Step By Step Procedure To Do Shortcut Sizing For Air Cooled Heat Exchanger And Estimate Finned Area And Power Required m1j13

Step by Step procedure to do shortcut sizing for Air Cooled Heat Exchanger and estimate finned area and power required.

An air-cooled exchanger is used to cool fluids with ambient air. A 1 inch OD tube is the most popular diameter and the most common fins are 1/2 inch or 5/8 inch high. Tube configuration used in this guideline : 1 inch OD tube, 5/8 inch fin height, 10 fins per inch and 2.5 inch triangular pitch. APSF - External area in ft²/ft² of bundle face area.

Rows

APSF

Face Velocity, ft/min

3

80.4

700

4

107.2

660

5

134.0

625

6

160.8

600

APF - Total external area/ ft of fintube in ft²/ft ~ 5.58. Face Velocity - Typical air face velocities (V result in optimum cost of exchanger.

Face

)

used in design are tabulated above, these value

Step 1

Obtain Process Parameters Obtain process duty (Q), hot process side inlet (T1) and outlet (T2) temperature. Select an overall heat transfer coefficient (U) from literature based on type of fluids. Select an air inlet temperature (t1) that is not exceeded during a certain percentage of time over the year (e.g. 95% of the time). Step 2

Calculate Air Density Density at air inlet temperature and site elevation ρo = 14.696*29 /(10.7316 * (t1 + 459.67)) ρAir / ρo = exp(-29 * z/ (1545 * (t1 + 459.67)))

where, t1: Air Inlet Temperature, °F. z: Elevation above sea level, feet Step 3

Calculate MTD Assume an Air outlet temperature (t2) and calculate LMTD. LMTD = ((T1-t2) -(T2-t1))/ ln((T1-t2)/(T2-t1)) R = (T1 - T2)/(t2 - t1) S = (t2 - t1)/(T1 - t1) LMTD Correction factor (F) is estimated based on following graphs Cross Flow Single

Cross Flow Two

Step 4

Calculate Air Flowrate Finned area is estimated using following equation : AFinned = Q / (U * F * LMTD) Bundle Face area is estimated using following equation : AFace = AFinned / APSF

Air Flow is estimated using following relation : VAir = AFace * VFace Air Mass flowrate is estimated : MAir = VAir * ρAir Air temperature rise is calculated : ΔT = Q / (MAir * Air )

Revised air outlet temperature is calculated : t2 = t1 + ΔT This temperature is again used in Step 3 to re-estimate air outlet temperature. Above steps are iterated till there is no change in air outlet temperature. Step 5

Air Cooler Dimension Air cooler width is calculated : W = AFace / LTube Number of Tubes are calculated : NTube = AFinned / (APF * LTube) Number of Tubes per Row are calculated : Nr Tube = NTube / No of Rows where, LTube : Length of Tube. Step 6

Air Side Pressure Drop Air Side pressure drop is calculated as following : ΔP Total = ΔPStatic + ΔPVelocity Static Pressure Drop Pressure drop across tube bundle. ΔP Static = FP * No of Rows / DR FP = 6*10-8 * ( GFace )1.825 where, DR : ρAir / ρAir at seal level and 70°F GFace : Air face mass velocity in lb/h.ft² face area ΔPStatic : Static pressure drop in inch of H2O Velocity Pressure Drop Typically 2 fans are used in air cooler. Fan area per fan (FAPF) is calculated as following :

FAPF = 0.4 * AFace / No. of Fans Fan Diameter is calculated : D = (4 * FAPF / π )0.5 ΔPVelocity = (ACFM / (4005 * (π* D2/4)) )2* DR where, ACFM : Air flowrate in Actual Cubic Feet per Minute ΔPVelocity : Velocity pressure drop in inch of H2O Step 7

Power Calculation Break power for fan is calculated as following : BHP = ΔP Total * ACFM / 6356 / ηFan Motor power is calculated as following : Power = BHP / ηMotor

References 1. Process Heat Transfer: Principles and Applications 2. Handbook of Chemical Engineering Calculations (McGraw-Hill Handbooks) 3. Rules of Thumb for Chemical Engineers: A Manual of Quick, Accurate Solutions to Everyday Process Engineering Problems 4. GPSA, Engineering Databook, 12th Edition FPS