Example 1 shows that at 70 F, the dscfm flow is very close to the fan industry scfm flow (only 1.6 percent variation). However, at more elevated temperatures, such as in Example 2, the fan industry scfm flow is 40 percent higher than the dscfm flow. ## Water Vapor Loads Due to Air FlowWater vapor loads on industrial buildings come from many sources. Listed below are a few of these sources of water vapor:People Permeation through walls, roofs, and floors Moisture from products and packaging materials Evaporation from open tanks or wet surfaces Product dryer leakage Open combustion From air flow • Air leakage through cracks and holes • Air leakage through conveyor openings • Intermittent door openings • Building-to-building air infiltration • Makeup air In many cases, water vapor loads by air flow are a major contributor to the total building vapor load. In the research work for this article, I came across a number of approximate equations that are used to calculate the water vapor load from air flow. Approximate equations can be fairly accurate as long as the air conditions are close to 70 F air temperature. [ back to top ] ## Industrial Dehumidification Part 3: Boiler & Plant Operationsby William G. AckerAccurate water vapor load equations for industrial dehumidification systems design are difficult to find, and terminology is not standard. This article provides a thorough review of both. The first article in this series, which covered air flow, can be read in HPAC Engineering's May 1999 issue. WATER VAPOR LOADS DUE TO AIR FLOW Water vapor loads on industrial buildings come from many sources. Listed below are a few of these sources of water vapor: ❒ People ❒ Permeation through walls, roofs, and floors ❒ Moisture from products and packaging materials ❒ Evaporation from open tanks or wet surfaces ❒ Product dryer leakage ❒ Open combustion ❒ Air flow • Air leakage through cracks and holes • Air leakage through conveyor openings • Intermittent door openings • Building-to-building air infiltration • Makeup air In many cases, water vapor loads by air flow are a major contributor to the total building vapor load. In the research work for this article, I came across a number of approximate equations that are used to calculate the water vapor load from air flow. Approximate equations can be fairly accurate as long as the air conditions are close to 70 F air temperature. For more information on water vapor permeation loads, consult the June 1998 issue of HPAC Engineering. The next few sections will compare approximate and exact equations for selected water vapor sources. MOISTURE FROM AIR LEAKAGE Equations (8) to (12) can be used to calculate moisture for air leakage through cracks, holes, and conveyor openings. Equations (8) to (11) were taken from engineering books or from manuals prepared by dehumidification companies. Notice that the engineering units do not properly cancel out, which is why they are considered approximate equations. Equations (11) and (12) are applied and compared in the example below to show how results from approximate and exact equations can vary under different conditions. Equation (11), which is approximate, calculates a water vapor load 20.18 percent over the exact equation (12). Equation (11) is more accurate if the entering air flow is close to 70 F. Engineers preferring the exact equation will need a psychrometric chart to obtain the entering air specific volume, or a psychrometric computer program that can calculate the air mixture properties. ## Example conditions:Room conditionsAir pressure: 29.921 in. Hg Dry bulb temp: 70 F Moisture level: 35 grains WV/lb dry air Relative humidity: 32.38 percent Entering air flow conditionsAir pressure: 29.921 in. Hg Dry bulb temp: 120 F Moisture level: 420 grains WV/lb dry air Relative humidity: 76.44 percent Specific volume: 16.02379 cu ft wet air/lb dry air Air flow acfm: 200 cu ft per min Equations (13) and (14). In this example, equations (13) and (14) produced water vapor loads that were 7.27 and 12.63 percent above the exact equation (12) water vapor load. The error is a direct result of the assumed entering air specific volume. Note that the makeup air specific volume will vary with the entering air psychrometric properties. Therefore, you cannot select a standard value for specific volume and expect the equation to be exact. For this reason, equations (13) and (14) are approximate equations. Equations (13) and (14), which can be found in many engineering books and dehumidification manuals, will be fairly accurate as long as the entering makeup air is close to the selected specific air volume. ## Example ConditionsInside room conditionsAir pressure: 29.921 in. Hg Dry bulb temperature: 70 F Moisture level: 35 grains WV/lb dry air Relative humidity: 32.38 percent Entering makeup airAir pressure: 29.921 in. Hg Dry bulb temperature: 100 F Moisture level: 280 grains WV/lb dry air Relative humidity: 93.67 percent Specific volume: 15.01722 cu ft wet air/lb dry air Air flow acfm: 2000 cu ft per min ## Water Vapor Removed by DehumidifiersDehumidification systems remove water vapor from the process air that travels through the unit. This section looks at the equations used to determine the humidity ratio of the process air entering and leaving the unit as well as equations used to estimate the amount of water vapor removed when the inlet and discharge humidity ratios and air flow are known. Note that some of the equations are the same as the equations used in preceding sections. The equations listed as approximate can be very accurate if the process air flow temperature is close to 70 F (Table 1). In this series of equations and calculations, approximate equations (15) and (11) produced water vapor removals that were 40 and 105 percent above the exact equations (16) and (17). The approximate equations can be fairly accurate if the air entering the dehumidifier is close to 70 F. Engineers that desire greater accuracy can use the psychrometric chart to get the specific volume needed to make the conversion from acfm to dscfm or purchase psychrometric programs that can calculate the value for them.## ConclusionMany of the flow diagrams presented in articles, books, engineering manuals, and proposals from dehumidification companies do not indicate the engineering units for the flows in the diagrams. As indicated in this article, the flows are illustrated in cfm or scfm with no explanation. In most cases, the flows are in dry standard cubic feet per minute.Over the years, I have been contacted by many engineers over the issue of calculated water vapor load variances from different equations. In most cases, the approximate equations are equations that have been shortened to make the calculations easier for engineers. If you have any questions on your equations, check the engineering units to make sure that they properly cancel out to grains of water vapor per hr, or lb of water vapor per hr. The author would like to thank his wife, Sandra, for her patience and assistance
during the preparation of these articles. He would also like to
thank Nels Strand, the author's mentor and close friend for over 20
years.[ back to top ] Bibliography (Part 3)1) Acker, W., Water Vapor Migration and Condensation Control in Buildings, HPAC Engineering, June 1998. 2) Acker, W., Industrial Dehumidification Water Vapor Load Calculations and System Descriptions, HPAC Engineering, March 1999. 3) Clifford, G. E., Modern Heating and Ventilating Systems Design, Prentice Hall, Englewood Cliffs, N.J., 1992. 4) Fan Engineering, 8th Ed., Edited by Robert Jorgensen, Buffalo Forge, Buffalo, N.Y., 1983. 5) 1997 ASHRAE Handbook: Fundamentals, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., Atlanta, Ga. |