Navigating The Cooling Process On The Psychrometric
Navigating the Cooling process on thePsychrometric ChartTwo examples of cooling will be reviewed in the following examples. The first is an example of sensible mechanicalcooling and the second is an example of sensible and latent mechanical cooling. All cooling calculation will assume thesame design conditions of 90 F Dry Bulb, 75 F Wet Bulb, and 2000 cfm of outside air.On the following psychrometric charts, different color lines represent different air properties:Blue Line Cooling ProcessYellow Line EAT Dry Bulb TemperatureRed Line EAT Wet Bulb TemperatureGreen Line EAT EnthalpyBlack Line LAT Dry Bulb TemperatureBrown Line LAT EnthalpyGrey Line EAT Humidity Ratio (Same as LAT on sensible cooling)Purple Line LAT Humidity RatioNavigating the psychrometric chart:1.2.3.4.5.6.7.8.9.Find the design dry bulb temperature on the bottom horizontal axis and follow the yellow line up.Find the design dry bulb temperature on the left curved axis and follow the red line down to the right.Where those 2 lines intersect is the design EAT point.Reading directly to the right from this point and following the grey line, will show the design humidity ratio onthe right vertical axis.Follow the green line from the design point to the upper left diagonal axis with generate the EAT enthalpy.The cooling process is defined by the blue line. Sensible cooling is shown by a horizontal line and latent coolingoccurs once the air reaches saturation and begins to follow the curved left axis to the lower left.Determine the desired leaving air dry blub (black line) and wet bulb (brown line) temperature from what isrequired for the specific application.Determine the LAT enthalpy (brown line), and the LAT humidity ratio (purple line) from the LAT point.Use the formulas below and follow the examples to determine the cooling capacity requirement.
Listed below are common cooling formulas used for both sensible and latent cooling calculations:Total Cooling Load (Btu/hr) Sensible Cooling Load (Btu/hr) Latent Cooling Load (Btu/hr)Where:Sensible Cooling Load (Btu/hr) CFM x Density Factor x (T1 – T2)Latent Cooling Load (Btu/hr) CFM x 0.69143 x (G1 – G2)Where:CFM Airflow in Cubic Feet Per Minute(70-Blower Temp) x .024Density Factor 1.08 10T1 Entering Air Drybulb Temperature ( F)T2 Leaving Air Drybulb Temperature ( F)G1 Grains of Moisture of Entering Air 7000 x humidity ratio (lbm moisture/lbm of dry air)G2 Grains of Moisture of Leaving Air 7000 x humidity ratio (lbm moisture/lbm of dry air)
Example 1: Sensible CoolingDesign Conditions (EAT):CFM 2000Dry Bulb 90 FWet Bulb 75 FFinal Conditions (LAT):Dry Bulb 75 FWet Bulb 70.5 FSensible Cooling Load (Btu/hr) CFM x Density Factor x (EAT Dry Bulb – LAT Dry Bulb)Density Factor 1.08 [(70 – Blower Dry Blub) x .024)/10]1 Ton 12000 Btu/hrDensity Factor 1.08 [(70 – 90) x .024)/10]Density Factor 1.032Sensible Cooling Load (Btu/hr) 2000 x 1.032 x (90 –75)Sensible Cooling Load 30960 Btu/hrRequired Cooling Tonnage (Btu/hr)/12000Required Cooling Tonnage 30960/12000Required Cooling Tonnage 2.58 TonsSee this example on the following psychrometric chart.
EXAMPLE 1 – SENSIBLE COOLINGDesign ConditionsDry Bulb 90 FWet Bulb 75 FEnthalpy 38.36 Btu/lbHumidity Ratio .015Final ConditionsDry Bulb 75 FWet Bulb 70.5 FEnthalpy 34.4Btu/lbHumidity Ratio .015
Example 2: Sensible and Latent CoolingDesign Conditions (EAT):CFM 2000Dry Bulb 90 FWet Bulb 75 FEnthalpy 38.36 Btu/lbHumidity Ratio .015Final Conditions (LAT):Dry Bulb 60 FWet Bulb 60 FEnthalpy 26.41 Btu/lbHumidity Ratio .011Sensible Cooling Load (Btu/hr) CFM x Density Factor x (EAT Dry Bulb – LAT Dry Bulb)Density Factor 1.08 [(70 – Blower Dry Blub) x .024)/10]1 Ton 12000 Btu/hrDensity Factor 1.08 [(70 – 90) x .024)/10]Density Factor 1.032Sensible Cooling Load (Btu/hr) 2000 x 1.032 x (90 –60)Sensible Cooling Load 61920 Btu/hrLatent Cooling Load (Btu/hr) CFM x 0.69143 x (G1 – G2)G1 Grains of Moisture of Entering Air 7000 x humidity ratio (lbm moisture/lbm of dry air)G2 Grains of Moisture of Leaving Air 7000 x humidity ratio (lbm moisture/lbm of dry air)G1 7000 x .015G1 105G2 7000 x .011G2 77Latent Cooling Load 2000 x 0.69143 x (105 – 77)Latent Cooling Load 38720 Btu/hrTotal Cooling Load (Btu/hr) Sensible Cooling Load (Btu/hr) Latent Cooling Load (Btu/hr)Total Cooling Load 61920 Btu/hr 38720 Btu/hrTotal Cooling Load 100,640 Btu/hrRequired Cooling Tonnage (Btu/hr)/12000Required Cooling Tonnage 100,640/12000Required Cooling Tonnage 8.38 TonsSee this example on the following psychrometric chart.
EXAMPLE 2 – SENSIBLE & LATENT COOLINGDesign ConditionsDry Bulb 90 FWet Bulb 75 FEnthalpy 38.36 Btu/lbHumidity Ratio .015Final ConditionsDry Bulb 60 FWet Bulb 60 FEnthalpy 26.41Btu/lbHumidity Ratio .011
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Psychrometric Chart Two examples of cooling will be reviewed in the following examples. The first is an example of sensible mechanical cooling and the second is an example of sensible and latent mechanical cooling. All cooling calculation will assume the same design conditions of 9
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