Air Coolers: Forced vs Induced draft 1

[Peregrino7]

I have specified an Air cooler with forced draft. But now, I have to change to Induced draft, keeping geometry, motor and fan power the same as in forced draft.
I know that the performance is not going to be the same, but the equipment working with induced draft is performing acceptable.

My question is regarding the difference in price... Does somebody know what would be the difference in price between a forced draft and an induced draft? (keeping the same bundle geometry and motor and fan power and geometry)Something like "Induced draft is ussualy 10% more expensive than forced", etc...

Thanks for your answer...!


GH

 

[Inthevalley]

Peregrino7,

In a fossil fired power station environment, the induced draught (ID) fans are usually around 1.5 times cost of the forced draught (FD) fans. In addition they use substantially more power. Larger motors also have major downstream effects e.g. switchgear, transformers etc. The reason for this is the reaction between carbon and oxygen to form large quantities of CO2. In addition the volume of the gas expands due to the increased temperature. The last factor is the amount of inleakage air (also called tramp air) which you allow for.

If I understand your application correctly, you don't seem to have major issue bar the changes you allow for in terms of temperature and tramp air affecting the volume. From a design point of view, the fans will thus be "similar", thus the cost for either FD or ID should be roughly the same. Just make sure you allow enough room for tramp air! Thus you may not be too far off the mark with your 10% guess...

 

[speco]

Peregrino7,

I would like to weigh in on this question with a slightly different answer. Part of the answer depends on whether your spec's for the project include API-661. The API spec requires that the motors and drives be mounted below the tube bundle, necessitating a long fan shaft to go from the drive to the top-mounted, induced-draft fan. This means that the fan shaft must go either through the tube bundle or between a pair of them and through a blank-off plate of some type. Note that this would be a slight change in the tube bundle geometry, since you will lose a few tubes from the bundle.

If gear drives are used, this adds a bit more to the complexity, since there would be an additional coupling between the gear and the fan shaft. On forced-draft coolers with gear drives, the fan can usually be mounted directly on the output shaft of the gear.

That is the major mechanical difference between most forced-draft and induced-draft coolers.

If your specs do not include API-661 or anything requiring this type of construction, then it may be possible to use top-mounted motors and drives. Outlet air temperatures should be considered here as well. In some cases it is just not a good idea to have mechanical equipment it a hot air stream.

The other aspect to consider is the fan horsepower. With the fan mounted in the exhaust stream, the air volume is increased proportionally to the absolute temperature of the air. This usually means a slightly higher fan horseposer requirement. Slightly is a relative term. If the air is very hot (say, over about 200 F), I would suggest rethinking the induced-draft idea.

The actual cost difference of the basic cooler should not be more than about 5-10% however. The mechanical differences are just not that great. This really depends on the manufacturing standards of the cooler maker. Some prefer induced draft, others forced. Generally the forced draft coolers are a bit easier to maintain due to their simplicity.

Regards,

Speco (

http://www.stoneprocess.com)