Chapter 7 Climate and environmental control

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Introduction

The quality of the environment in agricultural buildings includes such factors as temperature, light, moisture, air quality and movement, dust, odours, and disease agents. Environment affects animal comfort and health and ultimately production. It also influences the quality and longevity of stored products. From an engineering standpoint, environment can be closely controlled. However, economic factors often limit the extent to which control can be justified.

The particular region of the nation and the resulting climatic zone will influence the manner in which environmental requirements are met. A humid area may require homes with open construction to provide continual ventilation for comfort, whereas an arid region may need buildings of great thermal capacity to protect against daytime heat, and night chill.

As a general rule Tropical Climates are found within the tropics. However, the influence of the climate on structures makes the techniques used applicable to many regions outside the tropics, e.g. the middle east.

The following brief discussion of Africa's climatic zones is general and can be found worldwide in the tropics. It illustrates the wide variety of situations with which engineers are faced in designing environmentally suitable buildings for people, animals and products.

Climatic zones

There are several climatic zones on the African Continent with widely varying characteristics.

1 Low-latitude, wet equatorial: High rainfall, humid and close to 27°C mean temperature throughout the year. Congo Basin.

2 Monsoon and Trade-wind bittoral: Climate dominated by trade-winds. Maximum rain in high sun season; minimum following low sun season. Intense showers in eastern coastal zone. Warm throughout the year. Central and Western Africa and East Coast.

3 Wet-dry tropical: Typified by very wet high sun season and very dry low sun season. West and Southern Africa.

4 Dry tropical: Characterized by extreme heat at high sun season and cool at low sun periods. Gradually changes from arid to semi-arid and into wet-dry tropical zone: Sahara, South Africa.

5 Dry subtropical: A north-south extension of the dry tropical zone. Greater annual temperature range: North and South Africa.

6 Altitude modified wet-dry tropical: Increases in altitude generally result in an increase in precipitation and a reduction in mean temperatures. Precipitation is seasonal and varies from 500 to 1500mm depending on local conditions. Inland East and South-east Africa.

Climate can also vary greatly over relatively small areas, in particular where the country is hilly.

For design purposes, local climatic data from a nearby meteorological station should be obtained if possible.

A thorough knowledge of heat transfer, air-moisture temperature relationships and ventilation, combined with a knowledge of climatic conditions and the environmental requirements of animals and farm products, enables the engineer to design the best possible systems within economic constraints. For example, the maximum control reasonable for a cattle herd might be some artificial shade, whereas a high value fruit crop might justify the expense of a refrigerated storage.

Heat terminology

Heat is a form of energy. The molecules of a body are in constant motion and possess kinetic energy referred to as heat.

Temperature is intensity of heat i.e., the velocity of the molecules. In the Sl-system it is measured in degrees Celsius (centigrade) or Kelvin (absolute).

Ambient temperature is the temperature of the medium surrounding a body, e.g., air temperature within a building.

Quantity of heat is measured in Joules. One calorie of heat will raise one gram of water 1° Kelvin. This equals 4.1 87 Joules.

Sensible heat is the heat that causes a temperature change when there is a heat transfer, e.g., heat moving through the walls of a home causing a temperature rise.

Latent heat is the heat that causes a change in state but no change in temperature, e.g., heat that is absorbed when ice changes to water or when boiling water changes to vapor. However, water will evaporate to vapour over a wide range of temperatures. When air moves across the surface of water, some of the air's sensible heat is converted to latent heat causing the air temperature to drop. The latent heat of vaporization changes with temperature:

°C .....................kJ/ kg

0 ......................2500

30 ...................2430

100 .................2256

Thermal capacity is the ability of a material to absorb and hold heat. It is measured in J/( kg.°K). The thermal capacity of water is 4.187 kJ/( kg.°K) or 4.187J/(g.°K).

Specific heat is the dimensionless ratio between the thermal capacity of a material to that of water. However the actual thermal capacity measured in J/( kg.° K) is often listed as specific heat.

Total heat content. Bodies with great mass can store large quantities of heat, even at low temperatures, e.g., thick masonry walls are slow to warm up in the heat of the day and slow to cool down during a cool night. A match has a high temperature and little heat content. A large tank of water may have a low temperature but still have a large heat content.

Heat transfer

Basic to any discussion of insulation and ventilation is an understanding of the way heat is transfered. Heat moves from place to place by conduction, convection, radiation or some combination of these modes whenever a temperature difference exists.

Conduction

In Conduction, heat energy is passed from molecule to molecule in a material. For heat to be conducted it is essential that there be physical contact between particles and some temperature difference. Thermal conductivity is a measure of how easily heat is passed from panicle to particle. The rate of heat flow depends on the amount of temperature difference and the thermal conductivity of the material.

Convection

Heat is transferred by convection when a heated liquid or gas, often air, actually moves from one place to another, carrying its heat with it. The rate of heat flow depends on the temperature of the moving fluid and the rate of flow. Convection transfer can occur in any liquid or gas.

Radiation

Heat energy can be transferred in the form of electro-. magnetic waves. These waves emanate from a hot body and can travel freely only through completely transparent media. Heat cannot move by radiation through opaque materials, but instead is partially absorbed by and reflected from their surfaces. The atmosphere, glass and translucent materials pass a substantial amount of radiant energy, while absorbing some and reflecting some. Although all surfaces radiate energy, there will be always be a net transfer from the warmer to the cooler of two surfaces facing each other.

Thermal resistance of building components

The calculation of temperatures within buildings or of heating and cooling loads requires a knowledge of the thermal conductivity, specific heat capacity, and density of the materials of construction. The thermal resistance of air films adjacent to surfaces and, of air spaces are also required and as the latter are dependent on the emittances of surfaces, data on these parameters are also needed.

Table 7.1 contains a list of materials with their thermal properties. The thermal resistance, which is the quotient of thickness and thermal conductivity, has been given and where appropriate, for material thicknesses most commonly used. As there is a linear relationship between thickness and thermal resistance in most cases, other values are readily calculated.

This may not be the case for granular materials when the grain size becomes comparable with the thickness and therefore caution should be shown when assigning resistance values to such materials.

Insulating Materials

The choice of an insulating material will depend on the application, availability and cost. Loose granular materials work best when installed above a ceiling or poured into existing wall cavities. Batting or blanket materials are easiest to install as walls are constructed. Rigid insulating boards may be placed under concrete floors or cemented to masonry walls. Reflective surfaces such as aluminium foil or paint are most effective when exposed and not in contact with other materials. They are also more effective in preventing the downward flow of heat and in relatively high temperature applications.

Local natural materials such as straw, shavings, coffee hulls, etc., although not as high in resistance to heat flow as commercial insulations, may be the material of choice because of availability and low cost. A greater thickness will be required when using the natural materials and they may not be as fire and vermin resistant.

Surface Resistances

The values of surface resistances are influenced by several factors, the most important of which is the rate of air movement over the surface. Values for 3m/ s and 0.5m/ s of air movement and for still air are shown in Table 7.1.

Thermal Resistance of Pitched-Roof Spaces

The calculation of U values for a roof-ceiling combination requires a knowledge of the resistance of the airspace between the ceiling and the roofing material. Resistance values are given in Table 7.2 for four design combinations.

Overall Heat Transfer Coefficients

Table 7. 1 Thermal Properties of Building and Insulating Materials

The overall heat transfer coefficient or thermal conductance, U. is the rate of heat transfer through a unit area of a building element (wall, ceiling, window, etc.). When the building element is made of two or more different materials, the U value is calculated as the reciprocal of the sum of the resistances of the individual components of the elements as expressed in the equation:

R = t / c

R_T = R_si + R₁ + R₂ + ... + R_so where:

U= 1 / R_T

R =Thermal resistance of each homogenous material making up the building element.

R_T = Resistance to heat flow through a composite element.

R_si, R_so = Thermal resistance of inside and outside air surfaces of the building element.

U = Overall coefficient of heat transmission (air to air)

Using values from Tables 7.1 and 7.2 overall heat transfer coefficients (U) have been calculated for a number of composite wall and roof constructions. Although estimates were necessary for some materials, the U values are realistic. Table 7.3 shows several of the construction units.

The effect on U values and overall heat transfer of timber and metal frames in walls is in the order of 5% and may usually be ignored. However, local effects may be observed. The more rapid heat loss through the framing of a heavily insulated wall may result in a low enough wall temperature adjacent to the framing locations to cause condensation.

Table 7.2 Therrmal Resistance of Pitched-Roof Spaces

* dull, dark surfaces ** shiney, light surfaces

Table 7.3 Overall Heat Transfer Coefficients, U

Table 7.3 Overall Heat Transfer Coefficients, U

Rate of overall heat loss or gain from a building

Once the U values have been calculated for each element of the building (walls, ceiling, windows, doors, etc.), the area of each element is determined, and design temperatures for inside and outside are chosen. It follows then that for each building element:

Q=A x U x D T where:

Q = Total heat transfer rate through an element (W)

A = Area of building element (m²)

U = Coefficient of heat transfer for the element W/ (m².° K)

D T = Temperature differential across element (° K)

For the building as a whole the total heat exchange rate will equal the sum of the Q values. Total heat transfer in Joules for a given period may be found by multiplying kilowatts by 3.6 Megajoules times the number of hours. Figure 7.1 provides some rough approximations of maximum and minimum temperatures for design purposes. Temperature data for the immediate area in which the building will be constructed will provide the most accurate results.

Solar Load

In the countries of East and South-east Africa the effect of solar radiation can be appreciable during some seasons and at certain times of the day. The orientation, design, and materials used will all influence the amount of solar heat gain to which a building is subjected.

A method of determining the degree and extent of solar gain has been developed which is called sol-air. This concept provides a solar increment in the southern hemisphere to be added to the design air temperature used for horizontal roofs and northerly facing walls. These increments range from 10 to 30°C.

However, they apply for only a few hours per day and become of less significance if the building is designed to offset the effects of solar radiation. Two examples illustrate how this can be accomplished.

In an area of high diurnal-nocturnal temperature difference, the roof and walls of a building should be constructed of materials with a great deal of mass (adobe bricks or rammed earth). The resulting high thermal capacity will limit both daytime temperature rise and the nighttime temperature drop and thus the high solar-radiation effect is reduced to a minimum.

In the case of a refrigerated store, it would be desirable to use a roof design that allows attic ventilation and that is covered with a light-coloured reflective surface which when combined, will minimize the effect of solar radiation on the store to a minimum.

Figure 7.1 a Highest mean monthly maximum temperature ( ° C).

Figure 7.1b Lowest mean monthly minimum temperature ( ° C).

Example of Heat Loss from Buildings

Given: Two homes in Lesotho. One is constructed with adobe block walls and a thatch roof, while the other is made of hollow core concrete blocks with a sheet metal roof. Each house is 5 metres square, 2 metres high at the eaves, 3 metres at the ridge, has 1m² of window and 1.5m² of timber door. Find the heat lost from each house when the temperature is 0°C outside and is 15°C inside.

From Table 7.3, the U value for a sheet metal roof is 3.03 W/(m².°K); for a thatch roof, 0.26W/(m².°K); for an adobe wall, 2.5W/(m².°K); concrete block wall, 2.9W/(m².°K), and single glass, 6W/(m².°K).

The calculated U value for a 25mm timber door is 2.4W/ (m².° K).

Q = A x U x D T

Thatched roof

It is obvious that much more heat must be supplied to the metal roof house. A Ceiling with 50mm of Rockwool or Glasswool would provide a substantial saving.

Saving 3220 - 2191 = 1029W

While the "modern" house is almost as heat efficient as the traditional style house and should be more hygienic and durable, the traditional house can be constructed entirely from locally available materials and by local craftsmen and will thus require a minimum of cash expenditure.

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