Heat-transfer efficiency

But suppose we are operating a heat exchanger subject to rapid rates of initial fouling. The start-of-run heat-transfer coefficient U is 120 Btu/[(h)(ft2(°F)]. Four months later, the U value has lined out at 38. The calculated clean tube-side velocity is lV2 ft/s. This is too low, but what can be done  

It is possible to convert the two-pass tube bundle shown in Fig. 19,1 to the four-pass tube bundle shown in Fig. 19.6. This conversion is effected as follows  

The resulting four-pass tube bundle will have a tube-side velocity twice as high as it did when it was a two-pass exchanger 3 ft/s. Experience has shown that in many services, doubling this velocity may reduce fouling rates by an order of magnitude. That is fine. But what about pressure drop  

When we convert a tube bundle from two to four passes, the pressure drop increases by a factor of 8. For example, assume that the two-pass AP was 5 psig. With the same flow, the four-pass AP would be 40 psig. Let me explain  

Quite likely, even after several years of operation, the pressure drop of a four-pass exchanger, will be greater than the AP of a two-pass 

The center channel head pass partition baffle is cut out. 

Two off-center channel head pass partition baffles are welded in place, so that 25 percent of the tubes are above the upper baffle and 25 percent of the tubes are below the lower baffle. 

Both the channel head cover and the channel head tubesheet (see Fig. 26.1) must be remachined to accommodate the new baffles. 

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Reductive alkylations and aminations requite pressure-rated reaction vessels and hiUy contained and blanketed support equipment. Nitrile hydrogenations are similar in thein requirements. Arylamine hydrogenations have historically required very high pressure vessel materials of constmction. A nominal breakpoint of 8 MPa (- 1200 psi) requites yet heavier wall constmction and correspondingly more expensive hydrogen pressurization. Heat transfer must be adequate, for the heat of reaction in arylamine ring reduction is - 50 kJ/mol (12 kcal/mol) (59). Solvents employed to maintain catalyst activity and improve heat-transfer efficiency reduce effective hydrogen partial pressures and requite fractionation from product and recycle to prove cost-effective. 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

Countercurrent flow of gas and sohds gives greater heat-transfer efficiency with a given inlet-gas temperature. But cocurrent flow can be used more frequently to diy heat-sensitive materials at higher inlet-gas temperatures because of the rapid coohng of the gas during initial evaporation of surface moisture. 

Frame surface cooled (using the surrounding medium) The primary coolant is circulated in a closed circuit and dissipates heat to the secondary ccxilant. which is the surrounding medium in contact with the outside surface of the machine. The surface may be smooth or ribbed, to improve on heat transfer efficiency (as, in a TEFC or tube venulated motor (Figures 1.19 and 1.20) 4 ... 

Fouling of surfaces, resulting in decreased heat transfer efficiency... 

Figure 10-4A(3). Longitudinal fins resistance welded to tubes. The welding of the fins integral to the parent tube ensures continuous high heat transfer efficiency and the absence of any stress concentrations within the tube wall. (Used by permission Brown Fintube Co., A Koch Engineering Co., Bui. 80-1.)...

This reaction is responsible for the deposition of carbon in the reactor tubes in fixed-bed reactors and reducing heat transfer efficiency. 

Higher overall heat transfer coefficients are obtained with the plate heat exchanger compared with a tubular for a similar loss of pressure because the shell side of a tubular exchanger is basically a poor design from a thermal point of view. Considerable pressure drop is used without much benefit in heat transfer efficiency. This is due to the turbulence in the separated region at the rear of the tube. Additionally, large areas of tubes even in a well-designed tubular unit are partially bypassed by liquid and low heat transfer areas are thus created. 

Steam traps are automatic mechanisms that remove low heat-content air and condensate from the steam delivery system. The lack of steam traps or use of traps that fail to function properly leads to a gradual decline in heat-transfer efficiency, waterlogged heat exchangers, and water hammer (which may in turn result in ruptured pipes). When adequate maintenance of steam traps is neglected, this ultimately leads to a serious overall loss of operating efficiency. There are various types of steam traps, each designed for a specific function. Some common variations are discussed in the following sections. 

In any boiler plant system, the boiler itself typically is the single most expensive item of equipment and the source of generated steam. Thus, it tends to receive the highest percentage of whatever time, money, and human resources are available. This is understandable because any loss of heat transfer efficiency through deposition or other physicochemical waterside problems has the greatest impact here. 

Additionally, the surfactant properties of filmers reduce the potential for stagnant, heat-transfer-resisting films, which typically develop in a filmwise condensation process, by promoting the formation of condensate drops (dropwise condensation process) that reach critical mass and fall away to leave a bare metal surface (see Figure 11.2). This function, together with the well-known scouring effect on unwanted deposits keeps internal surfaces clean and thus improves heat-transfer efficiencies (often by 5-10%). 

Heat exchange surfaces must be kept clean deposits reduce heat transfer efficiency and promote various forms of under-deposit corrosion. It also is easier to keep a clean system clean than to prevent a dirty system from getting dirtier, so measurement of the dirt loading or deposit loading on a heat transfer surface is an important part of determining when a boiler needs cleaning. 

Where equipment has been in operational use for some length of time (say, several years), and despite the use of appropriate internal and external water treatment programs, it may become necessary to provide some form of basic offline cleaning to restore heat-transfer efficiency. For smaller HW and LP steam FT boilers, especially where external water treatment is minimal and internal water treatment is limited by cost considerations, the frequency of offline cleaning may be annual, perhaps at the end of the winter heating season. 

C06-0121. A home swimming pool contains 155 m of water. At the beginning of swimming season, the water must be heated from 20 °C to 30 °C. (a) How much heat energy must be supplied (b) If a natural gas heater supplies this energy with an 80% heat transfer efficiency, how many grams of methane must be burned (The heat of combustion of methane is -803 kJ/mol.)... 

Pyrolysis spectra become distorted with respect to their diagnostic features for two major sets of reasons. The first is variations in instrument operation (e.g., heat transfer efficiency from wire to sample, ion source temperature, MAB gas identity, analyzer calibration, tuning, and ion transmission discrimination attributable to contaminated optics). Most of these factors can be controlled... 

With this definition, a heat transfer efficiency of 100% implies that the temperature of the off-gas will be the same as the temperature of the bath. The HTE reported in the literature are in the 80-90% range (Ibaraki et al., 1990 Takahashi et al., 1992 Katayama et al., 1993b). Several authors (i.e., Gou et al., 1993, and Gudenau et al., 1993) have indicated that this definition has limitations because the heat losses to the furnace walls... 

In order to provide further insight into the post-combustion ratio and the heat transfer efficiency, the factors that affect the PCR and HTE will be delineated. The factors that affect the PCR and HTE will be discussed separately with the understanding that a complex relationship may exist between the two parameters. The factors that affect the PCR are shown in Table 4, and Fig. 3 demonstrates the primary conditions for postcombustion. The PCR should be kept relatively high, since the fuel consumption decreases with an increase in the PCR at the same HTE (Aukrust, 1993). However, as mentioned, high PCR may lead to problems due to increases in ... 

The heat transfer efficiency is significantly affected by the slag layer properties and behavior therefore, those factors other than slag phenomena that affect HTE are presented in Table 5. As for the PCR, it is desirable to keep the HTE as high as possible. An increase in HTE at the same PCR decreases fuel consumption (Fruehan et al., 1989 Keogh et al.,... 

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