Stream temperatures

In many applications the pressure drop available to drive the fluids through the exchanger will be set by the process conditions, and the available pressure drop will vary from a few millibars in vacuum service to several bars in pressure systems. 

8 kN/m2 0.1 x absolute pressure 0.5 x system gauge pressure 0.1 x system gauge pressure 

When a high-pressure drop is utilised, care must be taken to ensure that the resulting high fluid velocity does not cause erosion or flow-induced tube vibration. 

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Fig. 6.16 set to ATmin/2 below hot stream temperatures and ATn,in/2 above cold stream temperatures. 

Figure 7.6 now shows the stream population for each enthalpy interval together with the hot and cold stream temperatures. Now set up a table to compute Eq. (7.6). This is shown in Table 7.2. Thus the network area target for this problem for = 10°C is 7410 m. ... 

If U varies along the tube length or the stream temperature profile is not a smooth curve, then divide the entire tube length into a number of small heat-exchange elements, apply steps (2) through (8) to each element, and sum up the resulting area requitements 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. 

Alternative representations of stream temperature and energy have been proposed. Perhaps the best known is the heat-content diagram, which represents each stream as an area on a graph (3) where the vertical scale is temperature, and the horizontal is heat capacity times flow rate. Sometimes this latter quantity is called capacity rate. The stream area, ie, capacity rate times temperature change, represents the enthalpy change of the stream. 

The two portions of the feed stream recombine and flow into the high pressure separator where the Hquid is separated from the vapor and is fed into an intermediate section of the demethanizer with Hquid level control. The decrease in pressure across the level-control valve causes some of the Hquid to flash which results in a decrease in the stream temperature. The pressure of the vapor stream is decreased by the way of a turboexpander to recover... 

Feed characteri2ation, particularly for nondesalination appHcatioas, should be the first and foremost objective in the design of a reverse osmosis plant. This involves the determination of the type and concentration of the main solutes and foulants in the stream, temperature, pH, osmotic pressure, etc. Once the feed has been characteri2ed, a reaHstic process objective can be defined. In most cases, some level of pretreatment is needed to reduce the number and concentration of foulants present in the feed stream. Pretreatment necessitates the design of processes other than the RO module, thus the overaH process design should use the minimum pretreatment necessary to meet the process objective. Once the pretreatment steps have been determined and the final feed stream defined, the RO module can be selected. 

Framing. The framed bar process is by far the oldest and the most straightforward process utilized in the production of bar soaps. The wet base soap is pumped into a heated, agitated vessel commonly referred to as a cmtcher. The minor ingredients used in soap bars such as fragrance or preservative are added to the wet soap in the cmtcher or injected in-line after reduction of product stream temperature. The hot mixture is then pumped into molds and allowed to cool. 

To avoid decarburization and Assuring of the carbon and low-alloy steels, which is cumulative with time and, for all practical purposes irreversible, the limitations of the Nelson Curves should be followed religiously, as a minimum. Suitable low-alloy plate materials include ASTM-A204-A, B, and C and A387-A, B, C, D, and E, and similarly alloyed materials for pipe, tubes, and castings, depending upon stream temperatures and hydrogen partial pressures, as indicated by the Nelson Curves. 

Pollutant Loading Typical inlet concentrations to a wire-pipe ESP are 1 to 10 grams per cubic meter (g/m ) (0.5 to 5 gr/fl ). It is common to pretreat a waste stream, usually with a wet spray or scrubber, to bring the stream temperature and... 

Wet ESPs add to the complexity of a wash system, because of the fact that the resulting slurry must be handled more carefully than a dry product, and in many cases requires treatment, especially if the dust can be sold or recycled. Wet ESPs are limited to operating at stream temperatures under approximately 80 to 90°C (170 to 190°F), and generally must be constructed of noncorrosive materials (EPA, 1998 Flynn, 1999). 

In addition to the explosive aspects of the LEL, another issue is the heat energy given off during oxidation. An estimate of the exotherm is that there will be a 25 F rise per 1 % LEL in the stream. Hence, if the process air enters the oxidizer at a given temperamre, and if the stream has a concentration of 2% LEL, then a 50 F rise in process stream temperature is expected after oxidation. If the process stream were running at a 10% LEL, then a 250 F temperamre rise would be predicted. A maximum LEL of 25% yields a 625 F temperature rise of the process stream. 

The blades are usually fixed pitch up to 48-in. diameter with applications for adjustable pitch above this size. Fixed pitch is used up to 60-in. diameter with aluminum fen blades when direct-connected to a motor shaft. Variable pitch is used with belts, gears, etc., between the fen shaft and the driver to allow for the possibilities of slight unbalance between blades due to pitch angle variation. Aluminum blades are used up to 300°F, and plastic is limited to about 160°-180°F air stream temperature. 

In general, the thermal boundary layer will not correspond with the velocity boundary layer. In the following treatment, the simplest non-interacting case is considered with physical properties assumed to be constant. The stream temperature is taken as constant In the first case, the wall temperature is also taken as a constant, and then by choosing the temperature scale so that the wall temperature is zero, the boundary conditions are similar to those for momentum transfer. 

The procedure here is similar to that adopted previously. A heat balance, as opposed to a momentum balance, is taken over an element which extends beyond the limits of both the velocity and thermal boundary layers. In this way, any fluid entering or leaving the element through the face distant from the surface is at the stream velocity u and stream temperature 0S. A heat balance is made therefore on the element shown in Figure 11.10 in which the length l is greater than the velocity boundary layer thickness S and the thermal boundary layer thickness t. 

Comparison between the heat exchanged per unit of volume during oxidation experiment in the Shimtec reactor and the maximal heat exchanged in a classical batch reactor (with a double jacket) highlights the effectiveness of the former. Indeed, in oxidation reaction experiments, a mean value of the heat exchanged per unit of volume in the HEX reactor is estimated with utility stream temperature of 47 °C ... 

There is clearly scope for energy integration between these four streams. Two require heating and two cooling and the stream temperatures are such that heat can be transferred from the hot to the cold streams. The task is to find the best arrangement of heat exchangers to achieve the target temperatures. 

In Figure 3.20 the stream temperatures are plotted on the y-axis and the enthalpy change in each stream on the x-axis. For heat to be exchanged a minimum temperature difference must be maintained between the two streams. This is shown as Armin on the diagram. The practical minimum temperature difference in a heat exchanger will usually be between 10 and 20°C see Chapter 12. 

Figure 3.21. Hot stream temperature v. enthalpy (a) Separate hot streams (b) Composite hot streams...

Convert the actual stream temperatures Tact into interval temperatures Tml by subtracting half the minimum temperature difference from the hot stream temperatures, and by adding half to the cold stream temperatures ... 

In the correlations used to predict heat-transfer coefficients, the physical properties are usually evaluated at the mean stream temperature. This is satisfactory when the temperature change is small, but can cause a significant error when the change in temperature is large. In these circumstances, a simple, and safe, procedure is to evaluate the heat-transfer coefficients at the stream inlet and outlet temperatures and use the lowest of the two values. Alternatively, the method suggested by Frank (1978) can be used in which equations 12.1 and 12.3 are combined ... 

We will continue with the gas furnace to illustrate feedforward control. For simplicity, let s make the assumption that changes in the furnace temperature (T) can be effected by changes in the fuel gas flow rate (Ffuei) and the cold process stream flow rate (Fs). Other variables such as the process stream temperature are constant. 

Since we do not have the precise model function Gp embedded in the feedforward controller function in Eq. (10-8), we cannot expect perfect rejection of disturbances. In fact, feedforward control is never used by itself it is implemented in conjunction with a feedback loop to provide the so-called feedback trim (Fig. 10.4a). The feedback loop handles (1) measurement errors, (2) errors in the feedforward function, (3) changes in unmeasured load variables, such as the inlet process stream temperature in the furnace that one single feedforward loop cannot handle, and of course, (4) set point changes. 

Example 10.2 Consider the temperature control of a gas furnace used in heating a process stream. The probable disturbances are in the process stream temperature and flow rate, and the fuel gas flow rate. Draw the schematic diagram of the furnace temperature control system, and show how feedforward, feedback and cascade controls can all be implemented together to handle load changes. 

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