Units of Heat.
The unit of heat commonly used in the English system is the British thermal unit, or Btu, and is approximately equal to the quantity of heat that must be transferred to one pound of water in order that its temperature be raised one degree Fahrenheit. In laboratory work and throughout much of the world, the calorie is the common unit of heat. A gram calorie is the approximate quantity of heat that must be transferred to 1 gram (g) of water in order to raise its temperature by 1*C. The kilocalorie, sometimes called the kilogram calorie, is equal to 1000 gram calories.
The unit of heat commonly used in the English system is the British thermal unit, or Btu, and is approximately equal to the quantity of heat that must be transferred to one pound of water in order that its temperature be raised one degree Fahrenheit. In laboratory work and throughout much of the world, the calorie is the common unit of heat. A gram calorie is the approximate quantity of heat that must be transferred to 1 gram (g) of water in order to raise its temperature by 1*C. The kilocalorie, sometimes called the kilogram calorie, is equal to 1000 gram calories.
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The definitions above are indicated as being approximate because, over the temperature range from freezing to boiling points of water, different quantities of heat are required to produce a unit temperature change. For this reason, the calorie and the Btu have been defined in international units as
In most engineering work, it is sufficiently accurate to use 1 kg . cal = 3.968 Btu and 1 Btu = 0.252 kg . cal.
Units of Temperature.
The relative ‘‘hotness’’ or ‘‘coldness’’ of a body is denoted by the term temperature. The temperature of a substance is measured by noting its effect upon a thermometer or pyrometer whose thermal properties are known. The mercury thermometer is suitable for measuring temperatures from _39 to about 600_F. This limit may be extended to 1000_F if the capillary tube above the mercury is filled with nitrogen or carbon dioxide under pressure. High temperatures must be measured with thermocouples or optical pyrometers. The most commonly used thermometer scales are the Fahrenheit and the Celsius. Thermometer scales have as their bases the melting and boiling points of water, both measured at atmospheric pressure. The relation of the Fahrenheit and Celsius scales is as follows:
The relation between the two scales is
in which C is the reading on the Celsius scale and F is the reading on the Fahrenheit scale.
In certain calculations,
it is necessary to express the temperature in ‘‘absolute’’ units. The absolute temperature associated with the Fahrenheit scale is called the Rankine temperature, and that associated with the Celsius scale is termed the Kelvin temperature. The relationships among these scales are as follows:
where R and K designate absolute temperatures on the Rankine and Kelvin scales, respectively.
Specific Heat.
The specific heat of a substance is the quantity of heat required to produce a unit temperature change in a unit mass of that substance. Typical units are calories per gram per degree Celsius and Btu per pound per degree Fahrenheit. The numerical value of specific heat is a function of the process by which the unit temperature change is effected. If a gas expands at constant pressure owing to the addition of heat, work is done by the walls of the containing vessel on the surrounding atmosphere, and the heat addition must be greater than would have been required to cause the same temperature change at constant volume. The two most frequently used specific heats are those at constant volume and constant pressure, and they are represented symbolically as cv and cp, respectively.
The definition of specific heat given in the preceding paragraph is convenient for engineering applications. By thermodynamic analysis, it can be shown that the two specific heats referred to are given by
it is necessary to express the temperature in ‘‘absolute’’ units. The absolute temperature associated with the Fahrenheit scale is called the Rankine temperature, and that associated with the Celsius scale is termed the Kelvin temperature. The relationships among these scales are as follows:
where R and K designate absolute temperatures on the Rankine and Kelvin scales, respectively.
Specific Heat.
The specific heat of a substance is the quantity of heat required to produce a unit temperature change in a unit mass of that substance. Typical units are calories per gram per degree Celsius and Btu per pound per degree Fahrenheit. The numerical value of specific heat is a function of the process by which the unit temperature change is effected. If a gas expands at constant pressure owing to the addition of heat, work is done by the walls of the containing vessel on the surrounding atmosphere, and the heat addition must be greater than would have been required to cause the same temperature change at constant volume. The two most frequently used specific heats are those at constant volume and constant pressure, and they are represented symbolically as cv and cp, respectively.
The definition of specific heat given in the preceding paragraph is convenient for engineering applications. By thermodynamic analysis, it can be shown that the two specific heats referred to are given by
where u and h represent internal energy and enthalpy, respectively, and v and p indicate that volume or pressure remains constant during the measurement of the corresponding specific heat. The specific heats of most substances vary with temperature. For a general functional relationship, the mean value of specific heat over a temperature range from T1 to T2 is given by
If the algebraic relationship between specific heat and temperature is not known but the relation is available in the form of a graph or table, it is usually sufficiently accurate to evaluate the average or mean specific heat at the average of temperature over the temperature range in question.
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nice post on stainless steel capillary tubes can u write more on shims
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