Heat balances principles

Heating and cooling load calculation for HVAC system design is based on the heat balance principle. For the given building, room, or independent building zone, heat balance components should be established and analyzed. The ma or heat sources and sinks in industrial buildings are ... 

Three different principles govern the design of bench-scale calorimetric units heat flow, heat balance, and power consumption. The RC1 [184], for example, is based on the heat-flow principle, by measuring the temperature difference between the reaction mixture and the heat transfer fluid in the reactor jacket. In order to determine the heat release rate, the heat transfer coefficient and area must be known. The Contalab [185], as originally marketed by Contraves, is based on the heat balance principle, by measuring the difference between the temperature of the heat transfer fluid at the jacket inlet and the outlet. Knowledge of the characteristics of the heat transfer fluid, such as mass flow rates and the specific heat, is required. ThermoMetric instruments, such as the CPA [188], are designed on the power compensation principle (i.e., the supply or removal of heat to or from the reactor vessel to maintain reactor contents at a prescribed temperature is measured). 

The Contalab, initially supplied by Contraves, was purchased by Mettler-Toledo, which is now placing less emphasis on this design than on the RC1. Some comments here are appropriate, however, since it is another type of bench-scale calorimeter, and units continue to be used. Its measuring system is based on the heat balance principle, in which a heat balance is applied over the cooling/heating medium. For this purpose, both the flow rate of the coolant and its inlet and outlet temperatures must be known accurately. Figure 3.12 is a schematic plan of the Contalab. 

The major advantage of this type of calorimeter is that the heat balance principle can easily be applied to the reflux condenser as well, which enables a simpler investigation of processes under reflux conditions. Another advantage is its independence of the heat transfer coefficient at the reactor wall. 

The calorimeter that has been used to obtain the results presented in this section basically combines the power-compensation and heat-balance principles (see Sections 8.2.2.2 and 8.2.2.3). The heat-balance principle is implemented by Peltier elements [18]. This new... 

The noncontact measurement principle, usually called optical or radiation temperature measurement, is based on detecting electromagnetic radiation emitted from an object. In ventilation applications this method of measurement is used to determine surface temperatures in the infrared region. The advantage is that the measurement can be carried out from a distance, without contact with the surface, which possibly influences the heat balance and the temperatures. The disadvantages are that neither air (or other fluid) temperature nor internal temperature of a material can be measured. Also the temper-... 

The same very simple principles apply to the heat-balance equations for nonisothermal system and also to distributed systems as will be shown in the following subsections. The same principles also apply to heterogeneous systems. 

In this section we develop the heat-balance design equations for heterogeneous systems. Based on the previous sections it is clear how to use the heat-balance and heat-balance design equations that were developed earlier for homogeneous systems, as well as the principles that were used to develop the mass-balance and mass-balance design equations for heterogeneous systems for our purpose. We will start with lumped systems. 

Visentin, F., Zogg, A., Kut, O. and Hungerbtihler, K. (2004) A pressure resistant small scale reaction calorimeter that combines the principles of power compensation and heat balance (CRC.v4). Organic Process Research ej Development,... 

A practical approach of heat balance, often used in assessment of heat accumulation situations, is the time-scale approach. The principle is as in any race the fastest wins the race. For heat production, the time frame is obviously given by the time to maximum rate under adiabatic conditions. Then the removal is also characterized by a time that is dependent of the situation and this is defined in the next sections. If the TMRld is longer than the cooling time, the situation is stable, that is, the heat removal is faster. At the opposite, when the TMRld is shorter than the characteristic cooling time, the heat release rate is stronger than cooling and so runaway results. 

On the other hand, in a genuine DSC instrument, sample and reference are each heated individually. A null balance principle is employed, whereby any change in the heat flow in the sample, (e.g. due to a phase change) is compensated for in the reference. The result is that the temperature of the sample is maintained at that of the reference by changing the heat flow. The signal which is recorded (dH/dt) (the heat flow as a function of time (temperature)), is actually proportional to the difference between the heat input into the two channels as a function of time (temperature). 

The second important aspect of circnlation is the heat balance between absorption in the endothermic reaction and release in the exothermic zone. The circulation system design depends on whether the overall scheme is deactivation controlled or heat transfer controlled. The final eqnations, with a brief reference to the principles, are provided for the two extreme cases. 

Optimisation in flowsheeting implies by principle several variables, because a mono-variable optimisation can be solved easily by a sensitivity study. A flowsheeting optimisation problem is always constraint. Firstly, there are equality constraints, as for example the material and heat balances, but also phase equilibrium conditions, as the equality of component fligacities. Secondly, there are inequality constraints. Usually these consist of bounds on temperatures, pressures, flow rates, and concentrations, but they can express also performance limits, as minimum reflux ratio, temperature approach in heat exchangers, etc. 

The great advantage of this kind of evaluation of the heat balance is its complete independence of changes of the physicochemical properties of the reaction mixture. Consequently it is preferred for reaction systems for which this kind of changes in properties is known or to be expected. Especially polymerization reactions belong to this group. At the same time this calorimetric principle is independent of the product of heat transfer area and coefficient... 

The second term in the LHS accounts for the variation of the adsorbed concentration with respect to temperature. The above mass balance can, in principle, be solved for the concentration distribution if we know the temperature variation as a function of time. However, this temperature variation is governed by the interplay between the rate of mass transfer and the rate of energy dissipation. This means that mass and heat balances are coupled and their equations must be solved simultaneously. 

In industry many of the distillation processes involve the separation of more than two components. The general principles of design of multicomponent distillation towers are the same in many respects as those described for binary systems. There is one mass balance for each component in the multicomponent mixture. Enthalpy or heat balances are made which are similar to those for the binary case. Equilibrium data are used to calculate boiling points and dew points. The concepts of minimum reflux and total reflux as limiting cases are also used. 

In Chapter 1 elementary principles of mathematical and graphical methods, laws of chemistry and physics, material balances, and heat balances are reviewed. Many, especially chemical engineers, may be familiar with most of these principles and may omit all or parts of this chapter. 

An overall energy balance in reactive extrusion consists in principle of six components. There is a mechanical energy supply by the rotation of the screws, a heat flow into the extruder through the wall (either positive or negative), and a heat source due to the reaction. The mechanical energy is partly used for pressure generation, while the rest of the mechanical energy is converted directly into heat by internal friction. This heat is utilized to heat and melt the material. Written in a formula the macroscopic heat balance reads... 

The important difference between the DTA and DSC systems is that in the latter the sample and reference are each provided with individual heaters. This makes it possible to use a null-balance principle. It is convenient to think of the system as divided into two control loops. One is for average temperature control, so that the temperature, Tq, of the sample and reference may be increased at a predetermined rate, which is recorded. The second loop ensures that if a temperature difference develops between the sample and reference (because of exothermic or endothermic reaction in the sample), the power input is adjusted to remove this difference. This is the null-balance principle. Thus, the temperature of the sample holder is always kept the same as that of the reference holder by continuous and automatic adjustment of the heater power. A signal, proportional to the difference between the heat input to the sample and that to the reference, dH/ d, is fed into a recorder. In practice this recorder is also used to register the average temperature of the sample and reference. 

Let us first review the most general material and heat balance equations and all of the special cases which can be easily obtained from these equations. This will be followed by the basic idea of how to transform these material and energy balance equations into design equations, first for lumped systems and then followed by the same for distributed systems. We will use homogeneous chemical reactors with multiple inputs, multiple outputs, and multiple reactions. It will be shown in Chapter 6 how to apply the same principles to heterogeneous system and how other rates (e.g., rates of mass transfer) can systematically replace (or is added to) the rates of reactions. 

The overall tank heat balance (using similar principles as in Example 5.2) is... 

The overall heat balance for the coil is as follows. Using principles similar to those used in Example 5.2, we get... 

---