Static Pressure for Exhaust fan

helo iam sivabalasubramanian

working for ISRO-GOVT OF INDIA

IAM A HVACE ENGINEER

ANSWER

you have not mentioned the unit for static pressure i presume it is
= inchs SWG

Please refer to the friction chart for ducts availble in ASHRAE handbook or

design data hand book for hvac

you need two inputs

1.cfm of the duct

2. size of you duct

you get the pr drop for the above parameters

In genral for the exhaust system

1. Air change reaquirement for a given application (No of airchanges )

(this is available in the design data hand book or ventilation requirement as per ASHRAE standards 62-1989)

find out the cfm required based on the total volume of the room

use the 'ductulator'a mechanical device to form your duct layout

normally the duct layouts are done by keeping the total pressure drop of the entire system does not exceed between 0.1" SWG - 0.5 "SWG / 100 feet length of duct. (units in inches of standard water gauge)

Hence, you can select the fan with 0.125 swg for the cfm of your requirement so that it can take care maximum drop inthe duct layout. Provided it is made by using equal friction method

static pressure is, pressure builtup by the fan so that the fan can deliver the selected cfm against the SP ( pressure drop offered by the entire duct layout)

if you give your exact requirement like room size and application I can help you more on this

I hope that you have satisfied.

sivabala

thanks sivabalasubramanian for your quick reply,

Gr8 job at ISRO,

Here I would write more specific example,

Suppose I need to exhaust 400cfm from Toilet, and for that I need to select exhaust fan,

now from cataloge I have to choose Static pressure and I don't how to calculate static pressure for that, Ya unit is in inch. of WG, (i.e 0.125, 0.25 etc) which one is more suitable for exhaust fan,

I think it depends upon length of ductwork , I might be wrong,

For duct size selection, i use to take 0.08 inch. of wg friction losses,

now I have CFM, and duct size i have taken as per 0.08 inch. of wg friction losses, so for exhaust of 400 cfm how do we select static pressure,

please reply when u get the mesg,

thanks,

NJcool- In response there are two good sources for information. First is a set of ASHRAE (American Society of Heating, Refrigeration, and Air conditioning Engineers) manuals and the second is the Industrial Ventilation Manual (American Conference of Governmental Industrial Hygienists). Both cover aspects of room or equipment ventilation and fan selection.

More specific, for determining your static pressure requirements you must consider all parts of the air movement trail. This means pressure loss entering your "room" (gaps around doors, ventilation slats, ducting, etc.), loss across your "room" (related to room size, location of inlet and outlet air connections), expansion and contraction losses at room walls, ductwork losses (proportional to duct length), and loss/regain in pressure at the exhaust of the fan (does fan exhaust to atmosphere or thru additional ducting). Also, make sure your CFM is based on the minimum requirements for removal of heat, odor, or contaminants from your room (see ASHRAE guidelines).

Total static pressure losses are summation of each individual loss, and all branch lines must be balanced to ensure the static loss is equal in each branch. Loss for your example may be around 1" W.G. depending on your total system layout.