Open feed heaters

An open heat exchanger is a mixing chamber in which a hot fluid is mixed with a fluid at lower temperatures. As an example, open feed heaters (Figure 23.10) raise the temperature of boiler feed by mixing it with extracted steam. They have the added benefit of removing noncondensable gases from the feed stream, but require additional pumps to be installed in the system. Power plants typically have one open feedwater heater. 

Because heat loss to the environment is very small in comparison to the heat transferred within an open heat exchanger, the first law efficiency is close to unity. However, mixing streams is an irreversible process that generates entropy and thus destroys exergy. Assuming no heat loss to the environment, using Equation 23.42 yields the energy balance of the open feed heater. 

In this example, the effect of adding a single open feed heater (Figure 23.16) to the power plant in Example 2 (Section 23.6.2) is analyzed. The feed exiting the feed heater is assumed to be saturated liquid and all pump and turbine efficiencies are 85%. 

From a first law basis, the open feed heater is 100% efficient. On a second law basis using Equation 23.104,... 

In the case of an open feed heater (Figure 23.19) from Equation 23.103 where the vent flow is neglected yields. 

Determine the efficiency and power output of a regenerative Rankine cycle using steam as the working fluid and a condenser pressure of 80 kPa. The boiler pressure is 3 MPa. The steam leaves the boiler at 400° C. The mass rate of steam flow is 1 kg/sec. The pump efficiency is 85% and the turbine efficiency is 88%. After expansion in the high-pressure turbine to 400 kPa, some of the steam is extracted from the turbine exit for the purpose of heating the feed-water in an open feed-water heater, the rest of the steam is reheated to 400°C and then expanded in the low-pressure turbine to the condenser. The water leaves the open feed-water heater at 400 kPa as saturated liquid. Determine the steam fraction extracted from the turbine exit, cycle efficiency, and net power output of the cycle. 

Take two pumps, a boiler, a turbine, a reheater, another turbine, a splitter, a mixing chamber (open feed-water heater), and a condenser from the inventory shop and connect the devices to form the regenerating Rankine cycle. Switch to analysis mode. 

Plot the sensitivity diagram of cycle efficiency versus open feed-water heater temperature. 

Consider a steam power plant operating on the ideal regenerating Rankine cycle 1 kg/sec of steam flow enters the turbine at 15 MPa and 600°C and is condensed in the condenser at lOkPa. Some steam leaves the high-pressure turbine at 1.2 MPa and enters the open feed-water heater. If the steam at the exit of the open feed-water heater is saturated liquid, determine (1) the fraction of steam not extracted from the high-pressure turbine, (2) the rate of heat added to the boiler, (3) the rate of heat removed from the condenser, (4) the turbine power produced by the high-pressure turbine, (5) the turbine power produced by the low-pressure turbine, (6) the power required by the low-pressure pump, (7) the power required by the high-pressure pump, and (8) the thermal cycle efficiency. 

As shown in Figure 4.B.2, the Aspen Plus simulation file is opened first. In VBA, Dim statement is used for declaration of variables, whereas Set statement is used for creating new objects. Then, values of feed flow rate, feed heater outlet temperature, reflux ratio for column 1 and feed stage for column 2 and number of stages for column 3 are transferred from cells C3 to C7 in Excel worksheet named DV to Aspen Plus , to Aspen Plus... 

Feedwater heater A steam-to-water heat exchanger that heats the boiler feedwater, generally with steam extracted from a steam turbine. In a closed feedwater heater the two fluids are separated by the use of shell and tube construction. In an open feedwater heater the fluids are mixed. A feedwater heater that separates entrained gases from the feed-water by vigorous agitation with steam. 

The gas quality feeding the dry faee seal should be elean and dry. Due to the possibility of eondensation of the proeess gas in the seal eavity, it was deeided to use a seal gas heater. The heater eontrol was set to provide warm gas at 15°C above the dew point to ensure no eondensate entered the seal eavity. Also, a dual filter in series with 5 and 2 p filtration elements was ehosen to provide an ideal sealing environment and maintain the optimum performanee of the seal. To reduee the risk of seal damage during reverse rotation of the turboexpander, programming logie was set to open the eompressor bypass valve whenever a shutdown impulse was initiated. 

The BSD can either shut down the entire facility, or it can be designed for two levels of shutdown. The first level shuts down equipment such as compressors, lean oil pumps, and direct fired heaters, and either shuts in the process or diverts flow around the process by closing inlet/outlet block valves and opening bypass valves. The second level shuts down the remaining utilities and support facilities, including generators and electrical feeds. 

All single-screw extruders have several common characteristics, as shown in Figs. 1.1 and 1.2. The main sections of the extruder include the barrel, a screw that fits inside the barrel, a motor-drive system for rotating the screw, and a control system for the barrel heaters and motor speed. Many innovations on the construction of these components have been developed by machine suppliers over the years. A hopper is attached to the barrel at the entrance end of the screw and the resin is either gravity-fed (flood-fed) into the feed section of the screw or metered (starve-fed) through the hopper to the screw flights. The resin can be in either a solid particle form or molten. If the resin feedstock is in the solid form, typically pellets (or powders), the extruder screw must first convey the pellets away from the feed opening, melt the resin, and then pump and pressurize it for a down-... 

Feed-water Heaters.—Types include the open heater (Cochrane, Webster) in which the exhaust steam and water mix, and the closed heater, in which the water circulates in tubes surrounded by exhaust steam. Open heaters give slightly higher feed temperatures at the same back pressure, or slightly less back pressure at the same feed temperature. Filters, separators, etc., are provided to remove oil from the exhaust. The open heater forms a convenient receptacle for various drips, for the automatic introduction of any cold water make-up supply and for certain forms of feed-water treatment and purification. It may be of the thoroughfare type in which all exhaust steam in the pipe passes through the heater, or of the draw in type in which a branch from the auxiliary exhaust leads to the heater as a dead end. Open heaters must be located on the suction side of the feed pump and above (preferably 3 ft. or more above) the level of the feed-pump suction valves. 

The operation of the microreactors is monitored through the Microreactors tab on the main control panel. This panel displays the position of the SOVs, the feed gas flow rate to each microreactor channel, and whether or not the microreactor heaters are enabled. Additional information on the operation of each of the microreactor channels can be obtained by clicking on the Open Panel button next to the reactor name. In addition, pressing the Configure Reactor Control button opens a sub-panel where the operator can configure the temperature control mode for each of the microreactor channels to be either manual or automatic PID control. Some salient aspects of the reactor control panel are given below since this is the key system component. 

FIGURE 33.19 Aseptic open spray drying system (1, drying chamber 2, air dispenser 3, atomizer 4, prefilter 5, filter 6, heater 7, HEPA filter 8, sterile feed filter 9, feed pump 10, cyclone collector 11, packing room). (Courtesy of A/S Niro Atomizer, Soeborg, Denmark.)... 

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