Heating Fundamentals

Heating Fundamentals

There is still considerable disagreement about the exact nature of heat, but most authorities agree that it is a particular form of energy. Specifically, heat is a form of energy not associated with matter and in transit between its source and destination point. Furthermore, heat energy exists as such only between these two points. In other words, it exists as heat energy only while flowing between the source and destination. So far this description of heat energy has been practically identical to that of work energy, the other form of energy in transit not associated with matter. The distinguishing difference between the two is that heat energy is energy in transit as a result of temperature differences between its source and destination point, whereas work energy in transit is due to other, nontemperature factors.

British Thermal Unit

Heat energy is measured by the British thermal unit (Btu). Each thermal unit is regarded as equivalent to one unit of heat (heat energy).

Relationship Between Heat and Work

Energy is the ability to do work or move against a resistance. Conversely, work is the overcoming of resistance through a certain distance by the expenditure of energy. Work is measured by a standard unit called the foot-pound, which may be defined as the amount of work done in raising one pound the distance of one foot, or in overcoming a pressure of one pound through a distance of one foot

The relationship between work and heat is referred to as the mechanical equivalent of heat;

Heat Transfer

When bodies of unequal temperatures are placed near each other, heat leaves the hotter body and is absorbed by the colder one until the temperatures are equal to each other. The rate by which the heat

is absorbed by the colder body is proportional to the difference of temperature between the two bodies—the greater the difference in temperature, the greater the rate of flow of the heat.

Heat is transferred from one body to another at lower temperature by any one of the following means

1. Radiation

2. Conduction

3. Convection

Radiation, insofar as heat loss is concerned, refers to the throwing out of heat in rays. The heat rays proceed in straight lines, and the intensity of the heat radiated from any one source becomes less

as the distance from the source increases

The amount of heat loss from a body within a room or building through radiation depends upon the temperature of the floor, ceiling, and walls. The colder these surfaces are, the faster and greater

will be the heat loss from a human body standing within the enclosure. If the wall, ceiling, and floor surfaces are warmer than the human body within the enclosure they form, heat will be radiated from these surfaces to the body. In these situations a person may complain that the room is too hot.

Knowledge of the mean radiant temperature of the surfaces of an enclosure is important when dealing with heat loss by radiation. The mean radiant temperature (MRT) is the weighted average temperature of the floor, ceiling, and walls. The significance of the mean radiant temperature is determined when compared with the clothed body of an adult (80°F, or 26.7°C). If the MRT is below

80 F, the human body will lose heat by radiation to the surfaces of the enclosure. If the MRT is higher than 80°F, the opposite effect will occur.

Conduction is the transfer of heat through substances, for instance, from a boiler plate to another substance in contact with it . 

Conductivity may be defined as the relative value of a material, compared with a standard, in affording a passage through itself or over its surface for heat. A poor conductor is usually

referred to as a nonconductor orr insulator. Copper is an example of a good conductor. illustrates the comparative heat conductivity rates of three frequently used metals. The various materials

used to insulate buildings are poor conductors. 

It should be pointed out that any substance that is a good conductor of electricity is also a good conductor of heat.

Convection is the transfer of heat by the motion of the heated matter itself. Because motion is a required aspect of the definition of convection, it can take place only in liquids and gases.

Heat from the burning fuel passes to the metal of the heating surface by radiation, passes through the metal by conduction, and is transferred to the hand by convection as above

Specific, Sensible, and Latent Heat

The specific heat of a substance is the ratio of the quantity of heat required to raise its temperature one degree Fahrenheit to the amount required to raise the temperature of the same weight of

water one degree Fahrenheit (Figure 1). This may be expressed in the following formula

Specific heat = Btu to raise temp. of substance 1°F / Btu to raise temp. of same weight water 1°F

Sensible heat is the part of heat that provides temperature change and that can be measured by a thermometer. It is referred to as such because it can be sensed by instruments or touch.

Latent heat is the quantity of heat that disappears or becomes concealed in a body while producing some change in it other than a rise of temperature. Changing a liquid to a gas and a gas to a liquid are both activities involving latent heat. The two types of latent heat are:

1. Internal latent heat

2. External latent heat

Heat-Conveying Mediums

 several methods are used to classify heating systems. One method is based on the medium that conveys the heat from its source to the point being heated. When the majority of heating systems in use today are examined closely, it can be seen that there are only four basic heat-conveying mediums involved:

1. Air

2. Steam

3. Water

4. Electricity

Heating and Ventilating Systems

Many different methods have been devised for heating buildings.Each has its own characteristics, and most methods have at least one objectionable aspect (e.g., high cost of fuel, expensive equipment,

or inefficient heating characteristics). Most of these heating methods can be classified according to one of the following four criteria:

1. The heat-conveying medium

2. The fuel used

3. The nature of the heat

4. The efficiency and desirability of the method

The term heat-conveying medium means the substance or combination of substances that carries the heat from its point of origin to the area being heated. There are basically four mediums for conveying

heat. These four mediums are mentioned above 

1. Air

2. Steam

3. Water

4. Electricity

Different types of wood, coal, oil, and gas have been used as fuels for producing heat. You may consider electricity as both a fuel and a heat-conveying medium. Each heating fuel has its own characteristics; the advantage of one type over another depends upon such variables as availability, efficiency of the heating equipment (which, in turn, is dependent upon design, maintenance, and other

factors),and cost

The various heating methods differ considerably in efficiency and desirability. This is due to a number of different but often interrelated factors, such as energy cost, conveying medium employed, and type of heating unit. 

The integration of these interrelated components into a single operating unit is referred to as a heating system. Because of the different conditions met within practice, there is a great variety in heating systems, but most of them fall into one of the following broad classifications:

1. Warm-air heating system 

2. Hydronic heating systems 

3. Steam heating systems 

4. Electric heating systems 

You will note that these classifications of heating systems are based on the heat-conveying method used. This is a convenient method of classification because it includes the vast majority of heating systems used today.

ventilating is so closely related to heating in its various applications

The type and design of ventilating system employed depends on a number of different factors, including:

1. Building use or ventilating purpose

2. Size of building

3. Geographical location

4. Heating system used