend a pipe expansion joint for the application mentioned by Beazley. They have too much spring rate to them. Metallic duct work EJ's (convoluted type) have a small amount of spring rate to them too.

rmw

 

[Engineer6512]

Thanks very much for the responses. I am going to try to source some of the short rubber flange-type EJ that CESSNA1 has mentioned.