Heat loss corrections

L. D. Hansen, T. E. Jensen, S. Mayne, D. J. Eatough, R. M. Izatt, J. J. Christensen. Heat-Loss Corrections for Small Isoperibol Calorimeter Reaction Vessels. J. Chem. Thermodynamics 1975, 7, 919-926. 

A similar flat flame technique—one that does not require a heat loss correction—is the so-called opposed jet system. This approach to measuring flame speeds was introduced to determine the effect of flame stretch on the measured laminar flame velocity. The concept of stretch was introduced in attempts to understand the effects of turbulence on the mass burning rate of premixed systems. (This subject is considered in more detail in Section 4.E.) The technique uses two... 

What is meant by the specific heat of a substance (3) Where did most of the heat of the brass go (4) Where did the remainder go (5) Why was all possible speed used in transferring the brass from the beaker (6) Why was the time observed (8) How was the error of heat loss corrected for ... 

Millisecond experiments can be used to determine thermal and electrical properties of metals and alloys in the solid phase up to the melting point at around 3000 K. This moderately fast heating rate allows, e.g., the determination of phase transition temperatures. Nevertheless, millisecond e eriments are limited to the solid state as the rate of heating is still slow compared to the gravitational collapse of a liquid sample under gravitational forces once it becomes molten. Another limitation from the moderate heating rate is the need for heat loss corrections at elevated temperatures. 

In Fig. 4.40 a stirred liquid bench-scale calorimeter is displayed. It closely duplicates laboratory reaction setups. The information aboutheat evolved or absorbed is extracted from the temperature difference between the liquid return (Tj) and the reactor (T ). This difference is calibrated with electric heat pulses to match the observed effect at the end of a chemical reaction. In a typical example, 10 W heat input gives a 1.0 K temperature difference between Tj and Tr. The sample sizes may vary from 0.3 to 2.5 liters. The overall sensitivity is about 0.5 W. The calorimeter can be operated between 250 and 475 K. Heat loss corrections must be made for the stirrer and the reflux unit. The block diagram in Fig. 4.40 gives an overview of the data handling. 

Hnedkovsky, L, Hynek, V., Majer, V., and Wood, R.H. (2002) A new version of difierential flow heat capacity calorimeter tests of heat loss corrections and heat capacities of aqueous NaCl from T = 300 K to T= 623 K. /. Chem. Thermodyn., 34, 755-782. 

Correct the heat load for radiant heat losses, RL ... 

Sensible heat losses from thermal plant should be kept to a realistic minimum by the use of correctly specified insulation. There will be a point beyond which further... 

Heat transfer in micro-channels occurs under superposition of hydrodynamic and thermal effects, determining the main characteristics of this process. Experimental study of the heat transfer in micro-channels is problematic because of their small size, which makes a direct diagnostics of temperature field in the fluid and the wall difficult. Certain information on mechanisms of this phenomenon can be obtained by analysis of the experimental data, in particular, by comparison of measurements with predictions that are based on several models of heat transfer in circular, rectangular and trapezoidal micro-channels. This approach makes it possible to estimate the applicability of the conventional theory, and the correctness of several hypotheses related to the mechanism of heat transfer. It is possible to reveal the effects of the Reynolds number, axial conduction, energy dissipation, heat losses to the environment, etc., on the heat transfer. 

Non-adiabatic screening tests such as Carius tube111 and the Accelerating Rate Calorimeter (ARC ), corrected for sample heat losses due to thermal inertia, can also be used for screening. If it is known that the reaction is a vapour pressure system, DSC may be used. 

One example of a discontinuity is a drop in reaction rate due to depletion of reactants as the reaction nears completion. In this case, the step-wise method can still be used, but will tend to oversize. This is because heat losses in the full-scale reactor will cause the reaction to reach completion at a lower temperature/ pressure than was measured in an adiabatic small-scale test, corrected for thermal inertia (see Annex 2). It is not recommended to attempt to take account of heat losses from the full-scale reactor in sizing the relief system (e.g. by modelling them within a computer simulation). This is because a slight overestimation of the rate of heat loss could cause a large underestimation of the relief size required. 

If electronically excited species can be made to give up their excess energy to some active material, then their concentration may be determined by measurement of the rate of heat liberation. The method is, of course, well established for the measurement of oxygen and hydrogen atom concentrations, and the most accurate experimental technique is to use the isothermal hot-wire calorimeter developed by Tollefson and LeRoy.42 The amount of power needed to maintain a catalytic probe at a constant temperature is reduced if heat is liberated at the probe, and no correction is needed for heat losses. The flow of energy-rich species, [Pg.325]

Results pertinent to the theory of critical diameter are contained for the most part in earlier works by English authors. Despite his erroneous assumptions, Holm obtained the correct relation between the critical diameter and the flame velocity (1.4.6). The remarkable work by Daniell on the theory of flame propagation contains an analysis of the influence of heat losses. The losses enter directly into the equation describing the temperature distribution in the flame zone. A solution exists only for heat losses which do not exceed a certain limit, and under critical conditions (at the limit of propagation), the flame velocity drops to a certain fraction (40-50%) of the theoretical flame velocity. Daniell was also the first to indicate definitely that the flame velocity cannot be constructed from thermal quantities alone and by dimensional considerations must be proportional to the square root of the reaction rate. 

The adiabatic temperature rise method may also be used if the reaction is not very fast, by taking into account corrections for heat losses. The thermal energy balance may be written as (Rojas et al., 1981) ... 

The comparison of time scales of Figs 5.19 and 5.16, shows that the reaction proceeds at a much lower rate in the example reported in Fig. 5.19. This makes it necessary to correct the experimental curves for heat losses. 

The reaction is initiated by the addition of a reactant, which must be exactly at the same temperature as the Dewar contents, in order to avoid the sensitive heat effects. Then the temperature is recorded as a function of time. The obtained curve must be corrected for the heat capacity of the Dewar flask and its inserts, respective of their wetted parts, which are also heated by the heat of reaction to be measured. The temperature increase results from the heat of reaction (to be measured), the heat input by the stirrer and the heat losses. These terms are determined by calibration, which may be a chemical calibration using a known reaction or an electrical calibration using a resistor heated by a known current under a known voltage (Figure 4.2). The Dewar flask is often placed into thermostated surroundings as a liquid bath or an oven. In certain laboratories, the temperature of the surroundings is varied in order to track the contents temperature and to avoid heat loss. This requires an effective temperature control system. 

Note that the preceding equation is for ideal cases, in which the particles are monodis-persed, spherical, and totally elastic, and the contact surface is clean. In practice, the particles are usually nonspherical and polydispersed the collision could have involved some heat loss, plastic deformation, or even breakup and the contact surface may have impurities or contaminants. In these cases, a correction factor tj is introduced to account for the effects of these nonideal factors. The applicable form of the electric current through the ball probe is, thus, given by... 

The feedback portion of the loop has to do much less work in this configuration, as it only has to correct for minor load variables, such as heat losses to the atmosphere, steam enthalpy variations, and sensor errors. The feedback and feedforward portions of the loop complement each other. The feedforward portion is responsive, fast, and sophisticated, but inaccurate. The feedback portion is slower, but is capable of correcting the upsets caused by unknown or poorly understood load variations, and it is accurate. 

The assumption of no conductive, convective plus radiative heat loss from catalytic S02 oxidation converters is not perfectly correct. There is always some heat loss. Nevertheless ... 

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