Multipass condenser

When the condensation process is not exactly isothermal but the temperature change is small such as where there is a significant change in pressure, or where a narrow boiling range multicomponent mixture is being condensed the logarithmic temperature difference can still be used but the temperature correction factor will be needed for multipass condensers. The appropriate terminal temperatures should be used in the calculation. 

The APEX system (Element Scientific Inc., Omaha) as an improved Aridus nebulizer was introduced for ICP-MS in 2004 for more effective solution introduction at flow rates from 20-400 p,lmin-1.88 In this solution introduction system (see Figure 5.15), a microflow PFA nebulizer is combined with a heated cyclonic spray chamber followed by cooling of the nebulized aerosol in a condenser loop and using a multipass condenser cooled by a Peltier element. The APEX solution introduction system results in a significant increase of sensitivity (by a factor of ten in comparison to a standard nebulizer spray chamber arrangement) and a decreasing polyatomic formation rate.89... 

Figure 5.15 Experimental arrangement of microconcentric PFA nebulizer with heated cyclonic spray chamber and Peltier cooled multipass condenser APEX. (Reproduced by permission of Element Scientific Inc., Omaha).

Horizontal in-tube condensers. Typical applications of this condensation mode are air condensers and horizontal condenser-reboilers. These condensers are often made in single-pass or U-tube arrangements, but multipass arrangements are also used. With multipass arrangements, successive passes should be below one another (291). Multipass condensation may be troublesome with multicomponent mixtures as described in the preceding section. 

Air-cooled condensers are normally sloped toward the outlet to provide for better condensate drainage in the tubes. It is extremely important that air-cooled condensers be installed so that the outlet of the tubes in a pass is at least at the same elevation as the inlet to avoid condensate flooding preferably it should be a few inches lower. This is even more important when the freezing point of the condensate is higher than the lowest possible air temperature. Multipass condensers may be sloped in more than one direction at a fairly nominal cost. In some instances, the condensate outlet header is provided with a sump" by making the outlet header deeper. This prevents the bottom row of tubes from being flooded by condensate and avoids problems associated with Improper venting. 

Single-pass air-cooled condensers must usually be somewhat oversized resulting in subcooling of condensate. This results because the air temperature at each row of tubes is different consequently, each row condenses a different amount of vapor. In order to insure that all vapor entering the top row (highest air temperature) is condensed, it may be necessary to provide more area than required for all other rows. Multipass condensers avoid this problem, but pressure drop considerations do not always permit multipass designs. 

Microflow nebulizer with heated spray chamber and Peltier-cooled condenser An example of this design is the Apex inlet system from ESI. This unit includes a microflow nebulizer, heated cyclonic spray chamber (up to 140°C), and a Peltier multipass condenser/cooler (down to -5°C). A number of different spray chamber and nebulizer options and materials are available, depending on the application requirements. Also, the system is available with Teflon or Nafion microporous membrane desolvation, depending on the types of samples being analyzed. Figure 17.12 shows a schematic of the Apex sample inlet system with the ctoss-Aow nebulizer. 

Microflow nebulizer with heated spray chamber and Peltier-cooled condenser An example of this design is the Apex inlet system from ESI. This unit includes a microflow nebulizer, heated cyclonic spray chamber (up to 140°C), and a Peltier multipass condenser/cooler (down to -5°C). 

Condensers The vapor from the last effect of an evaporator is usually removed by a condenser. Surface condensers are employed when mixing of condensate with condenser coohng water is not desired. They are for the most part shell-and-tube condensers with vapor on the shell side and a multipass flow of cooling water on the... 

If one of the fluids is at constant temperature, as in a condenser, no difference exists among countercurrent flow, parallel flow, or multipass flow, and Eq. (11.15) applies to all of them. In countercurrent flow, AT2, the warm-end approach, may be less than ATj, the cold-end approach. In this case, for convenience and to eliminate negative numbers and logarithms, the subscripts in Eq. (11.15) may be interchanged. The LMTD is not always the correct mean temperature difference to use. It should not be used when U changes appreciably or when AT is not a linear function of q. As an example, consider an exchanger used to cool and condense a superheated vapor, with the temperature diagram shown in Fig. 11.6. 

To obtain larger velocities, higher heat-transfer coefficients, and shorter tubes, the multipass principle used in heat exchangers may also be used for the coolant in a condenser. An example of a two-pass condenser is shown in Fig. 15.8. 

Intermediate pressure. Noncorrosive vapor. Severe coolant or moderate vapor fouling. Can handle freezing condensate. Tube vibration problems. Multipassing and variable baffle spacing can be used for total condensation. Good venting control. 

The test results show the feasibility of controlling product output by varying the power input to a fixed multistage diffusion design operating at constant disk speed. Further analysis has shown that by design changes in the condenser plates —i.e., multipass within each plate—for a fixed condenser area, performance ratio can be traded for capacity. 

When the column feed passes through a heater (e.g., a refinery fractionator or vacuum tower), any water lying at low points in the coils must be blown out prior to startup. In multipass coils, water must be separately blown out of each pass block valves are sometimes installed on each pass to permit this (7). If blowing into the tower, it must be performed when the tower can still tolerate water. The coils must be kept hot and/or purged from then on to prevent condensation. One pressure surge incident (7) occurred when water accumulated in one heater pass entered a refinery vacuum tower which was under full vacuum and circulating 280°F oil. 

Condenser design determines the extent of Rayleigh fractionation. Vertical upflow condensers ("vent or "inerts condensers (e.g.. Fig. 15.14c,e see Sec. 15.11) are intended to operate as Rayleigh fractionators. Other condensers which exhibit this fractionation behavior are unbaffled vertical downflow shell-side condensers (if baffles are included, they would interrupt the free fall of liquid), emd multipass horizontal in-tube condensers where liquid is segregated at the end of each pass. One case was reported (381) where this behavior caused severe inerts blanketing in an unbafiled, downflow shell-side condenser. 

For shdl and tube heat exchange Numerous related topics including evaporation Section 4.1, distillation. Section 4.2, crystallization Section 4.6, freeze concentration Section 4.3, melt crystallization. Section 4.4, PFTR reactors Sections 6.5-6.12. Approach temperature 5 to 8°C use 0.4 THTU/pass design so that the total pressure drop on the liquid side is about 70 kPa. Allow 4 velocity heads pressure drop for each pass in a multipass system. Put inside the tubes the more corrosive, higher pressure, dirtier, hotter and more viscous fluids. Recommended liquid velocities 1 to 1.5 m/s with maximum velocity increasing as more exotic alloys used. Use triangular pitch for all fixed tube sheet and for steam condensing on the shell side. Try U = 0.5 kW/m °C for water/liquid U = 0.3 kW/m °C for hydrocarbon/hydrocarbon U = 0.03 kW/m °C for gas/ liquid and 0.03 kW/m °C for gas/gas. 

---