Condensing pressure

As stated in the introduction to the previous chapter, adsorption is described phenomenologically in terms of an empirical adsorption function n = f(P, T) where n is the amount adsorbed. As a matter of experimental convenience, one usually determines the adsorption isotherm n = fr(P), in a detailed study, this is done for several temperatures. Figure XVII-1 displays some of the extensive data of Drain and Morrison [1]. It is fairly common in physical adsorption systems for the low-pressure data to suggest that a limiting adsorption is being reached, as in Fig. XVII-la, but for continued further adsorption to occur at pressures approaching the saturation or condensation pressure (which would be close to 1 atm for N2 at 75 K), as in Fig. XVII-Ih. 

The efficiency of the Rankine cycle itself can be increased by higher motive steam pressures and superheat temperatures, and lower surface condenser pressures in addition to rotating equipment selection. These parameters are generally optimized on the basis of materials of constmction as well as equipment sizes. Typical high pressure steam system conditions are in excess of 10,350 kPa (1500 psi) and 510 °C. 

The optimum intermediate pressure for the two-stage refrigeration cycles is determined as the geometric mean between evaporation pressure (pi) and condensing pressure (p/, Fig. 11-79) ... 

The capacity could be controlled by (1) adjusting the prerotation vanes at the impeller inlet, (2) varying the speed, (3) vaiying the condenser pressure, aud (4) bypassing discharge gas. The first two methods are predominantly used. 

The price for an increase in heat-transfer characteristics is a more complex system with more anxihary eqnipmeut low-pressure receivers, refrigerant pumps, valves, and controls. Liquid refrigerant is predominantly pumped by mechanical pumps, however, sometimes gas at condensing pressure is used for pumping, in the variety of concepts. 

The condenser design, surface area, and condenser cooling water quantity should be based on the highest cooling water temperature likely to be encountered, if the inlet cooling water temperature becomes hotter then the design, the primaiy booster (ejector) may cease functioning because of the increase in condenser pressure. 

Pressures can be specified at any level below the safe working pressure of the column. The condenser pressure will be set at 275.8 kPa (40 psia), and all pressure drops within the column will be neglected. The eqnihbrinm curve in Fig. 13-35 represents data at that pressure. AU heat leaks will be assumed to be zero. The feed composition is 40 mole percent of the more volatile component 1, and the feed rate is 0.126 (kg-mol)/s [1000 (lb-mol)/h] of saturated liquid (q = 1). The feed-stage location is fixed at stage 4 and the total number of stages at eight. 

Example 1 Calculation of FUG Method A large hutane-pentane splitter is to he shut down for repairs. Some of its feed will he diverted temporarily to an avadahle smaller column, which has only 11 trays plus a partial rehoiler. The feed enters on the middle tray. Past experience on similar feeds indicates that the 11 trays plus the rehoiler are roughly equivalent to 10 equilih-rium stages and that the column has a maximum top-vapor capacity of 1.75 times the feed rate on a mole basis. The column will operate at a condenser pressure of 827.4 kPa (120 psia). The feed will he at its huhhle point (q = 1.0) at the feed-tray conditions and has the following composition on the basis of... 

Sidestream compressors are used in refrigeration processes where, for economy, the refrigerant is flashed off at different pressure levels. Ideally, separate compressors could be used to successively compress the gas back up to the condensing pressure level. The pressure ratio for each stage is low enough to enable this to be done with only one or two impellers in each section. Because of this, compressors can be made with all sections in one casing so that mixing of the streams takes place intern.ll I v. 

Gurgel and Grenier s results showed the bed conductivity to increase from 0.14 to 0.17 W/mK as the pressure was raised from 4 mbar (evaporating pressure) to 110 mbar (condensing pressure). The principle reason stated for this small variation is the reduction in the gas conductivity with decreasing pressure (Knudsen effect) in the macropores. The solid grain conductivity varied linearly from 0.61 to 0.65 W/mK as the methanol concentration varied from 0 to 31%. 

For the steam plant, the condenser pressure, the turbine and pump efficiencies are also specified there is also a single phase of water/steam heating, with no reheating. The feed pump work term for the relatively low pressure steam cycle is ignored, so that /ij, = /i. For the HRSG two temperature differences are prescribed ... 

Calculate condensing pressure, with to as bubble point (if subcooling exists, and to is below bubble point, use bubble point temperature for pressure calculation only). 

Ammonia vapors from a stripping operation are to be condensed. Select the condenser pressure, which sets the top of stripper pressure, and design a condenser. Water at 90°F is to be used. 

Tube Side Condensation Pressure Drop Kem recommends the following conservative relation ... 

In general, the number of boosters determines the operational flexibility of the unit with respect to the refrigeration load. A single booster unit operates continuously, regardless of load. A two booster unit can operate at 50% load by shutting off one unit at lower load levels it uses a pressure controller on the steam actuated by the condenser pressure. Because jets are not usually very flexible with respect to steam consumption and vacuum, load control may be in increments as compared to continuous variation. If a 100-ton unit is expected to operate an appreciable portion of the time at 25% of load, it may prove economical to install a four-booster unit and to operate only one for this period. Auxiliary ejectors remove uncondensed water vapor and air from the main condenser. 

Evaporator and condenser pressure, composition of compressed vapor, size of compressor, and weight of charge required. 

Step 3a. The calculations to determine condenser pressure are handled somewhat similarly to those of the evaporator except that they must be divided into two parts. 

The objective now is to assume a condenser pressure that will yield an average condenser temperature of 100°F between the top and bottom sections. Knowing the inlet vapor composition and pressure, the temperature may be determined and the system in the condenser completely defined. SX,i must = 1.000 when the correct temperature is assumed. The correct pressure assumption will not be known until the upper and lower temperatures have been averaged. 

Enthalpy,Btu/lb., h Temperature at 3 i Condensing Pi is Evaporating Pressure P2 is Condensing Pressure... 

To obtain the most efficient use of an economizer, studies are needed to balance economizer pressure, condenser pressure and temperature, evaporator surface area, and compression horsepower. These are all interrelated, and horsepower can be saved at the expense of surface areas, and vice-versa. 

Screw compressors have no clearance volume, and may work at high compression ratios without loss of volumetric efficiency . In all screw compressors, the gas volume will have been reduced to a pre-set proportion of the inlet volume by the time the outlet port is uncovered, and this is termed the built-in pressure ratio. At this point, the gas within the screws is opened to condenser pressure and gas will flow inwards or outwards through the discharge port if the pressures are not equal. 

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