Heat transfer, cost references

The primary reformer is essentially a process furnace in which fuel is burned with air to indirectiy provide the heat of reaction to the catalyst contained within tubes. This area of the furnace is usually referred to as the radiant section, so named because this is the primary mechanism for heat transfer at the high (750—850°C) temperatures required by the process. Reforming pressures in the range 3 —4 MPa (30,000—40,000 atm) represent a reasonable compromise between cost and downstream compression requirements. 

To meet the 1993 Energy Standards, the industry undertook, at considerable cost, the optimization of the various refrigeration system components. The most significant improvement was the increase in compressor efficiency, from an EER of about 4 to about 5.5. Other system improvements included more efficient fan motors, more effective heat transfer by the evaporator and the condenser, and less defrost energy. In the early 1980s, both the Whirlpool Corporation and White Consolidate Industries introduced electronic defrost controls. Heretofore, an electric timer initiated the defrost cycle, typically every t A elve hours, whether the evaporator needed it or not. With the electronic control the defrost inteiwal is more a function of frost accumulation than of time, and thus referred to as a variable defrost control or as adaptive defrost. It saves energy by being activated only when needed. 

Generally, a heat-transfer fluid should be noncorrosive to carbon steel because of its low cost. Carbon steel may be used with all the organic fluids, and with molten salts up to 450°C (842 °F) [6]. With the sodium-potassium alloys, carbon, and low-alloy steels can be used up to 540°C (1000 F), but above 540°C stainless steels should be used [6]. Stainless steels contain 12 to 30% Cr and 0 to 22% Ni, whereas a steel containing small amoimts of nickel and chromium, typically 1.85% Ni and 0.80% Cr, is referred to as a low alloy steel [6]. Cryogenic fluids require special steels. For example, liquid methane requires steels containing 9% nickel. To aid in the selection of a heat-transfer fluid. Woods [28] has constracted a tenperature-pressure chart for several fluids. 

Cg = constant, dimensionless defined in Table 4 Cj, = heat capacity, Btu/(IbX F) prime refers to process fluid C4 = installed cost of heat exchanger per unit of outside-tube heat-transfer area, /ft ... 

The two major costs associated with evaporators, as with any process equipment, are capital investment and operating costs. The best estimate of the installed cost of evaporation systems is, of course, a firm bid from a vendor. The installed cost, however, can be estimated based on the heat transfer surface area, as in Peters and Timmerhaus. Costs taken from published references must be adjusted for changes subsequent to the time of publication. To do this, one may use an index such as the Marshall and Swift allindustry index. The value of this index is published each month in Chemical Engineering, a McGraw-Hill publication. Further information on the use of this and other cost indices as well as their histories are available, for example, in Peters and Timmerhaus and Ulrich.f Variation of purchased evaporator costs with material of construction and pressure can also be found in Ulrich. ... 

Calculate the overall heat transfer coefficient and log mean temperature difference. (9) Calculate the heat transfer area required and compare it with the area available. (10) Vary Nj- or D. Repeat steps 1 to 9 until A/A = 1. (11) Cost evaluation since most steps are described in the previous case (reactant on the tube side), one case is presented briefly below (refer to case 10 in Table CS1.3). 

When the black box is a compressor, it is mechanical energy that makes the vapor more valuable, and the technique is referred to as mechanical vapor recompression (MVR). The steam temperature in the heating element of the evaporator must give a reasonable AT for heat transfer. This sets the condensing pressure, and the vapor from the evaporator must be compressed at least to that pressure. There is an economic balance to be struck between energy cost and evaporator surface area. Low compression ratios reduce compressor energy consumption but provide smaller temperature differentials and therefore require more heat-transfer surface. 

Note that in this equation, the heat transfer area must be in the English unit of ft. M S is the Marshall and Swift index and is a correction factor to do with the condenser type, operating pressure, and the material of construction. Please refer to Douglas to obtain the estimated value of the correction factor. With the known condenser heat removal and the inlet and outlet temperature difference on the cooling water side, the annual cost of the cooling water can also be easily estimated. 

Several advances on microscale devices and systems have taken place in the past few decades. These devices have taken advantage of low cost and superior performance for the augmentation in transport processes because of their small scale. However, there is a limited understanding of physical processes in these devices. More experimental and simulation studies are essential for further improvement and development of these microsystems. Therefore, I decided to pursue research on the emerging field of microfluidics and heat transfer. My first interest was to extend my prior expertise on experimental techniques for macroscale systems to microsystems. While initiating research on this topic, I also proposed an optional course at DT Kanpur to expose the students to this new exciting research area. I searched for a textbook on this topic but could not find a single book that satisfies all the requirements of my course proposal. Therefore, I had to refer to many reference books for the preparation of my class notes. This book is the result of several revisions of my class notes. 

For the establishment of the realistic limit, one has to take account of the rates of processes in which mass, heat, momentum, and chemical energy are transferred. In this so-called finite-time, finite-size thermodynamics, it is usually possible to establish optimal conditions for operating the process, namely, with a minimum, but nonzero, entropy generation and loss of work. Such optima seem to be characterized by a universal principle equiparti-tioning of the process s driving forces in time and space. The optima may eventually be shifted by including economic and environmental parameters such as fixed and variable costs and emissions. For this aspect, we refer to Chapter 13. 

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