Temperature approach

Direct Contact Heat Exchangers. In a direct contact exchanger, two fluid streams come into direct contact, exchange heat and maybe also mass, and then separate. Very high heat-transfer rates, practically no fouling, lower capital costs, and lower approach temperatures are the principal advantages. 

Approach temperature. The approach temperature, which is the difference between the process-fluid outlet temperature and the design dry-bulb air temperature, has a practical minimum of 8 to 14°C (15 to 25°F). When a lower process-fluid outlet teiTperature is required, an air-humidification chamber can be providea to reduce the inlet air temperature toward the wet-bulb temperature. A 5.6°C (10°F) approach is feasible. Since typical summer wet-bulb design temperatures in the United States are 8.3°C (15°F) lower than diy-bulb temperatures, the outlet process-fliiid temperature can be 3°C (5°F) below the dry-bulb temperature. 

Operating co.sts. Power requirements for air-cooled heat exchangers can be lower than at the summer design condition provided that an adequate means of air-flow control is used. The annual power requirement for an exchanger is a function of the means of airflow control, the exchanger seiwice, the air-temperature rise, and the approach temperature. 

As the feed-to-steam ratio is increased in the flow sheet of Fig. 11-125 7, a point is reached where all the vapor is needed to preheat the feed and none is available for the evaporator tubes. This limiting case is the multistage flash evaporator, shown in its simplest form in Fig. 11-125 7. Seawater is treated as before and then pumped through a number of feed heaters in series. It is given a final boost in temperature with prime steam in a brine heater before it is flashed down in series to provide the vapor needed by the feed heaters. The amount of steam required depends on the approach-temperature difference in the feed heaters and the flash range per stage. Condensate from the feed heaters is flashed down in the same manner as the brine. 

Approach Temperature—This is defined as the differenee between the saturation temperatures of the steam and the inlet water. Lowering the approaeh temperature ean result in inereased steam produetion, but at inereased eost. Conservatively high-approaeh temperatures ensure that no steam generation takes plaee in the eeonomizer. Typieally, approaeh temperatures are in the 10-20 °F (5.5-11°C) range. Figure 1-40 is the temperature energy diagram for a system and also indieates the approaeh and pineh points in the system. 

For cooling tow ers, one specifies the required cold water temperature and heat duty. Usually, the 95% summer hours maximum w et bulb temperature for the area is the starting point. To this, an allowance is added for recirculation by raising the wet bulb temperature (say, 1-3°F). After the design air wet bulb inlet temperature is set, the cold w ater approach temperature difference to this W et bulb temperature is specified (often, 10°F). 

Intercooler outlet temperature must be determined. If cooling water at 90°F and an approach temperature of 15°F are assumed, the gas outlet from the cooler returning to the compressor will be 105°F. 

In order to avoid the need to measure velocity head, the loop piping must be sized to have a velocity pressure less than 5% of the static pressure. Flow conditions at the required overload capacity should be checked for critical pressure drop to ensure that valves are adequately sized. For ease of control, the loop gas cooler is usually placed downstream of the discharge throttle valve. Care should be taken to check that choke flow will not occur in the cooler tubes. Another cause of concern is cooler heat capacity and/or cooling water approach temperature. A check of these items, especially with regard to expected ambient condi-... 

Approach temperature differences between the oudet process fluid temperature and the ambient air temperamre are generally in the range of 10 to 15 K. Normally, water cooled heat exchangers can be designed for closer approaches of 3 to 5 °K. Of course, closer approaches for air cooled heat exchangers can be designed, but generally these are not justified on an economic basis. 

Once the minimum utility cost has been identified, tradeoffs between operating and fixed costs must be established. This step is undertaken iteratively. For given values of minimum approach temperatures, the pinch diagram is used to obtain minimum cooling cost and outlet gas temperature. By ccmducting enthalpy balance around each unit, intermediate temperatures and exchanger sizing can be determined. Hence, one can evaluate the fixed cost of the system. Next, the minimum approach temperatures are altered, until the minimum TAC is identified. 

The temperature difference between the recooled water temperature and the inlet air wet bulb temperature is called the approach. The lower the ap proach, the more complex the tower s design becomes. The normally used minimum approach temperature is 2 °C. 

Unfired cycle This cycle is very similar to the mentary-fired case except there is no added fuel heat input. The approach temperature and pinch point are even more critical, and tend to reduce steam pressures somewhat. Similarly, the gas turbine exhaust temperature imposes further limits on final steam temperature. 

From Figure 26.7 it can be seen that for equal duties and flows the temperature difference for countercurrent flow is lower at the steam inlet than at the outlet, with most of the steam condensation taking place in the lower half of the plate. The reverse holds tme for co-current flow. In this case, most of the steam condenses in the top half of the plate, the mean vapor velocity is lower and a reduction in pressure drop of between 10-40 per cent occurs. This difference in pressure drop becomes lower for duties where the final approach temperature between the steam and process fluid becomes larger. 

Waste-heat boilers are often used to recover heat from furnace flue gases and the process gas streams from high-temperature reactors. The pressure, and superheat temperature, of the stream generated will depend on the temperature of the hot stream and the approach temperature permissible at the boiler exit (see Chapter 12). As with any heat-transfer equipment, the area required will increase as the mean temperature driving force (log mean AT) is reduced. The permissible exit temperature may also be limited by process considerations. If the gas stream contains water vapour and soluble corrosive gases, such as HC1 or S02, the exit gases temperature must be kept above the dew point. 

Determine the pinch temperature and the minimum utility requirements for the process set out below. Take the minimum approach temperature as 15 °C. Devise a heat exchanger network to achieve maximum energy recovery. 

Find the minimum utility requirements for this process, for a minimum approach temperature of 10 °C. 

Let the cooler-condenser outlet temperature be 40°C. The maximum temperature of the cooling water will be about 30°C, so this gives a 10°C approach temperature. 

As the amount of NO oxidised to NO2 in this unit has not been estimated, it is not possible to make an exact energy balance over the unit. However, the maximum possible quantity of steam generated can be estimated by assuming that all the NO is oxidised and the minimum quantity by assuming that none is. The plant steam pressure would be typically 150 to 200 psig 11 bar, saturation temperature 184°C. Taking the approach temperature of the outlet gases (difference between gas and steam temperature) to be 50°C, the gas outlet temperature will be = 184 + 50 = 234°C (507 K). 

Typical approach temperatures, flue gas to inlet process fluid, are around 100°C. 

A minimum approach temperature to the solvent freezing point is defined. 

The effectiveness of the regenerative Brayton cycle performance will depend on the efficiency of the fuel cell, compressor, and turbine units the pressure loss of gases flowing through the system the approach temperatures reached in the recuperative exchanger and, most importantly, the cost of the overall system. 

Figure lc reveals that this is not the case. Even if exchanger 1 had infinite area (ie., infinite overdesign factor), for a heat capacity flow rate of 1.359 kW/K the outlet temperature of stream Sh] cannot be decreased below 344 K. With a reasonable approach temperature difference of 10 K (Fig. Id), the minimum attainable outlet temperature for stream Shl is 375.4 K, corresponding to a target temperature violation of 52 K. If Sh] were the feed stream to a reactor, this design error could have serious consequences. 

---