PREMIER INDUSTRIES, INC.

FORMULAS, DEFINITIONS and TABLES

• Additional information about rigid media:

Virtually all formulas and tables presented here have to do with how the media performs.  It's cooling efficiency, static pressure drop across the media relative to the face velocity and cubic feet per minute of air flow is the measuring rod to which we apply all other calculations and determinations.  In simpler terms, the rigid media is the heart and soul of evaporative cooling.  Without an understanding of it's operation, it is difficult to design a cooling system or size cooling equipment for a building.

The part about applications and design are covered in the section "Applications and Design" which can be reached from the Technical Data section which is available from our home page.  To keep a surprisingly complex subject simple, only the formulas and tables that relate to this media are covered in this section.

• Abbreviations and Definitions:

Let's start with some abbreviations we use in our formulas:

     AC = Air Changes.  ( Usually expressed in changes per hour or per minute.  Air change is the number of times the air within a structure is exhausted and replaced during a specified period such as hour or minute).

     BTUH = British Thermal Units per Hour.  A measure of heat or the absence of heat  ("cold" can be defined as the absence of heat) in a volume of air or space. BTUH is not commonly used in evaporative cooling terminology but necessary to calculate heating and mechanical refrigeration.  (It is most often used as "heat of vaporization = 1043 BTU/lb in the formula for calculating evaporation rate and standard CFM).

     (S)CFM = (Standard) Cubic Feet per Minute.  Usually referred to as simply CFM.  This is a necessary ingredient in any formula involving evaporative cooling.  It is a measure of air volume movement in one minute.

     Fv (or) FV = Face Velocity.  Face velocity or "air velocity" is the measure expressed in feet per minute (FPM) the air is moving at the entry side (face) of the cooling media.  This is another necessary ingredient in any formula in evaporative cooling to determine efficiency.

     FPM = Feet Per Minute.  The measure of speed (velocity) of the air .

     ESP = External Static Pressure.  Expressed in inches, water column.  The pressure against which the air flow must move.  The pressure external to the cooling unit opposing air flow (i.e. restrictive ductwork, etc.)

     SPD = Static Pressure Drop.  Expressed in inches, water column.  The amount of pressure required to push the air through the media as measured with a magnehelic gage.  The difference between the pressure of the air flow at the intake of the media  and the discharge side of the media. The measure of pressure for any component through which air flow is measured at the intake and discharge.  This is an important consideration in some evaporative cooling applications.

     WG or w.g. or WC or w.c. = Water  gauge or water column in inches.  This is a measure of static pressure.  A Pitot Tube is used to take this measurement.  The Pitot Tube is a curved (U-shaped) glass tube with a prescribed amount of water and a scale. The tube is hollow.  When air is blown into one end the water column will be forced up the other side to some level.  The level to which the column of water rises is a measure, in inches, of the pressure of the force required.

     GPH = Gallons per hour.  A measure of liquid (usually water) moving during one hour.                                    

     GPM = Gallons per minute.  A measure of liquid (usually water) moving in one minute.

     (f) or (F) = Fahrenheit.  Temperature conforming to a thermometric scale on which water boils at 212 degrees and freezes at 32 degrees.  Named after Gabriel D. Fahrenheit, 1736.

     Db (or) DB = Dry Bulb temperature.  Measurement (usually in Fahrenheit) of  temperature taken by a standard thermometer or similar thermal indicator.

     Wb (or) WB = Wet Bulb temperature.  The lowest temperature that can be reached by evaporatively cooling the air.  This measurement is usually taken with a "sling psychrometer".  This device is a standard thermometer with a "wet sock" over the sensor bulb.  The psychrometer is slung in a circular motion rapidly enough to cause evaporation to occur around the sensor bulb to drop the temperature to it's lowest point possible with the evaporation process.

     Wbd (or) WBD = Wet Bulb Depression.  The difference between the Dry Bulb and the Wet Bulb temperatures.  This temperature is the total amount of cooling available through the evaporative cooling process.  At 100% cooling efficiency, the temperature drop would be equal to the Wet Bulb Depression. Also known as Wet Bulb Differential.

     SE = Saturation Efficiency.   This is the percent of the Wbd (Wet Bulb Depression) achieved by the cooling process.  I.E. At 100 degrees (f) Dry Bulb and 70 degrees (f) Wet Bulb, the Wbd would be 30 degrees (f).  If the actual temperature drop measured at the discharge side of the media was 73 degrees (f), the percent of saturation efficiency would be 90%.   This means that the air passing through the media has been saturated with water vapor (moisture) to 90% of its maximum.  "Cooling Efficiency" is the same as Saturation Efficiency and is most often used to define the performance level of the media.  Also called just "efficiency".

      Design = This term is used in many ways to define the parameters of an application or specifications.  Some common uses are as follows:  IDb =  Indoor Dry Bulb.  ODb = Outdoor Dry Bulb.  IWb = Indoor Wet Bulb.    OWb = Outdoor Wet Bulb. EDb = Entering Dry Bulb.  LDb = Leaving Dry Bulb.  EWb = Entering Wet Bulb.  LWb = Leaving Wet Bulb. This term is often used in conjunction with "conditions" such as "Climate Design Conditions".  In evaporative cooling, climate data is considered to be Dry Bulb and Wet Bulb levels.  It would require a  "Psychrometric Chart" to locate the juncture of the Dry Bulb and Wet Bulb lines to find the grains or pounds of moisture per pound of dry air or relative humidity (RH).  Refer to Table 1 for a psychrometric chart digitalized for easy reading of relational elements of Db, Wb and RH.

     RH = Relative Humidity.  Expressed in percent.  The percent of water vapor in the air compared to the amount of water vapor the same air could contain.  (i.e.  15% RH indicates the air is 15% saturated with water vapor)

• Formulas:

Leaving Dry Bulb = [ODb - (SE x (Odb-OWb)]

Leaving Wet Bulb = Normally considered same as entering  Wb.

Wet Bulb Depression = ODb - OWb

Evaporation Rate = [CFM x WBd x (SE / 8700)]   ( this is simple method)

Bleed - Off Rate = Evaporation Rate x .20 (prox)

(Recirculation) Water flow Rate = 3 times the evaporation rate (prox)

Standard CFM = Sensible BTU/hr / (1.08 x (IDb - Db) x Density Ratio
                           Where IDb = Indoor Design Dry Bulb (f)

CFM = Standard CFM / Density Ratio

BTU = CFM X Delta T x 1.08

Density Ratio = 1.325 x Barometric Pressure / (Db(f)                                                                                

Water weight (US gallon) = 8.33 pounds per gallon (based on distilled water)

Water volume (US gallon) = 7.481 gallons per cubic foot

Water weight (US gallon cubic foot) = 7.481 x 8.33 = weight of cubic foot of   water (62.288#)

Face Area =  Width x Height of open face  area through which air will flow (expressed in square feet.)

Face Velocity = CFM / Face Area (Sq Ft) (expressed  in Feet per minute  (FPM).                     

• Tables:

Table 1:  Psychrometric Chart

Notes:  Db is Dry Bulb temperature,  Wb is Wet Bulb temperature, #H2o  is Pounds of moisture per pound of dry air which is a measure of absolute humidity.  RH is Relative Humidity.

Read the chart by finding the known elements, such as Dry Bulb and Relative Humidity and move horizontal to find the Wet Bulb and pounds of moisture per pound of dry air.  I.E. The weather forecast is for 100 degrees at 15% Relative Humidity.  You will find that the Web Bulb is 66 degrees and the pounds of moisture is .006 per pound of dry air.  Knowing the Wet Bulb will allow you to determine the Wet Bulb depression.  

The Psychrometric chart will provide you the necessary information to design systems, predict outcomes and many other useful applications of the information!  This chart is digitalized to make it simpler to use.

It is necessary to interpolate and extrapolate accordingly for those in-between conditions not directly covered in the above chart.  I.E. if 105 degrees was the temperature to use, then it would be necessary to interpolate the available data to reach the right conclusion.  In this instance, the Wet Bulb would be 67.5 degrees.  This chart is intended to be reasonably accurate at sea level and should be within p/m 5%.  If greater accuracy is required, it is recommended that you use the proper Psychrometric chart for the elevation desired or make proper adjustments for elevation (barometric pressure).

• Table 2:  Evaporation Rate

To determine Gallons per Minute divide by 60.  Formula to determine evaporation rate is shown in Formulas section.

• Table 3:  Temperature drop across media (@ 500 FPM face velocity)

Note:  12" thick media @ 500 FPM face velocity is the preferred design .  This is the best trade-off between performance and cost.

• Table 3:  Air Density Ratio:  Density Ratio for Various Elevations and Temperatures.

• Table 4:  Air Changes suggested per hour.

Notes:  The "Air Change" method is a practical approach to assist in the determination of the size and efficiency of evaporative cooling equipment required for the structure.  The principle behind this method is to determine the difference between the inside temperature of the structure, without using evaporative cooling and the outside ambient temperature during its highest condition.  While this method is ideal for existing structures, new structures not yet built can be estimated on the same scale.

Leaving Air Temperature reflects the output of the evaporative coolers whether existing or planned.  The air change column indicates a range of frequency and is used in determining air volume requirements.  Other criteria are needed to complete the sizing of equipment.  Refer to the section on "Applications and Design" for more specific information on equipment sizing.

Copyright (c) Joe Fuller 1993