HOUSING
PIH-102
pork industry handbook
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA
Earth Tempering of Ventilation Air
Authors:
Warren D. Goetsch, University of Illinois
Larry Jacobson, University of Minnesota
Randall Reeder, Ohio State University
Dennis Stombaugh, Ohio State University
Reviewers:
Eldridge Collins, Jr., Virginia Polytechnic Institute and State University
Dexter Johnson, North Dakota State University
Richard Phillips, Pennsylvania State University
Harold and Dean Rogers, Petersburg, Illinois
David Shelton, University of Nebraska
Earth tempering of ventilation air for swine buildings is being considered by many producers because of the moderate fluctuations in soil temperatures at shallow depths. Depending on the season, incoming ventilation air is heated or cooled as it passes through a buried tube. The soil serves as a heat sink in the summer and as a heat source in the winter, thus giving almost year-round temperature modification. It has the potential to significantly reduce heating costs during winter and provide zone cooling during summer.

Soil Temperature
Soil temperature is one of the most important factors affecting design and performance of earth-tube heat exchanger systems. Soil temperatures vary with soil type, depth, moisture content, time of year, and geographic location.
The mean annual ground temperatures for various locations in the United States are given in Figure 1 .* In the central U. S., these mean annual ground temperatures range from 49°F. in St. Paul, Minnesota, to 58°F. in Lexington, Kentucky, and from 52°F. in Ames, Iowa to 55°F. in Columbus, Ohio. The variation of ground temperature from this yearly mean at any site is suggested by Figure 2. The amount of temperature variation decreases as depth increases. For example, at a depth of 6 ft., the yearly variation of a typical clay soil can be expected to range from 11 degrees above to 11 below
the mean annual ground temperature, or a total yearly variation of approximately 22 degrees. At a depth of 10 ft., this variation is reduced to plus or minus 6 degrees F. or a total variation of 12 degrees.

The time of year when the ground temperature is at the extreme is also important in the design and performance of a system. Soil temperature fluctuations lag behind surface temperature changes due to the heat storage capacity of the soil. The soil surface reaches maximum temperature during the heat of the summer, but soil 10-12 ft. deep may not reach its peak temperature until almost three months later. This thermal lag at the 10 ft. depth (Fig. 3) helps both the heating and cooling performance of these systems. During the winter, soil temperatures at this depth are at the fall season level, making the soil near the mean annual ground temperature, thus adding to the heating capabilities of a system. The reverse is true during the summer months, when the soil temperatures at the 10-12 ft depth are springlike and can cool the ventilation air.

Soil types and moisture content also affect the ground temperature variation. Soils with increasing sand content tend to have larger temperature variations at deeper depths than clay soils. Soil moisture and ground water elevation also affect soil temperature. Seasonal temperature variation is larger in very moist soils as compared to very dry ones due to the increase in heat transfer through soils whose voids are filled with water.

System Design
The typical earth-tube tempering or heat exchanger system consists of a heat exchanger field, a collection
* The drawings in Figures 1. 2. and 3 first appeared in 'Underground Building Climate by Kenneth Labs, in the October 1979 issue of Solar Age, c 1979 SolarVision. Inc.. Harrisville, NH 03450. All rights reserved Reprinted and published by permission
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana. Purdue University and U. S. Department of Agriculture cooperating. H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to its programs and facilities without regard to race, color, sex, religion, national origin, age or handicap.