big fan, little fans 2

[GOTWW]

This is probably a dumb question, But, to get the biggest CFM/Watt is alot of little efficent fans better than one big fan? This is for a solar powered evaporative cooler. I have had pretty good luck using a 1/4 hp dc motor driving a ~3000 cfm fan. I will be re-doing the system and am re-evaluating the options. The static back pressure at the above mentioned approx cfm was ~ 1/8". Thanks

 

[lilliput1]

For high volume low CFM, propeller fans are best. You can use the formula below & compare fifferent fans to see which has the highest efficiency.

Fan bhp = (CFM x In wg TP)/(6344 x Eff)

Where Eff = fan efficiency in decimal. Typical is 0.65

In wg TP = fan total pressure in inches water gauge

CFM = Cubic feet/minute air flow

 

[pennpoint]

lilliput1
I think you intended to say "For high volume low Static Pressure, propeller fans are best"
CFM=volume.

Regards
pennpoint

 

[lilliput1]

Yes. Thanks Pennpoint

 

[wilg]

If you are moving air through
a condensing coil
(tube and fin heat exchanger),
cfm per watt might not be your only
design variable, over a large
surface you may want to look
at air distribution over
the coil area, more than one
fan may be required to move
air flow over the
heat exchange area to get
homogenous velocity distribution.

 

[GOTWW]

I salvaging a nice cast aluminum 4-blade fan w/ventura frame. I notice that the direct connected motor is 850/750 rpm, 0.5/2HP, 3-phase motor. Now a 2HP motor? is that overkill, or what?

 

[GOTWW]

The above fan diameter is 24", BTW

 

[lilliput1]

Are you sure it is not 1/2hp? What is the full load amps & voltage?

 

[GOTWW]

The RPM was 850/1750, the motor is 200-230V, E-Plus, (208-three phase 2-speed) 7.8-6.8 Amps.

 

[BravoCompany]

A solar powered evaporative cooler—very interesting. Are these units common?

Are you indicating a direct current motor or direct connect motor?

For small fan direct driven fans are typically more efficient as there is an energy loss associated with the drive train of a belt driven setup.

Using the analogy of pumps the smaller you go typically the less efficient you become due to scale relation between unit size and tolerances associated with fabrication of parts. This may not be nearly as pronounced in fans as in pumps but I would imagine the same still applies.

Bottom line you have to compare a fixed set of alternatives to make a decision. I believe that it is difficult to qualify a blanket statement committing to smaller fans or larger fans being more efficient. Does anyone else agree?

 

[lilliput1]

GOTWWW,
The FLA indicates it is indeed a 2 hp motor. Since Hp varies as the cube of the rpm, hp at 850 rpm = 2 x (850/1750)^3 = 0.23 but nameplate indicates 0.5 hp.

I looked at the ACME fan cattalog & saw that it list a 24" diameter propeller fan at a max duty of 1.32 bhp 4150 CFM 0.625"wg 1237 rpm with 1 hp motor. Using the above formula this fan will require 3.74 bhp at 1750 rpm. Thus it seems that at 1750 rpm the fan will need more than the rated capacity of the 2 hp motor! If the air cools the motor it is possible to operate it over. Hence the 1.32 bhp with 1 hp motor.

 

[fredt]

Reverting to the basic question. There is a fundamental difference using many small fans vs, one larger fan. It really depends on the resistance of the system. It may be easier and more efficient to produce a high volume flow at low pressure with a number of smaller fans in parallel.

Fan efficiency varies dramatically from the optimum operating volume-pressure EG it could take three times as much power to achieve the same flow with a badly selected fan compared with a correctly selected fan(s). Given that you want maximum flow with minimum power it is worth getting this right.

 

[GOTWW]

Bravo, The different variations of DC powered evaporative coolers have been around quite some time, and mainly associated with the RV'ers, with some for the remote homes.

There is one brand quote from a website

"Solar Chill 3024HP”

http://www.affordable-solar.com/solarchill.html

Solar Chill's30 Inch, 24V, 5200 Cubic Feet per Min, 50 watt, High Performance Evaporative Cooler"

THAT IS 5200 CFM, FOR 50 WATTS. SOUNDS TO GOOD TO BE TRUE.
MY MECHANICAL FRIEND SAYS THIS IS BASICALLY THE HP MAX THEORETICAL LIMIT FOR AIR MOVEMENT. I believe there is a footnote, for export only. (Wonder why?)

Anyway, I did hobble an imperfect adaptor (5/8 motor to 7/8 fan bore) to my 1800 RPM, 1/4 HP dc motor and ran it, open air outside. It ran well, the kids pretended to be getting blowed away. My hobbled 1/2" split brass pipe adapter was not perfect and there was to much vibration to run to long.

I think I will not run it direct, the poor little motor may not live long. I am planning to proceed with a belt drive - pillow block bearings, such as to slow it down and lower the sones.

The question about many small versus one big was brought up because large motors are more efficient than small motors. I woundered if this also applied to fans.

 

[gaffatape]

don't know if this will help but i have also been trying to find a fan in the past week or so. Most of the ones i have looked at have had quite the overkill with the hp on the motor, leading me to believe that they must be quite ineffiecent designs, with motors large enough just to do the job with brute force.
i just got off the phone to Nicotra in Italy, and their fans seem to be by far the most compact and efficient that i have found. Seems to be they may be a good option for people to check out when trying to source a good fan, capable of larger dfm, for a lower horse power motor.

 

[GOTWW]

gaffatape, do you have a link, or distributer?

I was impressed with EBM backward curved impeller blowers. I was lucky to get a few from a surplus outfit. However, When I want another model, ebm would not return my calls. I finally found a distributer, they wanted a kings ransom for one of these fans, they must be mighty proud of them.

As for using many little fans at a specified pressure, every time I have added the CFM's, and required amps. They always seem to fall short of one big fan.

 

[fredt]

I am not too impressed with the trial and error approach. Selecting fans IS for practical purposes an exact science and there is a right fan or fans for every application. Every fan has an optimum efficiency operating point on its pressure-volume curve and this is where the fan should be selected. For this reason there is no simple answer to the question. As I have previously stated, the wrong selection can draw three times the power of a correctly selected fan or fans. Try to determine the overall system resistance (pressure drop for a given flow) and I can probably give you some guidance on suitable fan types.

 

[ko99]

GOTWW,

I've used the EBM backward impellers. They perform as advertised, BUT any significant restriction to the exhaust air will drop performance dramatically.

Most impellers have a built-in shroud to direct the exhaust, and this restriction is accounted for in the performance curve. Unless your backward impeller can exhaust into open air, you must add the pressure loss due to any restrictions you have (ie, subtract the pressure loss from the performance curve).

ko (

http://www.ecooling.biz)

 

[ko99]

GOTWW,

To answer your original question, I copied the appropriate fan laws:

-----------------------------------------------------------

1. VOLUME of air flow varies as the (fan diameter)³ and as the rpm
or: New Volume = Old Volume × ( new impeller diameter ) ³ × ( new rpm )
old impeller diameter old rpm

or: Q2 = Q × ( D2 ) ³ × ( n2 )
D1 n1



2. PRESSURE developed varies as the (fan diameter)² and as the (rpm)²
or: New Pressure = Old Pressure × ( new impeller diameter ) ² × ( new rpm )
old impeller diameter old rpm

or: P2 = P1 × ( D2 ) ² × ( n2 ) ²
D1 n1



3. POWER ABSORBED by the fan varies (fan diameter)5 and as (rpm)³
or: New Power = Old Power × ( new fan diameter ) 5 × ( new rpm³ )
old fan diameter old rpm

or: W2 = W1 × ( D2 ) 5 × ( n2 ) ³

Q = volume flow rate
P = pressure (total, static or dynamic)
p = gas density
n = fan rotational speed
D = impeller diameter
W = impeller power

------------------------------------------------------------

Therefore if you have limited space available, a large fan is better than a lot of little efficient fans. Doubling the diameter is equiv to 8X airflow and 4X pressure, which is like 32 fans (8in parallel X 4 in series).

However, if you're more concerned about power, small fans are better. Doubling the diameter is equiv to 64X power.


ko (

http://www.ecooling.biz)

 

[GOTWW]

Ko99, I will try to verify your 2X power number using commercially available fans.

 

[gaffatape]

GOTWW,

go to

http://www.nicotra.com

then click on whatever language you want. The phone number on the front page takes you straight to the manufacter in Italy, who spoke good English actually and was extremely helpful. It depends where you are in the world, but no doubt they can help you out.