A study of the effect of temperature and relative humidity on the drying rate and equilibrium moisture content of hay

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Virginia Polytechnic Institute

The curing or hay on the farm to retain its maximum feeding value, including minerals and vitamins, is of recognized importance. For several years, agricultural workers have realized that a limiting factor in the production of high quality hay in the Southeastern United States is the difficulty of proper curing. The climatic conditions found in the southern states are among the nations most favorable for the production of hay, but these same conditions make it almost impossible to cure high quality hay in the field.

The average total rainfall for any one month during the hay curing season is not excessive, but numerous light showers at short intervals make the field curing of hay very difficult. In 1942 a survey of hay losses was made on 215 farms in nine Southwest Virginia counties. The results of the survey showed that 25% of the crop had been lost or damaged. Virginia's annual production of tame hay is about 1,500,000 tons. If the same ratio of loss was applied to the total crop of the state, it would mean that 375,000 tons of the annual hay crop were lost or damaged.

Hay and grain drying by forced ventilation has been practiced successfully by farmers in the Southeast for the past decade. At the close of the 1948 hay drying season, there were 500 driers in operation on Virginia farms.

Available data on the analysis of a number of barn and field dried samples indicate a decided advantage in favor of the barn dried hay, when considering the amount of leaves, green color, carotene, and protein retained. The design and testing of the drying equipment and forced ventilation systems have become an important part of the research work of the Agricultural Engineering Department, Virginia Agricultural Experiment Station, in cooperation with the U.S. Department of Agriculture.

With the increased use of supplemental heat in forced ventilation drying, a definite need has arisen for data showing the effect of the temperature and relative humidity or the air supply on the drying rate and equilibrium moisture content of hay. The equilibrium moisture content is that moisture content at which no further moisture will be removed from the hay by a particular combination of temperature and relative humidity.

The elementary laws of thermodynamics and heat transfer show that the condition of the drying media has a definite effect on the rate at which a substance may be dried. A thorough investigation showed that the literature on this particular phase of hay drying is very limited; however, a number of articles have been published on closely related subjects, such as the humidity equilibrium of wheat, flour, flaxseed and various other common substances. Of the literature available the most applicable reports were those by Mr. Davis¹, "Supplemental Heat in Mow Drying of Hay - Part Two", and Dexter, Sheldon and Waldron², "Equilibrium Moisture Content of Alfalfa Bay".

In Mr. Davis I report the drying rate factor for the 68°F. dew point only was given, and the study by Dexter, Sheldon and Waldron was limited to a constant temperature condition throughout the humidity range. Since practically all hay drying installations operate with varying dry bulb temperatures and varying relative humidities, this investigation was proposed for the purpose of providing data that would constitute an important contribution to the further study and design of hay drying systems.

  1. Davis, R. B. Jr., "Supplemental Heat in Mow Drying of Hay - Part Two", U.S.D.A., V.P.I., Agr. Engr. Journal, June 1948. 2. Dexter, S. T., Sheldon, W. H., and Waldron, Dorothy I., Michigan State College. "Equilibrium Moisture Content of Alfalfa Hay". Agr. Engr. Journal, July 1947.