People have always struggled with the problem of being comfortable in their environment.
First attempts were to use fire directly to provide heat through cold winters. It was only in recent times that interest and technology permitted development of greater understanding of heat and heating, and substantial improvements in comfort were made. Comfort heating now is a highly developed science and, in conjunction with air conditioning, provides comfort conditions in all seasons in all parts of the world.
As more was learned about humidity and the capacity of the air to contain various amounts of water vapor, greater achievements in environmental control were made. Control of humidity in buildings now is a very important part of heating, ventilation, and air conditioning, and in many cases is extremely important in meeting manufacturing requirements. Today, it is possible to alter the atmosphere or environment in buildings in any manner, to suit any particular need, with great precision and control.
Thermometers and Scales
Energy in the form of heat is transferred from one material or substance to another because of a temperature difference that exists between them. When heat is applied to a material or substance, there will be an increase in average velocity of its molecules or electrons, with an increase in their kinetic energy. Likewise, as heat is removed, there will be a decrease in the average molecular velocity and, therefore, also the electron or molecular kinetic energy.
A thermometer is used to measure the degree of heat in a substance or material.
The thermometer includes an appropriate graduated scale to indicate the change in temperature of the substance. The change in temperature as read on a thermometer is a measure of heat transferred to or from the substance. A unit of temperature is called a degree and is equivalent to one graduation on the scale.
By convention, the scale is an interval scale. The Celsius thermometer is a metric system of measuring temperature; 0C is assigned to the temperature at which water freezes and 100C to the temperature at which water boils at normal atmospheric conditions. Hence, on a Celsius thermometer, there are 100 intervals or graduations, called degrees, between the freezing and boiling temperatures. Each interval or degree is called 1 Celsius degree.
In the Fahrenheit system, 32F is used to designate the freezing temperature of water and 212F the boiling temperature at normal atmospheric pressure. Hence, on the Fahrenheit scale, a degree is equal to 1⁄180 of the distance on the scale between the freezing and boiling temperatures. Conversion formulas used for each scale are as follows:
Thermal Capacity and Specific Heat
The thermal capacity of a substance is indicated by the quantity of heat required to raise the temperature of 1 lb of the substance 1F. In HVAC calculations, thermal capacity is usually expressed by the British thermal unit (Btu).
One Btu is the amount of heat that is required to increase the temperature of 1 lb of water 1F at or near 39.2F, which is the temperature at which water has its maximum density. Conversely, if 1 Btu is removed from 1 lb of water, its temperature will be reduced by 1F.
Various quantities of heat will produce changes of 1F per pound of substances other than water. Thus, thermal capacity is entirely dependent on the specific heat of the substances.
The specific heat of a substance is the ratio of the heat content or thermal capacity of a substance to that of water. And by definition, the specific heat of water is unity.
It is customary in HVAC calculations to use specific heat in lieu of thermal capacity, because of the convenience of using the Btu as a unit of heat quantity without conversions. Specific heats of air and some common building materials are shown in Table 13.1. Data for other substances may be obtained from tables in the ‘‘ASHRAE Handbook—Fundamentals,’’ American Society of Heating, Refrigerating and Air-Conditioning Engineers. An examination of Table 13.1 indicates that the specific heat of these materials is less than unity and that, of all common substances, water possesses the largest specific heat and the largest thermal capacity.
When heat energy is added to or taken away from a substance, the resulting changes in temperature can be detected by the sense of touch, or sensibly. Therefore, this type of heat is called sensible heat. Since sensible heat is associated with a change in temperature, the quantity of sensible heat energy transferred in a heat exchange is usually calculated from
Laws of Thermodynamics
The application of the laws of thermodynamics to HVAC calculations is usually limited to two well-known laws. These laws can be expressed differently, but in equivalent ways. A simplification of these laws as follows will permit an easier understanding.
The first law of thermodynamics states that when work performed produces heat, the quantity of the heat produced is proportional to the work performed. And conversely, when heat energy performs work, the quantity of the heat dissipated is
proportional to the work performed. Work, ft-lb, is equal to the product of the force, lb, acting on the body for a distance, ft, that the body moves in the direction of the applied force.
Hence, this first law of thermodynamics can be expressed mathematically by the following equation:
Experiments have shown that the mechanical equivalent of heat, known as Joule’s constant, is equivalent to 778 ft-lb /Btu. The first law is also known as the law of conservation of energy.
The second law of thermodynamics states that it is impossible for any machine to transfer heat from a substance to another substance at a higher temperature (if the machine is unaided by an external agency). This law can be interpreted to imply that the available supply of energy for doing work in our universe is constantly decreasing. It also implies that any effort to devise a machine to convert a specific quantity of heat into an equivalent amount of work is futile.
Entropy is the ratio of the heat added to a substance to the absolute temperature at which the heat is added.
The definition of entropy given above involves the concept of absolute temperature measured on a ratio scale. The unit of absolute temperature is measured in degrees