Tower bypass

Figure 1. Flow diagram of a typical single-stage, seed cotton drying system equipped with a tower bypass...

Airflow through drying towers Drying towers bypassed... 

The same procedure maybe used at other pump flows to permit plotting the series of balance-point curves as has been done in Fig. 29-61. From such curves, one can establish the maximum lean pump at any total tower outflow, and combining this with the semilean-pump performance curve results in Fig. 29-55. Bypass flow plotted in Fig. 29-55 is obtained by adding simultaneous lean- and semilean-pump flows and subtracting the recovery pump-turbine flow required to make the balance point at that lean-pump flow. 

One method of maximizing the LCO end point is to control the main fractionator bottoms temperature independent of the bottoms pumparound. Bottoms quench ( pool quench ) involves taking a slipstream from the slurry pumparound directly back to the bottom of the tower, thereby bypassing the wash section (see Figure 9-9). This controls the bottoms temperature independent of the pumparound system. Slurry is kept below coking temperature, usually about 690°F, while increasing the main column flash zone temperature. This will maximize the LCO endpoint and still protect the tower. 

To lower the tower pressure, the hot-vapor bypass pressure recorder controller (PRC) valve is closed. This forces more vapor through the condenser, which, in turn, lowers the temperature in the reflux drum. As the liquid in the reflux drum is at its bubble point, reducing the reflux drum temperature will reduce the reflux drum pressure. As the stripper tower pressure floats on the reflux drum pressure, the pressure in the tower will also decline. 

Hot-vapor bypass pressure control. A more modern way of controlling a tower s pressure is shown in Fig. 13.6. This is the hot-vapor bypass method. When the control valve on the vapor bypass line opens, hot vapors flow directly into the reflux drum. These vapors are now bypassing the condenser. The hot vapors must condense in the reflux drum. This is because there are no vapors vented from the reflux drum. So, at equilibrium, the hot vapors must condense to a liquid on entering the reflux drum. They have no other place to go. 

Leaking hot-vapor bypass valve. Let s assume that the hot-vapor bypass valve, shown in Fig. 13.7, is leaking. It is leaking 10 percent of the tower overhead flow. A good rule of thumb is then... 

Hydrocarbons. For each 20°F temperature difference between the cooler condenser outlet and the warmer reflux pump suction, 10 percent of the tower s overhead vapor flow is leaking through the hot-vapor bypass valve. 

As the hot-vapor bypass valve opens, the condensate level in the shell side of the condenser increases to produce cooler, subcooled liquid. This reduces the surface area of the condenser exposed to the saturated vapor. To condense this vapor, with a smaller heat-transfer area, the pressure of condensation must increase. This, in turn, raises the tower pressure. This then is how opening the hot bypass pressure-control valve increases the tower pressure. 

In general, flooded condenser pressure control is the preferred method to control a tower s pressure. This is so because it is simpler and cheaper than hot-vapor bypass pressure control. Also, the potential problem of a leaking hot-vapor bypass control valve cannot occur. Many thousands of hot-vapor bypass designs have eventually been converted—at no cost—to flooded condenser pressure control. 

Sometimes we see tower pressure control based on feeding a small amount of inert or natural gas into the reflux drum. This is bad. The natural gas dissolves in the overhead liquid product and typically flashes out of the product storage tanks. The correct way to control tower pressure in the absence of noncondensable vapors is to employ flooded condenser pressure control. If, for some external reason, a variable level in the reflux drum is required, then the correct design for tower pressure control is a hot-vapor bypass. 

It is important that no bypassing of the two phases occurs in the tower. A uniform liquid distribution across the tower s cross section is essential. This is accomplished by spraying the water over the top of the tower. Spray nozzles also help to create droplet surface and increase contact time. 

The value of a cannot be directly determined as it consists of both droplet and film surface area. Film surface is independent of the thickness of the water film however, the droplet surface is a function of both the liquid loading generating drops and the size distribution of droplets formed. We can bypass the difficulty in measuring a by measuring the product Ka for the entire tower at specific operating conditions. This is discussed at greater length later. 

Fill bypass systems, which are capable of diverting the entire hot water flow directly into the tower basin, comprise the first. 

There is, however, a danger for ice formation in the fill if too much water is bypassed. In the majority of cooling tower operations, standard practice is to open and close the bypass valves in a cyclic fashion to maintain a desired average basin water temperature while minimizing fill ice formation. 

Mode III Operation is the one in which the tower is zon d and the ice prevention ring fully activated. Again, the fill bypass can, be operated within specified limits. 

Mode IV Operation functions with the hot water flow to the tower entirely diverted to the fill bypass and ice prevention ring. Figure 9.4 summarizes the thermal performances of the four iee prevention operating modes. 

Mode I Operation is the normal cooling tower operating fashion in which hot water is distributed evenly over the entire fill plan area. In this operation, all the valves to the ice prevention ring are in the closed position. The fill bypass can be operated if needed to maintain an optimum basin water temperature. 

Where sand filters are installed to remove suspended matter in recirculating cooling water, it is usually sufficient to provide capacity based on 1 to 5% of the pumped flow rate. Smaller systems with highly rated cooling towers or evaporative condensers, or systems with high cycles of concentration, will tend to need filters installed in a bypass configuration at 4 to 5% of... 

NOTE Where an actual recirculating water pH is required for an in situ LSI calculation, never take a sample of the water falling through the tower fill, as it is over-concentrated due to the evaporation effect. Samples should preferably be taken from a bypass, return line, or well-mixed basin. 

Ozone generators convert only a small percentage of oxygen to ozone. This ozone/air mixture is dosed to a cooling water bypass loop via a venturi injector. A static mixer then ensures intimate contact of the ozone with the water before it returns to the cooling tower basin. 

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