Heat exchangers effectiveness

Hence, the heat-exchanger effectiveness can be written as follows ... 

The importance of equations 37—39 is that once the heat-exchanger effectiveness, S, is known for a given heat exchanger, one can compute the actual heat-transfer rate and outlet stream temperatures from specified inlet conditions. This process is known as rating a given heat exchanger. 

Fig. 4. Heat-exchanger effectiveness where numbers on the curves represent the ratio flow (b) counterflow (c) parallel...

There are two relative temperature changes for a heat exchanger. 1 he greatest of them is the heat exchanger effectiveness, c ... 

The heat exchanger effectiveness shows how close the heat exchanger is operating to the maximum heat transfer performance. Equation (9.5) is valid for any type of heat exchanger. 

Counterflow Heat Exchanger and the Heat Exchanger Effectiveness... 

This heat exchanger effectiveness is one of the important parameters that describes the performance of a counterflow heat exchanger. 

Heat exchanger Effectiveness (or thermal ratio) e = Temperature rise (cold side (/maximum temperature difference between entry (hot side) and entry (cold side)... 

For the closed recuperative cycle [CHTXJi, with states 1,2,X,3,4, Y as in the T,s diagram of Fig. 3.10, the net specific work is unchanged but the heat supplied has to be reassessed as heat qj is transferred from the turbine exhaust to the compressor delivery air. Using the heat exchanger effectiveness, e = (Tx — — Ti) the heat supplied... 

A set of calculations using real gas tables illustrates the performance of the several types of gas turbine plants discussed previously, the [CBT]ig, [CBTX]ig, [CBTBTX]ig, [CICBTXIig and [CICBTBTX]ig plants. Fig. 3.15 shows the overall efficiency of the five plants, plotted against the overall pressure ratio (r) for = 1200°C. These calculations have been made with assumptions similar to those made for Figs. 3.13 and 3.14. In addition (where applicable), equal pressure ratios are assumed in the LP and HP turbomachinery, reheating is set to the maximum temperature and the heat exchanger effectiveness is 0.75. 

Before eonsidering the effects of water injection in an EGT type plant, it is worthwhile to refer to the earlier studies on the performanee of some dry recuperative cycles. Fig. 6.6 shows the T..s diagram of a [CBT i X r cyele, with a heat exchanger effectiveness of unity. It is implied that the surface area for heat transfer is very large, so that the outlet temperature on the cold side is the same as the inlet temperature on the hot side. However, due to the higher specific heat of the hot gas, its outlet temperature is higher than the inlet temperature of the cold air. 

In practice, however, the heat exchanger effectiveness will not be unity for these dry cycles, but the above analysis does suggest that for practical plants ... 

The discussion of the last section is then useful in considering the evaporative cycles. We shall see that the effect of water injection downstream of the compressor (and possibly in the cold side of the heat exchanger) may lead towards the [CBTJiXr type of plant, with increased cold side effective specific heat and hence increased heat exchanger effectiveness. Water injection in the compressor may lead to a plant with isothermal compression. 

We also give calculations of the performance of some of these various gas turbine plants. Comparison between such calculations is often difficult, even spot calculations at a single condition with state points specified in the cycle, because of the thermodynamic assumptions that have to be made (e.g. how closely conditions in a chemical reformer approach equilibrium). Performance calculations by different inventors/authors are also dependent upon assumed levels of component performance such as turbomachinery polytropic efficiency, required turbine cooling air flows and heat exchanger effectiveness if these are not identical in the cases compared then such comparisons of overall performance become invalid. However, we attempt to provide some performance calculations where appropriate in the rest of the chapter. 

Charpy Izod impact test, 19 580-581 Charts, heat-exchanger effectiveness, 13 255-256... 

Carnot efficiency of, 24 654 thermodynamics of, 24 653-654, 655 Heaters, feedwater, 23 218 Heat exchange, 10 144 Heat exchanger design equation, 13 189 Heat-exchanger effectiveness method,... 

Paraformaldehyde, 2 623 22 123 Paraformaldehyde solids, 22 119 para-hydrogen, 23 759, 760—761, 764 vapor pressure of, 23 764 Parainfluenza vaccine, 25 498 Paraldehyde, 2 103 Parallax errors, in liquid-in-glass thermometers, 24 465 Parallel-counterflow exchanger, heat-exchanger effectiveness for, 23 255 Parallel flow heat exchangers heat-exchanger effectiveness for,... 

Single-pass counterflow heat exchanger, heat- transfer equation for, 13 251 Single-pass heat exchangers, heat-exchanger effectiveness for, 13 254 Single-photon emission computed... 

We noted earlier that in a boiling or condensation process the fluid temperature stays essentially constant, or the fluid acts as if it had infinite specific heat. In these cases Cmm/Cmax 0 and all the heat-exchanger effectiveness relations approach a single simple equation,... 

A shell-and-tube heat exchanger is used as an ammonia condenser with ammonia vapor entering the shell at 50°C as a saturated vapor. Water enters the single-pass tube arrangement at 20°C and the total heat transfer required is 200 kW. The overall heat-transfer coefficient is estimated from Table I0-I as 1000 W/m2 °C. Determine the area to achieve a heat exchanger effectiveness of 60 percent with an exit water temperature of 40°C. What percent reduction in heal transfer would result if the water flow is reduced in half while keeping the heat exchanger area and V the same ... 

Return to Eq. (7.12) and let this equation be equal to a product eCA7 where the proportionality constant e is the so-called heat-exchanger effectiveness and AT is a temperature difference. Since the inlet temperatures are given, let AT = Tm — Tci, which is the largest temperature difference in a heat exchanger. Ch or Cc are two possibilities for C. Consider the smaller of the two and designate it as Cjnin. Then, for any heat exchanger, we have... 

For gas-turbine recuperators and for mostgas-to-gas heat exchangers, the C /Cm ratio is approximately equal to unity. Show for this case that the heat-exchanger effectiveness for parallel flow is Eq. (7.48) and for counter flow is Eq. (7.49). 

FIGURE 17.23 Heat exchanger effectiveness E as a function FIGURE 17.24 Heat exchanger effectiveness as a function... 

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