Total heat, enthalpy

TOTAL HEAT (ENTHALPY) - Total heat is the sum of the sensible heat and latent heat in an exchange process. In many cases, the addition or subtraction of latent and sensible heat at terminal coils appears simultaneously. Total heat also is called enthalpy, both of which can be defined as the quantity of heat energy contained in that substance. 

Enthalpy. Enthalpy is the thermodynamic property of a substance defined as the sum of its internal energy plus the quantity Pv//, where P = pressure of the substance, v = its specific volume, and J = the mechanical equivalent of heat. Enthalpy is also known as total heat and heat content. 

Sensible Heat Factor. The ratio of the change in sensible (constant moisture content) cooling enthalpy to the change in total cooling enthalpy. 

If the substitute fuel is of the same general type, eg, propane for methane, the problem reduces to control of the primary equivalence ratio. For nonaspiring burners, ie, those in which the air and fuel suppHes are essentially independent, it is further reduced to control of the fuel dow, since the air dow usually constitutes most of the mass dow and this is fixed. For a given fuel supply pressure and fixed dow resistance of the feed system, the volume dow rate of the fuel is inversely proportional to. ypJ. The same total heat input rate or enthalpy dow to the dame simply requires satisfactory reproduction of the product of the lower heating value of the fuel and its dow rate, so that WI = l- / remains the same. WI is the Wobbe Index of the fuel gas, and... 

Binary minimum reflux so calculated implies feed enthalpy just equal to the above started vapor V and liquid L. Any increase or decrease in that enthalpy must be matehed by inerease or decrease in total heat content of overhead reflux. Note that the Underwood" binary reflux equation essentially computes the flash versus specifi-eation composition relationship along with enthalpy correction. 

If a phase change occurs in the process stream for which heat duties are being calculated, it is best to perform a flash calculation and determine the heat loss or gain by the change in enthalpy. For a quick hand approximation it is possible to calculate sensible heat for both the gas and liquid phases of each component. The sum of all the latent and sen -i-ble heats is the approximate total heat duty. 

Overall enthalpy transfer coefficient, lb/ (hr) (ft- transfer area) (lb water/lb dry air or, mass transfer coefiRcient, lb water/(hr) (ft ) Enthalpy coefficient of total heat transfer,... 

Pressure, temperature, and enthalpy or total heat values may be obtained from tables or diagrams covering each particular refrigerant. Table 11-12 presents a few comparative values of boiling points (evaporator temperature) and corresponding pressures as taken from such data. 

The specific enthalpy (or total heat) of the mixture can be taken from 0 K (- 273.15°C) or from any convenient arbitrary zero. Since most air-conditioning processes take place above the freezing point of water, and we are concerned mostly with differences rather than absolute values, this is commonly taken as 0°C, dry air. For conditions of 25°C, saturated, the specific enthalpy of the mixture, per kilogram of dry air, is... 

ALL CHANGES IN PHASE involve a release or absorption of calories. One reason for this is that each solid has its own heat capacity. That is, there is a characteristic heat content for each material which depends upon the atoms composing the solid, the nature of the lattice vibrations within it, and its structure. The total heat content, or enthalpy, of each solid is defined by ... 

The heat absorbed in a process at constant pressure is equal to AH, the increase in the enthalpy of the system. It can thus be said that the heat change accompanying a chemical reaction is equal to the difference between the total heat content of the products and that of the reactants, at constant pressure and temperature conditions. This quantity is called the heat of reaction, AH, and can be expressed as follows... 

If the heat exchange involves desuperheating as well as condensation, then the exchanger can be divided into zones with linear temperature-enthalpy profiles in each zone. Figure 15.12a illustrates desuperheating and condensation on the shell-side of a horizontal condenser. The total heat transfer area is the sum of the values for each zone ... 

Solution The conditions for the inlet steam are fixed, but the conditions of the outlet steam will depend on the performance of the turbine. First, estimate the steam flowrate from the process heating duty. A good approximation is that the sum of the heat content of the superheat and latent heat is constant from inlet to outlet. At the expander inlet, the heat content of the superheat is higher than that of the outlet, but the latent heat is lower in the inlet than in the outlet. The two trends tend to cancel each other out, with the total heat content of superheat and latent heat being approximately constant across the turbine. From steam properties, the enthalpy of the superheated steam HSup, enthalpy of the saturated steam HSat and enthalpy of the saturated condensate Hi at 100 barg are ... 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

It is usually assumed that the total heat released when one mole of ATP is hydrolysed is equal to the enthalpy change (-21 kJ) but this is only in vitro. It ignores the fact that hydrolysis of ATP in vivo demands simultaneous generation of ATP to maintain its concurrence, which requires aerobic metabolism, which also expends energy. When this is taken into account, the hydrolysis of one mole of ATP is actually responsible for release of 90 kJ of heat (calculated on the basis that oxidation of one mole of glucose generates 30 moles ATP). 

A simple enthalpy balance gives the total heat load per unit mass as... 

Differential Scanning Calorimetry (DSC) This is by far the widest utilized technique to obtain the degree and reaction rate of cure as well as the specific heat of thermosetting resins. It is based on the measurement of the differential voltage (converted into heat flow) necessary to obtain the thermal equilibrium between a sample (resin) and an inert reference, both placed into a calorimeter [143,144], As a result, a thermogram, as shown in Figure 2.7, is obtained [145]. In this curve, the area under the whole curve represents the total heat of reaction, AHR, and the shadowed area represents the enthalpy at a specific time. From Equations 2.5 and 2.6, the degree and rate of cure can be calculated. The DSC can operate under isothermal or non-isothermal conditions [146]. In the former mode, two different methods can be used [1] ... 

Closely related to the problem of the structure of the effective rate constant is the above-mentioned problem of the compensation mechanism. Without a knowledge of this mechanism, it would be impossible to understand why in such a complicated epoxyamine system one can frequently observe relatively simple kinetic principles, viz., a weak dependence of the effective rate constant on conversion, simple dependences of the initial rate on reagent concentrations, a linear dependence of the total heat release on conversion and almost equal values of the heat release and enthalpy of the epoxy ring opening. The latter two aspects have been discussed above, whereas the first two problems can be understood, say, from a consideration of a noncatalytic reaction. 

Enthalpy The sum of the internal and pressure energies of a substance or system. Formerly called Total Heat or Heat Content. 

The integral heat (enthalpy) of solution A/7soln is defined as the total heat liberated (under constant-P conditions) when a solution is formed from its pure components A, B ... 

The term specific heat refers to the sensible-heat content of either vapor or liquid. The specific heat is the amount of heat needed to raise the temperature on one pound of the vapor or liquid by 1°F. The term latent heat refers to the heat of vaporization, or the heat of condensation, needed to vaporize or condense one pound of liquid or vapor at constant temperature. Note that the heat of condensation is equal to the heat of vaporization. Each is referred to as the latent heat. The sum of the sensible heat, plus the latent heat, is called the total heat content, or enthalpy. 

Calculate the total heat absorbed, given the enthalpies of the inlet and outlet process streams and the heat of reaction... 

Figure 20 shows the phase diagram of polyethylene119). The existence range of the condis crystals increases with pressure and temperature. The enthalpy of the reasonably reversible, first order transition from the orthorhombic to the hexagonal condis phase of polyethylene is 3.71 kJ/mol at about 500 MPa pressure 121) which is about 80 % of the total heat of fusion. The entropy of disordering is 7.2 J/(K mol), which is more than the typical transition entropy of paraffins to their high temperature... 

Enthalpy—Total heat content the sum of the sensible heat of the air and water vapor and of the latent heat of vaporization. 

Consider a countercurrent operation in which cold water contacts hot air. There are two unknowns the outlet air humidity and the water loading to the tower, Even though the outlet-air temperature may be known, we cannot determine its enthalpy unless its humidity is known. Also. without the outlet humidity, the total heat load, qt, cannot be determined as well as the water,... 

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