Units of heat

The SI heat energy unit is the joule (J), but four other units still commonly encountered are the calorie (cal), kilocalorie (kcal), British thermal unit (Btu) and the centigrade heat unit (chu). The old definitions of these four units are 

The definitions of these units are linked to the basic SI unit, the joule  

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Of all the fossil fuels, the use of natural gas results ia the formation of the least amouat of CO2 per unit of heat energy produced. On a constant energy basis, natural gas combustion produces approximately 30% less CO2 than Hquid petroleum fuels and approximately 45% less CO2 than coal and other soHd fossil fuels. 

Plants for briquette production exist in the eastern part of Germany, AustraHa, and India. German transport costs pet unit of heating value ate about 40% less for briquettes than for lignite. 

Application of this result shows that if 100 units of heat are needed to maintain a household at 24°C (297°K) by pumping heat from the outside surroundings at 0°C (273°K), it would require a minimum of (24 x 100/297) = 8.08 units of work energy. 

The unit of heat is the watt. However, the imperial unit should be understood, as it will still be met, particularly outside Europe. The ton of refrigeration is derived from an ability to remove sufficient heat from a short ton (2000 lbs) of water at 32°F to turn it to ice at the same temperature in the course of 24 hours. This amounts to a heat extraction rate of 3.517kW. 

The measurement of reaction heats is called calorimetry—a name obviously related to the unit of heat, the calorie. You already have some experience in calorimetry. In Experiment 5 you measured the heat of combustion of a candle and the heat of solidification of paraffin. Then in Experiment 13 you measured the heat evolved when NaOH reacted with HC1. The device you used was a simple calorimeter. 

For the purpose of measuring quantities of heat we require a unit of heat, and the preceding considerations would lead us to define a quantity of heat in terms of the change of temperature which it produces in a given mass of a particular substance under specified conditions. 

It is not essential, however, that the unit of heat should be defined in terms of the rise of temperature produced when heat is absorbed by a standard body, say unit mass of water. Any effect of heat absorption which is capable of measurement and numerical expression might be used, and the method of measurement would in all cases be consistent with the axiom that if two identical systems are acted upon by heat in the same way so as to produce two other identical systems, the quantities of heat supplied to the systems are equal. Lavoisier and Laplace (1780-84) took as unit that quantity of heat which must be absorbed by unit mass of ice in order to convert it completely into water. This unit is of course different from the one we adopted, but if a quantity of heat A has been found to raise from lo ° to 16 ° twice as much water as another quantity of heat B, then A will also melt twice as much ice as B. 

If equal masses of lead, iron, mercury, and glass, all having a temperature of 0° C., are dropped into vessels containing, say, 100 grams of water at 50° C., it will be found that the temperature falls in each case, but when the temperatures have again become steady they are all different. In accordance with our definition, we say that in each case a certain number of units of heat has passed from the water to the body, but the number so passing before the temperature of the body is equal to that of the (somewhat cooled) water is different for the different substances. We... 

If Q units of heat are required to raise the temperature of a body 1°, then 2Q will be required to raise the temperature of two such bodies through 1°, and so on. Hence the heat capacity of a homogeneous body is proportional to its mass. The heat capacity of unit mass of a homogeneous body may therefore be... 

The variation of specific heat with temperature was discovered by Dulong and Petit in 1819. It explains why so many different heat units exist (cf. 5), and requires the definition of specific heat to be so framed as to allow for this variation. For this purpose we replace the finite changes by infinitesimal ones. If SQ units of heat are absorbed when unit mass of a substance is raised in temperature from 6— SO) to 0- - SO) underspecified conditions, the true specific heat at the temperature 0 is ... 

If Q units of heat appear (or disappear) in any process, and A units of work disappear (or appear) simultaneously, then,... 

Let the engine take Qi units of heat from the source, produce A units of work, and give up Q2 units of heat to the refrigerator then the efficiency is N = A/Qi. a)... 

The fall of temperature per unit increase of volume in adiabatic expansion is equal to the increase of pressure per mechanical unit of heat supplied at constant volume, multiplied by the absolute temperature. 

The rate of increase of volume per mechanical unit of heat absorbed at constant pressure is equal to the adiabatic rate of rise of temperature with pressure, divided by the absolute temperature. 

In the SI system, the unit of heat is taken as the same as that of mechanical energy and is therefore the Joule. For water at 298 K (the datum used for many definitions), the specific heat capacity Cp is 4186.8 J/kg K. 

Prior to the now almost universal adoption of the SI system of units, the unit of heat was defined as the quantity of heat required to raise the temperature of unit mass of water by one degree. This heat quantity is designated the calorie in the cgs system and the kilocalorie in the mks system, and in both cases temperature is expressed in degrees Celsius (Centigrade). As the specific heat capacity is a function of temperature, it has been necessary to set a datum temperature which is chosen as 298 K or 25°C. 

In the British systems of units, the pound, but never the slug, is taken as the unit of mass and temperature may be expressed either in degrees Centigrade or in degrees Fahrenheit. The units of heat are then, respectively, the pound-calorie and the British thermal unit (Btu). Where the Btu is too small for a given application, the therm (— 105 Btu) is normally used. 

System Mass unit Temperature scale (degrees) Unit of Heat... 

C, you have also defined your standard unit of heat (and called it... 

The amount of heat released by the complete combustion of one mole of a substance is defined as the heat of combustion, AAVcomb The amount of heat released may be measured in calories (cal) or in joules (J). A calorie is the amount of heat needed to raise the temperature of one gram of water one degree Celsius. The SI unit of heat is the joule. One joule is equal to 4.184 calories. 

Analysing Equations (3.6) and (3.7) helps us remember how the SI unit of heat capacity Cy is J K-1. Chemists usually cite a heat capacity after dividing it by the amount of material, calling it the specific heat capacity, either in terms of J K-1 mol-1 or J K-1g-1. As an example, the heat capacity of water is 4.18 J K-1g-1, which means that the temperature of 1 g of water increases by 1 K for every 4.18 J of energy absorbed. 

Heat energy will flow from an object of a high temperature to an object of a lower temperature. An object with a high temperature does not necessarily contain more heat energy than one with a lower temperature as the temperature change per unit of heat energy supplied will depend upon the specific heat capacity of the object in question. 

A counter-flow heater as shown in Fig. 7.3a heats helium at 101 kPa from a temperature of 20°C to 800°C. The temperature of the heating flue gas (air) entering and leaving are 1800°C and 1200°C at 101 kPa. Find (A) the LMTD, rate of helium flow, and heat transfer based on a unit of heating flue gas, and (B) the LMTD, rate of helium flow, and heat transfer for a parallel-flow heat exchanger under these identical operating conditions. 

Be sure that your units of heat, mass, and temperature match those used in your specific heat capacity before attempting any calculations. 

The unit of heat in the Si system is Joule (J). Calorie (cai) is another heat unit widely used. 

Useful quantitation of heat q as a quantity of energy can be traced to the studies of Joseph Black around 1803. Black recognized that different substances vary in their capacity to absorb heat, and he undertook systematic measurements of the heat capacity C (the ratio of heat absorbed to temperature increase) for many substances. He recognized that a fixed quantity of any pure substance (e.g., 1 g of water) has a unique value of C, which can be chosen as a calorimetric standard for defining quantity of heat in a convenient way. In this manner, he introduced the calorie as a unit of heat ... 

If heat is applied to a substance, the temperature is raised if heat is withdrawn, the temperature is lowered. The unit of heat is the calorie (cal), which is defined as the quantity of heat required to raise the temperature of 1 g of water 1 degree Celsius. We shall not deal with problems so accurately as to be concerned about the very small difference between a 15° calorie" (the heat needed to raise 1 g of water from 14.5°Cto 15.5°C), or the "mean calorie" (1 one-hundredth of the heat needed to raise 1 g of water from 0°C to 100°C). 

Since plate-and-frame exchangers are made by comparatively few concerns, most process design information about them is proprietary but may be made available to serious enquirers. Friction factors and heat transfer coefficients vary with the plate spacing and the kinds of corrugations a few data are cited in HEDH (1983, 3.7.4-3.7.5). Pumping costs per unit of heat transfer are said to be lower than for shell-and-tube equipment. In stainless steel... 

BRITISH THERMAL UNIT (Btu). A unit of heat. The heat required to warm 1 pound of pure water through an interval of 1 degree Fahrenheit. 

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