Noncondensibles accumulation

Inerts. Accumulation of even a small fraction of noncondensables can impair condensation heat transfer. The mechanism by which noncondensables accumulation reduces heat transfer is described in Sec. 15.10. Inerts problems are most common in shell-side condensation, where gases can segregate in pockets, and are difficult to remove unless sufficient pressure drop is used to force them to the vent outlet (291, 381). Cross-flow condensers are particularly prone to inerts accumulation. Numerous cases where inerts venting was a problem have been reported (28,134, 239, 381). Adequate venting facilities are needed at all locations where noncondensables are likely to accumulate. Guidelines for venting are presented in Sec. 15.9. 

Accumulation of noncondensables in the drum may unflood the drum and interrupt the control action. If noncondensables accumulation is infrequent, manual venting from the top of the drum will be sufficient to restore satisfactory operation. If noncondensables accumulate frequently, or the column is nm imattended, automatic venting is required. 

Figure 17.5c illustrates an automatic vent system that worked well (203). A second pressure controller (PC No. 2), a level controller, and a control valve in the vent line are added. The set point of PC No. 2 is lower than that of the normal pressure controller (PC No. 1). When the drum is full, the level controller keeps PC No. 2 tripped off, and the vent valve is closed. Drum unflooding (due to noncondensable accumulation) is sensed by a drop in drum level. The lower level activates PC No. 2. Since the set point of PC No. 2 is lower than PC No. 1, it opens the vent valve. As the pressure falls, PC No. 1 closes, helping to build up the drum level. As soon as the drum refills, the level controller trips PC No. 2, and the vent valve closes. 

In a paper machine, the dryers are arranged in groups that have a common steam-supply header. Each group has to operate in such a condition that neither the condensate nor the noncondensables accumulate in the dryer. There are basically two approaches to design the dryer steam and the condensate system. One is the recirculation system in which most of the blow-through is recirculated and the other is the cascade system in which the blow-through steam from higher-pressure sections is used in lower-pressure sections. The recirculation system is more flexible whereas the cascade system is simpler and cheaper [4],... 

Air is usually the basic load component to an ejector, and the quantities of water vapor and/or condensable vapor are usually directly proportional to the air load. Unfortunately, no reliable method exists for determining precisely the optimum basic air capacity of ejectors. It is desirable to select a capacity which minimizes the total costs of removing the noncondensable gases which accumulate in a process vacuum system. An oversized ejector costs more and uses unnecessarily large quantities of steam and cooling water. If an ejector is undersized, constant monitoring of air leaks is required to avoid costly upsets. 

Accumulation of Noncondensihles - Noncondensibles do not accumulate under normal conditions since they are released with the process vapor streams. However, with certain piping configurations, it is possible for noncondensibles to accumulate to the point that a condenser is "blocked". Such a condition could occur if an automatic vent control valve failed closed for a period of time. This effect is equal to a total loss of coolant, and thus need not be considered separately. 

Non-Condensable Gas Accumulation If noncondensible gases are not removed, overpressure can result when a heat exchanger surface becomes blanketed or pressure drop through the condensers is increased by the presence of the non-condensable gas. 

Steam produced from demineralized water is free of carbonates. Steam produced from lime-softened water will be contaminated with carbonates that decompose in the boiler to carbon dioxide. As the steam condenses in a reboiler, the C02 accumulates as a noncondensable gas. This gas will be trapped mainly below the channel head pass partition baffle shown in Fig. 8.6. As the concentration of C02 increases, the C02 will be forced to dissolve in the water ... 

Vapor binding, or air lock, is another common cause of household radiator malfunction. Often, the vapor accumulating in the radiator is CO,2, rather than air. The C02 originates from the thermal decomposition of carbonates in the boiler. Regardless, air and C02 form a noncondensable vapor in the radiator. These noncondensables mix with the steam in the radiator. The noncondensables then reduce the concentration of the steam, by dilution. The diluted steam has a lower partial pressure than pure steam. The lower the partial pressure of the steam, the more difficult it is to condense. As the rate of condensation of the steam drops, so does the heat radiated by the radiator. 

To restore the efficiency of a radiator, suffering from the accumulation on noncondensables inside its condensing coils, the noncondensable gases have to be removed. The air vent shown in Fig. 13.1 serves this purpose. 

The gas that accumulates inside the surface condenser is called the noncondensable load to the steam jets. Some of the noncondensable load consists of C02 accidentally produced when the boiler feedwater is vaporized into steam. Air leaks through piping flanges and valves are other sources of noncondensable vapors. But the largest source of noncondensable vapors is often air drawn into the turbine case, through the shaft s mechanical seals. To minimize this source of leaks, 2 or 3 psig of steam pressure is ordinarily maintained around the seals. However, as the turbine s shaft seals deteriorate, air in-leakage problems can overwhelm the jet capacity. This will cause a loss of vacuum in the surface condenser. 

Finally, the receiver will accumulate any noncondensable (or hard-to-condense) components that have accidentally entered the system. Air left in the vessels on start-up is one such example. Traces of methane and ethane in a propane refrigerant system is another. These light vapors may be vented from the top of the receiver during normal operations. 

The next table, Table 8.30, is similar to the previous, but without any condenser. A steam condensate accumulator with condensate pumps is required however, many installations simply return the exhaust steam in an existing LP header directly, without any supplemental equipment other than piping added to the noncondensing turbine driver. Piping is... 

The value of the coefficient of heat transfer from steam to sheet is determined by the conditions prevailing on the inside and on the surface of the dryers. Low coefficients may be caused by (1) poor removal of air or other noncondensables from the steam in the cylinders, (2) poor removal of condensate, (3) accumulation of oil or rust on the interior of the drums, and (4) accumulation of a fiber lint on the outer surface of the drums. In a test reported by Lewis et al. [Pulp Pap. Mag. Can., 22 (February 1927)] on a sulfite-paper dryer, in which the actual sheet temperatures were measured, a value of 187 W/(m °C) [33 Btu/(h-fr-°F)] was obtained for the coefficient of heat flow between the steam and the paper sheet. 

In some cases, as with pulp-mill liquors, the evaporator vapors contain constituents more volatile than water, such as methanol and sulfur compounds. Special precautions may be necessary to minimize the effects of these compounds on heat transfer, corrosion, and condensate quality. They can include removing most of the condensate countercurrent to the vapor entering an evaporator-heating element, channeling vapor and condensate flow to concentrate most of the foul constituents into the last fraction of vapor condensed (and keeping this condensate separate from the rest of the condensate), and flashing the warm evaporator feed to a lower pressure to remove much of the foul constituents in only a small amount of flash vapor. In all such cases, special care is needed to properly channel vapor flow past the heating surfaces so there is a positive flow from steam inlet to vent outlet with no pockets, where foul constituents or noncondensibles can accumulate. 

Accumulators are not separators. In one application, an acciunulator placed after a total condenser provides reflux to a fractionator and prevents column fluctuations in flow rate from affecting downstream equipment. In this application the accumulator is called a reflux drum. A reflux drum is shown in Figure 6.3. Liquid from a condenser accumulates in the drum before being split into reflux and product streams. At the top of the drum is a vent to exhaust noncondensable gases that may enter the distillation column. The liquid flows out of the drum into a pump. To prevent gases from entering the pump, the drum is designed with a vortex breaker at the exit line. 

The process includes a condenser for continuous separation of the mixture of gases and the mixture of liquid fuels. The condensed heavier fractions are collected in the accumulator and the uncondensed gases are collected and stored separately through the vent gas stream consisting of noncondensable with gases up to C4. The liquid hydrocarbons are subjected to fractional distillation. 

The trapped noncondensibles were released directly to the overhead accumulator through a small (1 in. dia.) added line. The gases included the light hydrocarbons and any noncondensible acid gases. 

Most condensers used in steam power plants operate at pressures well below the atmospheric pressure (usually under 0.1 atm) to maximize cycle thermal efficiency, and operation at such low pressures raises the possibility of air (a noncondensable gas) leaking into the condensers. Experimental studies show that the presence of noncondensable gases in the vapor has a detrimental effect on condensation heat transfer. Even small amounts of a noncondensable gas in the vapor cause significant drops in heat transfer coefficient during condensation. Eor example, the presence of less than 1 percent (by mass) of air in steam can reduce the condensation heat transfer coefficient by more than half. Therefore, it is common practice to periodically vent out the noncondensable gases that accumulate in the condensers to ensure proper operation. 

Electric power loss Accumulation of noncondensibles Failure of automatic controls Loss of heat in series fractionation Volatile material entering system Heat exchanger tube failure... 

Condensation of a vapor in the presence of a noncondensable gas is treated elsewhere in this chapter. Figure 14.3 describes the added thermal resistance that occurs due to mass diffusion of the vapor through a noncondensable, gas-rich layer next to the condensate. The case of two condensing vapors is similar to that depicted in Fig. 14.3. Both vapor components condense, but the more volatile one accumulates at the interface and provides a barrier for the less volatile one, similar to a noncondensable gas. Similar effects are also found with multi-component mixtures. 

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