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Fan Laws and Formulas used in Performance Calculations
Fan efficiencies remain constant for symmetrical design. When one or more conditions change, the other conditions vary according to certain fan laws for an established fan size, system of ductwork and air density.

When fan speed is varied:

  1. Fan’s air-delivery will vary directly as the RPM ratio.

  2. Developed fan pressures will vary as the RPM ratio squared.
  3. Horsepower absorbed by fan will vary as the RPM ratio cubed.

When fan pressure varies:

  1. Fan’s air-delivery and RPM will vary as the square root of the pressure ratio.
  2. Horsepower absorbed by fan will vary as the square root of the pressure ratio cubed.

    Note: Most fan curves and ratings are based upon standard air of 0.075 pounds mass per cubic foot. For air densities at other temperatures and elevations.

When density of air varies:

  1. Since the fan is essentially a constant volume machine when run at a constant speed, the volume will not vary with a change in density, but the horsepower absorbed and the static, velocity and total pressures will vary directly with the density ratio and inversely as the absolute temperature.
  2. For constant pressure - fan speed, air-delivery and horsepower absorbed vary inversely as the square root of the density ratio.
  3. For constant air-delivery and fan speed - horsepower absorbed by fan and pressure developed vary directly as the air density ratio.
  4. For constant amount of air by weight - air-delivery, fan speed and developed pressure vary inversely as the density ratio.
    For constant amount of air by weight – horsepower absorbed by fan varies inversely as the square of the density ratio.


Performance of a Fan in a System:


When a system is designed for moving air, the air flow rate (CFM) and the static pressure (SP) seldom fall conveniently on the RPM curve shown for fan performance. In this case, you can use the fan laws to determine the exact fan RPM and BHP needed.


Suppose you need 2750 CFM at 1.5" SP. Using the fan curve shown, you can see that this point (indicated as “A”) falls between 2518 and 2747 RPM. Going to the BHP curves, you can see that the BHP will be between 1.4 BHP and 1.8 BHP. The motor HP needed may be 11/2 or it may be 2. (Point B)


Algebraically rearranging the fan laws to show:


You may calculate a system curve based on 2750 CFM and 1.5" SP (Point A). This system curve crosses the 2518 RPM curve at 2625 CFM.


Using the fan laws, we see:

This is the operating RPM for this fan at the specified performance point. Following the 2625 CFM line up to the BHP curves, we see that we intersect the 2518 RPM curve at 1.4 BHP.

Again, using the fan laws, we find


To obtain 2750 CFM at 1.5" SP, you would have to operate this fan at 2638 RPM and would use 1.61 BHP. You would need a 2 HP motor with a 1.0 service factor or a 11/2 HP motor with a 1.15 service factor. The 2 HP model code would be 46-16DJ3.


Average Air Changes Required For Good Ventilation


Assembly Halls 2-10
Engine Rooms 1-3
Laboratories 1-5
Auditoriums 2-10
Factories 2-5
Laundries 1-3
Bakeries 2-3
Forge Shops 2-5
Markets 2-10
Boiler Rooms 1-5
Foundries 1-5
Offices 2-10
Bowling Alleys 2-10
Garages 2-10
Packing Houses 2-5
Churches 5-15
Generator Rooms 2-5
Plating Rooms 1-5
Dairies 2-5
Gymnasiums 2-10
Toilets 2-5
Dance Halls 2-10
Kitchens - Hospital 2-5
Transformer Rooms 1-5
Dry Cleaners 1-5
Kitchens - Restaurant 1-3
Warehouses 2-10


NOTE: Measured in minutes per change. The above chart is a general guideline only.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hours of Operation:
Main Office: Monday thru Friday
8:00 a.m. to 5:00 p.m.

Warehouse: Monday thru Friday
7:30 a.m. to 4:30 p.m.

Type of Sales:
Cash, Check, Visa, Mastercard, Corporate Charge Cards and Net 30 Credit Accounts
All Non Cash Sales Require a Purchase Order.

Prices:
All prices are subject to change without notice. Merchandise will be billed at the prices in effect at time of shipment and are F.O.B. original shipping point. Prices do not include applicable Federal, State and/or local taxes. Air Design will make a written quotation on specified merchandise. Such quotations are binding for forty five (45) days after issue, unless special consideration is stated.

Terms:
All invoices are due within 30 days of receipt, with prior approved credit. Thereafter, a 1.5% service charge will be assessed for each 30 days until paid.

Claims:
The responsibility of Air Design Incorporated ceases upon the delivery of the equipment in good order to the carrier. Since all goods are shipped at the customer’s risk, claims for freight damage or shortages must be filed by the customer against the carrier.

Warranties:
Air Design Incorporated does not issue product warranties. Any and all product warranties are the responsibility of the manufacturer and limited to the manufacturer’s standard warranty policies. Copies of manufacturer’s warranty are available


General:

  1. Air Design is entitled to rely upon the receipt of a written purchase order of the Buyer in sufficient detail to enable it to fulfill said order.
  2. The Buyer is liable, should an order be cancelled through no fault of Air Design, for any and all expenses incurred by Air Design in connection with the order.
  3. All repairs are made on an F.O.B. original shipping point basis and transportation charges on materials returned for repair must be prepaid.
  4. Merchandise may not be returned except by permission of Air Design.
  5. Air Design shall not be liable for failure or delay in making delivery when such failure or delay is beyond its reasonable control.
  6. If Air Design initiates legal action to enforce payment for merchandise, buyer agrees to pay reasonable attorney’s fees and costs as may be incurred.

Tag and Hold:
We allow products in our stock to be held for a customer for up to 7 days. After that time the customer must accept shipment, or the product will be placed back in our stock. All orders not picked up after 7 days will be returned to stock.

Stock Returns
Where the return is due to error on the part of the manufacturer or Air Design, the customer will be credited at full invoice price. In all other cases, the following policy will apply.

  1. Products may not be returned for credit without prior authorization.
  2. Products returned must be in first-class resalable condition and of current model. Any cartons from the original shipment must be provided. The Air Design invoice number and date on which the originally purchased must be furnished.
  3. No returns will be accepted after 30 days from delivery.
  4. A handling charge may be assessed equaling 25% of the sell price.
  5. Products will not be picked up from a customer without prior authorization. Our driver is not permitted to pick up merchandise for return without authorization.
  6. Special ordered and custom fabricated items may not be returned.

 

 

 

 

 

 

 

 

 

 

 

 


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