Cooling-water throttling

Controlling reflux drum temperature by throttling cooling water to condenser caused boiling of cooling water when control valve cloeed. This resulted in atmospheric product release. 

Pay attention to low-rate operation of control systems throttling cooling water to condensers. 

Throttling cooling water flow to water-cooled condensers is generally not recommended because of fouling and silting which frequently occur at reduced water velocities. Consequently, water-cooled condensers should be provided with tempered-water systems as described in Chapter 13. Tempered-water systems result in better control, reduced maintenance, and reduced pumping costs. 

As mentioned in Chapter 3, some plants like to control condensate temperature by throttling cooling water as shown in Figure 3.2. This seldom works well. 

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-... 

Figure 6-32 illustrates ejector systems with large condensable loads which can be at least partially handled in the precondenser. Controls are used to maintain constant suction pressure at varying loads (air bleed), or to reduce the required cooling water at low process loads or low water temperatures [2]. The cooler W ater must not be throttled below the minimum (usually 30%-50% of maximum) for proper contact in the condenser. It may be controlled by tailwater temperature, or by the absolute pressure. 

A proposal is made to use a geothermal supply of hot water at 1500 kPa and 180°C to operate a steam turbine. The high-pressure water is throttled into a flash evaporator chamber, which forms liquid and vapor at a lower pressure of 400 kPa. The liquid is discarded while the saturated vapor feeds the turbine and exits at lOkPa. Cooling water is available at 15°C. Find the turbine power per unit geothermal hot-water mass flow rate. The turbine efficiency is 88%. Find the power produced by the geothermal power plant, and find the optimized flash pressure that will give the most turbine power per unit geothermal hot water mass flow rate. 

The oldest, most direct method of pressure control is throttling on the cooling-water supply. This scheme is shown in Fig. 13.5. Closing the water valve to the tube side of the condenser increases the condenser outlet temperature. This makes the reflux drum hotter. The hotter liquid in the reflux drum creates a higher vapor pressure. The higher pressure in the reflux drum increases the pressure in the tower. The tower pressure is the pressure in the reflux drum, plus the pressure drop through the condenser. 

Throttling on the cooling water works fine, as far as pressure control is concerned. But, if the water flow is restricted too much, the cooling-water outlet temperature may exceed 125 to 135°F. In this tempera-... 

Figure 13.5 Tower pressure control using cooling-water throttling.

Figure 3.28. Schematic of laser-induced cold-wall CVD reactor [57] (1 C02 laser, 2 reflector, 3 laser beam, 4 GaAs lens, 5 cooling water, 6 reactor, 7 nozzle, 8 reaction flame, 9 particle plume, 10 board, 11 window, 12 throttling valve, 13 powder collector, 14 pump, 15 pressure gauge, 16 water-cooled Cu block, 17 temperature controller, 18 oven, 19 heater, 20 precursor vessel, 21 liquid HMDS, 22 needle valve, 23 flow meter, 24 preheating tube, 25 co-axial protection gas, 26 lens protective gas)...

Focus attention on exchanger duties which are calculated from small temperature differences (e.g., a condenser duty calculated from the inlet and outlet temperatures and cooling water flow, where the temperatures are less than 10°F apart). Often, the flow can be throttled to increase the temperatm-e difference if this is impractical, high-accuracy temperature indicators may be required. 

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