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Will the static pressure of a fan change with the RPM & flow rate?

If, Yes, then how the Static pressure of the fan is maintained constant in the Variable Air Volume system(VAV)?

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Question ajoutée par Jamsith Ahmed S , Project Engineer , Blue Line Establishment LLC
Date de publication: 2017/05/03
chetan sharma
par chetan sharma , Senior project engineer , trinity engineering services

Pease refer to law of affinity

Law 1. With impeller diameter (D) held constant:

Law 1a. Flow is proportional to shaft speed:[1]

Q 1   Q 2 = ( N 1 N 2 ) {\\displaystyle {Q_{1} \\over \\ Q_{2}}={\\left({N_{1} \\over N_{2}}\\right)}} {Q_{1} \\over \\ Q_{2}}={\\left({N_{1} \\over N_{2}}\\right)}

Law 1b. Pressure or Head is proportional to the square of shaft speed:

H 1 H 2 = ( N 1 N 2 ) 2 {\\displaystyle {H_{1} \\over H_{2}}={\\left({N_{1} \\over N_{2}}\\right)^{2}}} {H_{1} \\over H_{2}}={\\left({N_{1} \\over N_{2}}\\right)^{2}}

Law 1c. Power is proportional to the cube of shaft speed:

P 1 P 2 = ( N 1 N 2 ) 3 {\\displaystyle {P_{1} \\over P_{2}}={\\left({N_{1} \\over N_{2}}\\right)^{3}}} {P_{1} \\over P_{2}}={\\left({N_{1} \\over N_{2}}\\right)^{3}}

Law 2. With shaft speed (N) held constant: [1]

Law 2a. Flow is proportional to the impeller diameter:

Q 1   Q 2 = ( D 1 D 2 ) {\\displaystyle {Q_{1} \\over \\ Q_{2}}={\\left({D_{1} \\over D_{2}}\\right)}} {Q_{1} \\over \\ Q_{2}}={\\left({D_{1} \\over D_{2}}\\right)}

Law 2b. Pressure or Head is proportional to the square of the impeller diameter:

H 1 H 2 = ( D 1 D 2 ) 2 {\\displaystyle {H_{1} \\over H_{2}}={\\left({D_{1} \\over D_{2}}\\right)^{2}}} {H_{1} \\over H_{2}}={\\left({D_{1} \\over D_{2}}\\right)^{2}}

Law 2c. Power is proportional to the cube of impeller diameter (assuming constant shaft speed):

P 1 P 2 = ( D 1 D 2 ) 3 {\\displaystyle {P_{1} \\over P_{2}}={\\left({D_{1} \\over D_{2}}\\right)^{3}}} {P_{1} \\over P_{2}}={\\left({D_{1} \\over D_{2}}\\right)^{3}}

where

  • Q {\\displaystyle Q} Q is the volumetric flow rate (e.g. CFM, GPM or L/s),
  • D {\\displaystyle D} D is the impeller diameter (e.g. in or mm),
  • N {\\displaystyle N} N is the shaft rotational speed (e.g. rpm),
  • H {\\displaystyle H} H is the pressure or head developed by the fan/pump (e.g. psi or Pascal), and
  • P {\\displaystyle P} P is the shaft power (e.g. W).[2]

So according to the the two laws depending on if you keep diameter size constant  then the ESP will change .

 

Utilisateur supprimé
par Utilisateur supprimé

No. The static pressure is computed for the run of the duct and duct fitting losses that the fan is selected to overcome and deliver the air.

Akram Mohye
par Akram Mohye , HVAC Engineer , china state co

yes the friction losses increase the static pressure 

Muhammad Arsalan Siddiqui
par Muhammad Arsalan Siddiqui , Associate Director (MEP) , JLL

ESp for a fan will remain same with same rpm and flow rate.

In constant air flow systems the ESP has been calculated for specific flow rate and duct routing what ever it is. During the installation and balancing the system shall be balanced for a specific flow and ESP than for that specific fan the system shall remain under same conditions for ever untill unless the system parameters are same the flow rpm and ESP shall be same.

Mohammed Shebl
par Mohammed Shebl , Contracts Manager , SALFO - Engineering & Management Consultants

yes static pressure can be high or low debending on rpm

Imad Melhem Zghaib
par Imad Melhem Zghaib , Senior Mechanical Engineer./ MEP Manager/ Coordinator , GHD Global

yes, if fan impeller shaft rpm increase => shaft power increase the air flow increase, the friction losses increase then ESP increase

shameem pulikkaparambil
par shameem pulikkaparambil , Principal mechanical engineer , www.krameralbrecht.com

compared to the incompressible fluid such as water, the friction loss due to the air movement is negligible in straight ducts.

Static pressure again depends on the size of duct, fittings, rpm and flow rate.

the question is not sufficient to justify the "Yes"

 

 

 

 

ISSAM DAGHER
par ISSAM DAGHER , OWNER , ISSAM DAGHER MECHANICAL ENGINEERING

No.not affect the only thing affect the type of duct.

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