Blower output flow greater than intake?

[skuntz]

We are moving air using a 7.5 HP blower. On the suction side we have a filter and an evaporator coil. We are measuring flow using two identical air velocity transducers - one at suction and one at discharge. Both compute SCFM and output a signal to our PLC. We are getting readings of approximately 1400 SCFM at discharge but only about 1000 SCFM at the intake. At one point we put both on the suction line and the readings were the same withing 5% so we are sure it is not instrument error. Both discharge and intake are at about the same temperature and at atmospheric pressure. As far as we can determine the minor differences in pressure and temperature could not account for this. We have been diligently trying to find leaks on the intake side. Other than air leaking into the blower on the suction side, is there any other phenomena that could account for this great discrepancy?

 

[DrRTU]

Do you have a density change? You stated you you have an evaporator installed. Are you only using one (1) velocity probe for inlet and discharge. Probe placement in the stream is the most likely cause of error. 7.5 hp for 1400 CFM is large. The velocity gradient across the duct my be large.

 

[MintJulep]

A velocity transducer measures the velocity at one point. Velocity is not uniform across the full area of the duct.

 

[ChasBean1]

Agree. Adding to what these guys said — if it’s a 0-5 inch w.c. transducer and one location is in a fan bellmouth at 10,000 feet per minute you’re right in the transducer’s wheelhouse (midrange). If the same device is asking to do a coil profile at 500 fpm it’s running at 2.5% of range, which will give pretty lousy accuracy… so the question is, are the velocities at the two measured locations about equal and both suitable to the transducer range?

 

[willard3]

If you run at higher pressures, you will have density and temperature changes.

 

[ChasBean1]

mmm..., velocity reading difference due to density is only about +1.3% even at 5" w.c. static across the fan. For a fan with the motor in the airstream there's only about a 2°F rise, which would be about -0.4% difference and the two would offset each other (net difference maybe about 0.9%)... not the order of magnitude OP's seeing.