Blower speed

[dignam]

I have a FA4A Bryant fan coil and want to add a dehumidification by running the fan at 50% of the slowest speed. The blower is a 3-tap, 3/4 hp 240V single phase motor, and runs at 1600 cfm at the lowest speed.

I have looked into installing a variable speed drive and build the remaider of the control ckt myself to control the blower, but it (the VFD) seems overkill, and pricey, for this application.

Any suggestions?

Thanks

[sadlier]

Here's a cheaper option: Replace the motor with a Trip-Saver motor and add a relay to switch between the lower-HP capacitor.

Another option might be to add a motorized bypass damper so that the air within the ducts will recirculate more, thus lowering the CFM and the humidity level.

And another option might be to add a motorized damper that will simply cut the airflow down. Actuators can be set for different amounts of opening thus the damper will not completely cut off the air flow if properly set. Use a spring return actuator (Belimo if you want really good quality) and set it for power-close/spring-open; This way if the motor fails it will not affect the airflow. The return is the best spot for dampering air flow.

[dash]

1600 cfm on low is a lot of cfms,what model ,and what External Static Pressure?



50% of the lowest speed is likely to ice the coil.


Hoe do you know it's 1600 cfm on low speed?

[dignam]

Thanks for the information. Can you recomend a Trip-saver motor? I thought about adding control dampers but wanted to keep the noise down.

The unit short cycles during low sensible heat gain.

Thanks again

[sadlier]

The Trip-Saver motors are motors which can range in HP ratings depending on which capacitor is used. I have purchased them before for 1/2 HP (which includes options such as 1/3HP, 1/4HP, 1/5HP) but have not looked for a 230V 3/4HP; That is something which would need to be sought at the local distributor. Wiring the 2 capacitors up for different HP ratings would require a double pole double throw relay/contactor.

[dash]

What tonnage or model is the outdoor unit?

How many cfms per ton are you trying to get?

[dash]

Try http://www.fanhandler.com for variable speed
,sort of.

[dignam]

This is a 4 ton unit. The cfm rating is from the mfg; external sp was running at ~ 1.05" wc. I've increased it to .2, but any higher and the system gets noisy. Also, the returns are dampened down just before the condensate "floats" around the coil.

I've installed a freeze stat to keep the coil from freezing.

Thanks for all the suggestions

[dash]

Originally posted by dignam
This is a 4 ton unit. The cfm rating is from the mfg; external sp was running at ~ 1.05" wc. I've increased it to .2, but any higher and the system gets noisy. Also, the returns are dampened down just before the condensate "floats" around the coil.

I've installed a freeze stat to keep the coil from freezing.

Thanks for all the suggestions

So is the air handler a 5 ton?

Static was 1.05,but increased it to .2.Check the decimal point,think you have a typo.



Where,in the system, are you testing the static?

[dignam]

Bryant model NO: FA4B 048 with a matching 4 ton condensor.

Yes, I did have a typo. I checked the sp just before the first take off ~ 4' downstream of the fancoil.

[dash]

Question,you staed it was 1.05"wc and you incresed it to .2,which is actually a reduction in static,decemimal point error???


You need to take the supply and return static,entering and leaving the air handler ,add them together ,ignoring the +/- signs,to get the "total".


FA048, on low speed is 1605 cfms at .10 ,"total" static,and 1360 cfms at .50 "total" static(ESP).


.50 should not be noisey and I wouldn't suggest any less on the cfms,without adding a TXV.

[dignam]

Thanks.

[dash]

To get the accurate ESP(static),be sure to read and adjust for mfrs. ,footnotes,filter,wet coil,electric heaters,etc. ,etc..

If you aren't sure about any of this ,let us know.