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AE13 Winter Ventilation And Heating Requirements Of PRIVATE Fiberglass Greenhouses For Condensation Control1D. E. Bungton, R. A. Bucklin, R. W. Henley and D. B. McConnell2 1. This document is AE13, one of a series of the Agricultural and Biological Engineering Department, Florida Cooperative Extension Service, Institute of F ood and Agricultural Sciences, University of Florida. Original publication date January 1992. Revised July 2002. Reviewed December 2011. Visit the EDIS website at http://edis.ifas.u.edu 2. Pr ofessor and Head, Department of Agricultural and Biological Engineering, The Pennsylvania State University, University Park, PA 16802; Associate Professor, Agricultural Engineering Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611; Professor, Cooperative Extension Service, Institute of Food and Agricultural Sciences, Central Florida Research and Education Center, Apopka, FL; and Professor, Environmental Horticulture Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL.The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or aliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A&M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim DeanCondensation forming on the inside surfaces of greenhouses is of considerable economic signicance. Economic problems associated with condensation in greenhouses are fungus diseases; diculty in maintaining a clean greenhouse; more rapid deterioration of structural components; and damp, uncomfortable environmental conditions for the workers. Furthermore, the presence of condensation is unsightly and a nuisance. Condensation occurs when warm, moist air in a greenhouse comes in contact with a cold surface such as glass, berglass, plastic or structural members. e air in contact with the cold surface is cooled to the temperature of the surface. If the surface temperature is below the dew point temperature of the air, the water vapor in the air will condense onto the surface. For example, condensation will occur if air in a greenhouse at 70F and 70 percent relative humidity comes in contact with a surface that is 60F or colder. Most of the condensation problems in greenhouse occur when the minimum outside temperatures drop below 50F. is occurs between the months of November and March, except for unenvironmental circumstances. Condensation will form heaviest in greenhouses during the period from sundown to several hours aer sunrise. During the daylight hours, there is sucient heating in the greenhouse from solar radiation to minimize or eliminate condensation from occuring except on very cold, cloudy days. e time when greenhouses are most likely to experience heavy condensation is sunrise or shortly before. At this time, the outdoor air temperature is usually at a minimum. Four general methods exist for controlling condensation: air, Of these four methods, only exhausting moist air and replacing it with heated outside air is really eective in eliminating condensation. e other methods reduce and minimize the amount of condensation that may occur, but in themselves are not solutions to eliminating condensation
2 formation. is publication deals with ventilation and heating requirements of greenhouses to prevent conformation.MATERIALS AND METHODSEnvironmental conditions were recorded inside and outside a large, double-vaulted commercial, berglass greenhouse in Apopka, Florida,during two consecutive cold seasons. Inside surface temperatures, bed temperatures, and air temperatures were measured with copperthermocouples at 16 locations. ese measurements were recorded on a thermocouple recorder. Inside and outside air temperatures and relative humidities were measured and recorded with hygrothermographs.RESULTS AND DISCUSSIONe nine inside surface temperatures that were recorded at the time of the minimum temperature of the day were averaged to obtain the mean inside surface temperature. Based on the measured data points obtained during the two cold seasons, the mean inside surface temperature was found to be simply the average of the inside and outside air temperatures, within 2F. is result appears to be reasonable when one considers that the berglass material, about 1/32 in. thick, has essentially no thermal resistance. e only resistance to heat ow is that oered by the inside and outside air lms. ese two air lms would have approximately the same thermal resistance at the time of the minimum temperature of the day, because then the outdoor air is usually very still. Having established the relationship between inside surface temperature and inside and outside air temperatures, the minimum ventilation requirement to prevent condensation can be calculated. e calculated ventilation rates for the stated inside and outside air temperatures in columns 1 and 2 are presented in column 3 of Table 1. To greatly increase the application of the calculated values for ventilation and heating to maintain desired conditions, the rates are expressed per unit of greenhouse bed area. Ventilation is expressed in units of cfm (cubic feet of air per minute) per square foot of actual bed area of the greenhouse. e ventilation rate was calculated by analyzing a conservation of mass equation; that is, the moisture entering the greenhouse in the ventilation air plus the moisture produced within the greenhouse equals the moisture leaving the greenhouse in the ventilation air. In equation form, the moisture conservation balance becomes: M = mass ow rate of ventilation air, pounds of dry air per hour per square foot of bed area W0 = humidity ratio of outside air, pounds of water per pound of dry air Wprod = moisture production rate within the greenhouse, pounds of water per hour per square foot of bed area Wi = humidity ratio of inside air, pounds of water per pound of dry air e ventilation rate was calculated from Equation 1 by rst solving for M, the mass ow rate of the ventilation air. e mass ow rate of air was then converted to ventilation rate, with units of cfm per square foot of bed area, by multiplying air mass ow rate and specic volume of 13.33 cubic feet per pound and dividing by 60 minutes per hour. e conversion factor obtained was 0.222. In order to solve Equation 1 for M, the value of Wi was determined from a psychrometric chart such that the dew point temperature of the inside air mixture was 4F below the predicted inside surface temperature. By choosing Wi in such a manner incorporated a modest safety factor into the values presented in Table 1. e 4 F margin also takes into consideration that the surfaces of the greenhouse with an unobstructed view of the sky will be cooler than other surfaces of the greenhouse due to the radiation losses. e values of W0 and Wi can be obtained most conveniently from a psychrometric chart. e rate of moisture production by the plants and soil was estimated to be 0.00275 inches of water per hour based on calculations involving data collected in the greenhouse and on results published by Stewart and Mills (1). e resulting relative hufor the recommended ventilation rate is presented in column 5 of Table 1. is value was also obtained from a psychrometric chart. e heating required to prevent condensation was calculated by analyzing a conservation of energy equation; namely, the heat content (enthalpy) of the ventilation air entering the greenhouse plus the heat added by the heating equipment equals the heat content of the ventilation air leaving plus the heat lost by conduction through the exposed surface area of the green In equation form, the total heat balance equation is:
3 where: M = mass ow rate of ventilation air, pounds per hour per square foot of bed area h0 = enthalpy of outside air, Btu per pound of dry air Qhtr = heating rate within greenhouse, Btu per hour per square foot of bed area hi=enthalpy of inside air, Btu per pound of dry air Qcond = heat lost by conduction, Btu per hour per square foot of bed area e rate of heat conduction through the exsurface areas of the greenhouse was calculated according to: where: S=ratio of exposed surface area of the greenhouse to the bed area of the greenhouse T=temperature dierence between inside and outside air temperatures, F R=overall resistance to heat ow, hour-square footF per Btu. A constant value of 1.0 (hour-square foot-er Btu was used throughout this study. Having determined the mass ow rate from Equation 1, the heat loss by conduction from Equation 3, and ho and hi from a psychrometric chart, the heating rate required to prevent conwithin the greenhouse was calculated from Equation 2. e results, in terms of S, are presented in column 4 of Table 1. e value of S generally ranges from 1.5 to 3.0, depending on the conguration of a greenhouse. A large multi-vaulted greenhouse will have a S-value closer to 1.5 or 2.0. e value of S must be calculated for each particular greenhouse in order to determine the total heating required to prevent condensation from Table 1. e greenhouse that was monitored for this study was used for propagating cuttings. Consequently, most of the water produced inside the greenhouse during the early morning hours was evaporation from the bed media. In greenhouses used for growing larger plants, transpiration from the plants would produce more water within the greenhouse. Under such circumstances, the ventilation and/or heating rates may need to be increased to prevent condensation. Only experience with a particular greenhouse operation can dictate whether an increase would be necessary. An alternative to increasing the ventilation and/or heating rates presented in Table 1 would be to add air mixing devices, such as turbulators, inside the greenhouse. A turbulator is eective in reducing thermal stratication, thereby insuring a more uniform air temperature throughout the entire greenhouse (2). Turbulators, or similar air mixing devices, would be most needed in those greenhouses having a large portion of their surareas exposed to an unobstructed view of the sky. When a surface has an unobstructed view of the sky, its temperature will drop several degrees below ambient temperature on clear nights because of radiant cooling. In any greenhouse, some ventilation will occur because of natural inltration. e magnitude of the rate of natural inltration depends on the openness of the greenhouse. e more open the greenhouse, the more inltration will take place. However, it is most dicult to try to eectively eliminate condensation by manipulating the openness of the greenhouse, especially when desiring to control temperature and to minimize amount of heating fuel required. To illustrate the use of the data presented in Table 1, consider the following example.EXAMPLEe cross-section dimensions of a large, double vaulted berglass greenhouse are shown in Figure 1. e greenhouse is 372 feet long. By using basic arithmetic methods, the exposed surface area is calculated for the dierent structural components, and then totaled as shown in Figure 1. Note that the oor is not considered as part of the exposed surface area. e bed area, assumed to be 75 percent of the total oor area, is 21,204 square feet (.75 x 372 x 76). erefore, the S-value equals 1.67 (35,480 square feet/21,204 square feet). Consider the case when the outside temperature is forecast to reach a low of 30F. Assume you desire to maintain an
4 inside temperature of 65F and that you want no condensation to occur. Referring to the values presented in Table 1, the ventilation rate is 0.97 cfm per square foot of bed area, or 20,600 cfm (0.97 x 21,204); the required heating rate is 110.65 Btu per hour per square foot of bed area (52.2 + 35 x 1.67); the total heating required is 2,345,000 Btu per hour (21,204 x 110.65); and the resulting inside relative humidity would be about 46%.SUMMARYTemperature and humidity measurements were recorded inside and outside a commercial greenhouse in Apopka, Florida, during two consecutive cold seasons. Predictive equations were developed to determine the minimum ventilating and heating requirements to prevent condensation from forming inside the greenhouse. e equations were formulated based on the environmental conditions, ventialtion rates, physical characteristics of greenhouses, and principles of heat and mass transfer. Calculations from the predictive equations are tabulated to present the ventilating and heating requirements for a wide range of inside and outside enviromental conditions.REFERENCESStewart, E. H. and W. C. Mills. 1967. Eect of depth to water table and plant density on evaporate in Southern Florida. Transactions of the ASAE 10(6): 746 Walker, J. N. and G. A. Duncan. 1974. Eectiveness of recommended greenhouse air circulation systems. Transactions of the ASAE 17(2): 371 Figure 1. Cross-section view of double-vaulted greenhouse used in the example.
5 Table 1. Minimum ventilation and heating rates required to prevent condensation inside berglass greenhouses. Outside Temperature,FInside Temperature, F Ventilation Rate,cfmHeating Rate*,Btu/hResulting InsideRelativeHumidity,% 10 50 1.76 92.0 + 40S** 38 55 1.48 87.8 + 45S 36 60 1.23 81.8 + 50S 34 65 1.11 82.3 + 55S 30 20 55 1.38 68.1 + 35S 43 60 1.11 62.8 + 40S 41 65 0.97 62.7 + 45S 38 70 0.85 61.5 + 50S 35 30 55 1.39 52.2 + 25S 54 60 1.17 53.5 + 30S 50 65 0.97 52.2 + 35S 46 70 0.85 51.1 + 40S 43 40 55 1.71 43.3 + 15S 65 60 1.31 44.3 + 20S 60 65 1.01 42.8 + 25S 56 70 0.82 42.2 + 30S 52 75 0.67 40.8 + 35S 48 50 60 1.71 34.7 + 10S 73 65 1.17 34.3 + 15S 67 70 0.89 34.9 + 20S 62 75 0.72 35.4 + 25S 57 60 65 2.02 27.5 + 5S 80 70 1.23 30.1 + 10S 74 75 0.89 31.4 + 15S 68 80 0.65 30.2 + 20S 63 70 70 2.47 11.1 88 75 1.31 23.0 + 5S 80 80 0.82 24.7 + 10S 74 *Ventilation rate and heating rate are expressed per ft2 of greenhouse bed area. **S = exposed surface area of greenhouse, ft2/bed area of greenhouse, ft2.