Heat counter-current

The best known use of the hairpin is its operation in true counter-current flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted me advantages gained from countercurrent flow. 

Fields of Application One of the major advantages of the gravity-bed technique is that it lends itself well to true intimate counter-current contacting of solids and gases. This provides for efficient heat transfer and mass transfer. Gravity-bed contacting also permits the use of the sohd as a heat-transfer medium, as in pebble heaters. 

Counter-current rinsing and rinse-water reuse are useful tips for reducing usage. Counter-current contact systems are more efficient in promoting heat and mass exchanges, which are important to gas absorption, extraction, and many types of chemical reactions. 

BATCH COOLING EXTERNAL HEAT EXCHANGER (COUNTER-CURRENT FLOW), NON-ISOTHERMAL COOLING MEDIUM... 

Batch heating/cooling of fluids external heat exchanger (counter-current flow) non-isothermal cooling medium... 

Heat exchanger, counter flow or counter current A heat exchanger in which the flow inlet of one fluid is adjacent to the outlet of the second fluid and vice versa the fluids flow in opposite directions. 

Although two fluids may transfer heat in either counter-current or cocurrent flow, the relative direction of the two fluids influences the value of the LMTD, and thus, the area required to transfer a given amount of... 

L the shell-side fluid makes one pass from inlet to outlet. With a longitudinal baffle, and with the nozzles placed 180° around the shell, the shell-side fluid would be forced to enter at the left, flow to the right to get around the baffle, and flow to the left to reach the exit nozzle. This would be required to approximate true counter-current flow, which was assumed in the heat transfer equations of Chapter 2. 

In the basic heat transfer equation it is necessary to use the log mean temperature difference. In Equation 2-4 it was assumed that the two fluids are flowing counter-current to each other. Depending upon the configuration of the exchanger, this may not be true. That is, the way in which the fluid flows through the exchanger affects LMTD. The correction factor is a function of the number of tube passes and the number of shell passes. 

Figure 7-5 shows a typical hot carbonate system for gas sweetening. The sour gas enters the bottom of the absorber and flows counter-current to the potassium carbonate. The sweet gas then exits the top of the absorber. The absorber is typically operated at 230°F therefore, a sour/ sweet gas exchanger may be included to recover sensible heat and decrease the system heat requirements. 

Continuous production ol charcoal is typically performed in multiple hearth furnaces, as illustrated in the Herreshoff patent shown in Figure 2. Raw material is carried by a screw conveyor to the uppermost of a series of hearths, /kir is supplied counter-currently and burns some of the wood to supply process heat. As the layers of wood carbonize, they are transported to the lower (hotter) hearths by rakes. The hot charcoal product is discharged onto a conveyor belt and cooled with a water spray. 

For counter-current flow of the fluids through the unit with sensible heat transfer only, this is the most efficient temperature driving force with the largest temperature cross in the unit. The temperature of the outlet of the hot stream can be cooler than the outlet temperature of the cold stream, see Figure 10-29 ... 

For heat exchangers in true counter-current (fluids flowing in opposite directions inside or outside a tube) or true co-current (fluids flowing inside and outside of a tube, parallel to each other in direction), with essentially constant heat capacities of the respective fluids and constant heat transfer coefficients, the log mean temperature difference may be appropriately applied, see Figure 10-33. ... 

In air-cooled heat exchangers, the air flows upward umixed across the finned tubes/bundle, and the tube-side process fluid can flow back and forth and downward as established by the pass arrangements. At 4 or more passes, the flow is considered counter-current, and the F factor = 1 0 216 q-pg other fewer-passes correction factors are given in Figures 10-187A, 10-187B, 10-187C. 

Describe the advantages and disadvantages of the following reactor types with reference to heat and mass transfer. For each reactor discuss one reaction for which it may be appropriate to use that reactor, (a) fluidized bed reactor, (b) A continuous counter-current flow reactor, (c) A monolith reactor. 

The reformer feeds and combustor air flow in a counter current manner as shown in Fig. 2. In order to transfer heat to the reformer evenly throughout the interface between reformer and combustor, the combustor is designed to feed the fuel through the holes distributed over the combustor. In this manner, the feed will mix with air incrementally and generate heat throughout the combustor plate evenly. The combustor plate is packed with a Pd catalyst and the reformer uses a Ni/Al203 catalyst. 

Figure 2.30 Schematic drawing of a counter-current micro heat exchanger.

Figure 2.31 Characteristic temperature profiles in a counter-current micro heat exchanger for a very low (left), intermediate (middle) and very high (right) thermal conductivity of the wall material and equal volume flows inside the two channels, reproduced from [125],...

Figure 2.52 2-D model of a counter-current heat-exchanger reactor with a nanoporous catalyst layer deposited on the channel wall.

TeGrotenhuis et al. studied a counter-current heat-exchanger reactor for the WGS reaction with integrated cooling gas channels for removal of the reaction heat. The computational domain of their 2-D model on the basis of the finite-element method... 

The cost of recovery will be reduced if the streams are located conveniently close. The amount of energy that can be recovered will depend on the temperature, flow, heat capacity, and temperature change possible, in each stream. A reasonable temperature driving force must be maintained to keep the exchanger area to a practical size. The most efficient exchanger will be the one in which the shell and tube flows are truly countercurrent. Multiple tube pass exchangers are usually used for practical reasons. With multiple tube passes the flow will be part counter-current and part co-current and temperature crosses can occur, which will reduce the efficiency of heat recovery (see Chapter 12). 

Before equation 12.1 can be used to determine the heat transfer area required for a given duty, an estimate of the mean temperature difference A Tm must be made. This will normally be calculated from the terminal temperature differences the difference in the fluid temperatures at the inlet and outlet of the exchanger. The well-known logarithmic mean temperature difference (see Volume 1, Chapter 9) is only applicable to sensible heat transfer in true co-current or counter-current flow (linear temperature-enthalpy curves). For counter-current flow, Figure 12.18a, the logarithmic mean temperature is given by ... 

When the fluid being vaporised is a single component and the heating medium is steam (or another condensing vapour), both shell and tubes side processes will be isothermal and the mean temperature difference will be simply the difference between the saturation temperatures. If one side is not isothermal the logarithmic mean temperature difference should be used. If the temperature varies on both sides, the logarithmic temperature difference must be corrected for departures from true cross- or counter-current flow (see Section 12.6). 

The temperature correction factor, Ft, will normally be higher with plate heat exchangers, as the flow is closer to true counter-current flow. 

Fig. 5.11. A schematic counter-current heat exchanger. Tl is the temperature of the input stream, T2 of the output stream. Superscripts H and C signify hot end and cold end respectively.

---