The fans are 3300 cfm each and 40 amps each the thermostat in it is 185 and that's where the car used to sit temp wise all the time. With my mechanical fan. I have an ac condenser in the way now yes and that's my main concern/issue i believe not enough air flow....never got warm with the mechanical fan I went to serp kit and had to go electric fans due to space. I'm going to remove my condenser and see if my issue goes away
But what TYPE of fan(s) are you using? Probably a propeller type? You mentioned that the fans are 3300 cfm....but at what static pressure? Since the fans are "puller" type, any resistance to air flow, measured in inches/wg, (inches of water) will reduce the actual air capacity the fan will deliver. Power consumed increases as either: volume of air required to move, at standard conditions, at temperature increases, the air density decreases the hp required will be less. Propeller fans are very inefficient at high static pressure requirements. The only way to know is ask the fan supplier for the fan curves of your selection. The fan curve will show: CFM vs Static Pressure including hp required at the best operating point.
Fan Law 1: CFM is directly proportional to RPM.
Formula: CFM2 = CFM1 X (RPM2 ÷ RPM1)
or RPM2 = RPM1 X (CFM2 ÷ CFM1)
What it means: As you increase fan RPM, CFM increases at a 1:1 ratio. So if you need to increase CFM by 10%, your RPM has to increase by 10%. Since it is a 1:1 ratio, we can interchange RPM for CFM in Fan Laws 2 and 3. We use Fan Law 1 all the time in the field. If we need to change the airflow, we change fan speed by changing a speed tap, VFD output, pulley diameter, or other means.
Apply it in the field: If your blower is moving 1000 CFM at 1100 RPM, and you need to decrease airflow by 10% to 900 CFM, Fan Law 1 says your RPM must decrease by 10% also. Let’s put that in the formula:
RPM2 = RPM1 X (CFM2 ÷ CFM1)
RPM2 = 1100 X ( 900 ÷ 1000)
RPM2 = 990 This is your new RPM.
We also need to understand that for us to make predictions using this fan law and fan laws 2 and 3, everything else about the air and the system needs to stay the same, including air temperature and density. System friction must also stay constant, so these fan laws cannot be used with automatic dampers that self-adjust to maintain flow.
Fan Law 2: Total Static Pressure changes with the square of CFM (or RPM).
Formula: SP2 = SP1 X (CFM2 ÷ CFM1)²
or SP2 = SP1 X (RPM2 ÷ RPM1)²
What it means: A 10% increase in CFM will result in a 21% increase in static pressure. Think about that. A small increase in airflow creates a significant increase in duct pressure. This increased pressure will be evenly distributed across components like coils and filters. So, this fan law can be applied to total static pressure or a static pressure drop across a single component in the system. That matters because some components have static pressure limitations that affect their performance. Air filters work best when they have a low-pressure drop across them. This usually means the air velocity is low enough to allow for “dwell time” through the filter material, catching more particulates. Condensate traps that are already close to their limit may have to be made deeper so that they don’t get overwhelmed. Air proving switches must be adjusted so that they do their job at the new CFM and static pressure.
Apply it in the field: At 1000 CFM, you read a 0.9″w.c. pressure drop across a media filter. You need to increase your airflow to 1200 CFM. What will be the new pressure drop?
SP2 = SP1 X (CFM2 ÷ CFM1)²
SP2 = 0.9 X (1200 ÷ 1000)²
Manufacturers provide performance specifications to allow designers to select the right fan for their system. In residential design, we size the duct friction based on the fan performance of the air handler we have pre-selected based on the tonnage our load calculation calls for. But in commercial design, we size the fan based on the friction of the duct system we have already designed. In either case, we must consult the manufacturer’s fan performance data to verify the fan is a good match for the load.
Exercise: Select the better exhaust fan for our commercial application. 1000 CFM @0.5″ w.c. We have 2 choices: Greenheck Model SQ-130-B or a smaller model SQ-100-VG.
FYI.....typical example of a fan curve....to make a intelligent decision you should talk with you supplier, using the above examples of his recommendation for your specific application. PM me if you want any more information about fans....most people say this is not required but if you want specific information you need to convey your requirements.......
BOB RENTON
