8.6. CONDENSERS FOR EVAPORATORS

8.6A. Introduction

In multiple-effect evaporators the vapors from the last effect are usually leaving under vacuum, that is, at less than atmospheric pressure. These vapors must be condensed and discharged as a liquid at atmospheric pressure. This is done by using cooling water to condense the vapors. The condenser can be a surface condenser, where the vapor to be condensed and the cooling liquid are separated by a metal wall, or a direct-contact condenser, where the vapor and cooling liquid are mixed directly.

8.6B. Surface Condensers

Surface condensers are employed where actual mixing of the condensate with condenser cooling water is not desired. In general, they are shell-and-tube condensers, with the vapor on the shell side and cooling water in multipass flow on the tube side. Noncondensable gases such as air, CO2, N2, or another gas are usually present in the vapor stream. They may have entered as dissolved gases in the liquid feed or occur because of decomposition in the solutions. These noncondensable gases may be vented from any well-cooled point in the condenser. If the vapor being condensed is below atmospheric pressure, the condensed liquid leaving the surface condenser can be removed by pumping and the noncondensable gases by using a vacuum pump. Surface condensers are much more expensive and use more cooling water, so they are usually not used in cases where a direct-contact condenser is suitable.

8.6C. Direct-Contact Condensers

In direct-contact condensers cooling water directly contacts and condenses the vapors. One of the most common types of direct-contact condenser is the countercurrent barometric condenser shown in Fig. 8.6-1. The vapor enters the condenser and is condensed by rising upward against a shower of cooling water droplets. The condenser is located on top of a long discharge tailpipe. The condenser is high enough above the discharge point in the tailpipe that the water column established in the pipe more than compensates for the difference in pressure between the low absolute pressure in the condenser and the atmosphere. The water can then discharge by gravity through a seal pot at the bottom. A height of about 10.4 m (34 ft) is used.

Figure 8.6-1. Schematic of barometric condenser.


The barometric condenser is inexpensive and economical of water consumption. It can maintain a vacuum corresponding to a saturated vapor temperature within about 2.8 K (5°F) of the water temperature leaving the condenser. For example, if the discharge water is at 316.5 K (110°F), the pressure corresponding to 316.5 + 2.8 or 319.3 K is 10.1 kPa (1.47 psia).

The water consumption can be estimated by a simple heat balance for a barometric condenser. If the vapor flow to the condenser is V kg/h at temperature TS and the water flow is W kg/h at an entering temperature of T1 and a leaving temperature of T2, the derivation is as follows:

Equation 8.6-1


where HS is the enthalpy from the steam tables of the vapor at TS K and the pressure in the vapor stream. Solving,

Equation 8.6-2


The noncondensable gases can be removed from the condenser by a vacuum pump, either a mechanical pump or a steam-jet ejector. In the ejector high-pressure steam enters a nozzle at high speed and entrains the noncondensable gases from the space under vacuum.

Another type of direct-contact condenser is the jet barometric condenser. High-velocity jets of water act as both a vapor condenser and an entrainer of the noncondensables out of the tail pipe. Jet condensers usually require more water than the more-common barometric condensers and are more difficult to throttle at low vapor rates.

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