Page 184 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

182 Chapter 6 Evaporators

In some cases, the evaporator type is selected based on material of construction, e.g. for a sulfuric
acid evaporator where the acid concentration can reach 50% uses graphite tubular heat exchangers and
nonmetallic separators and piping.
Typical materials of construction for common evaporator applications are shown in Table 6.3.

Table 6.3 Material of construction for different services.
Product Material of construction
Most dairy and food products 304/316 stainless steel
Most fruit juices 316 stainless steel
Sugar products Carbon steel/304/316
Foods containing high salt Titanium/Monel
(NaCl)
High alloy stainless steels
Duplex stainless steels
Caustic soda <40% Stress relieved carbon steel
Caustic soda high concentration Nickel
Hydrochloric acid Graphite/rubber lined carbon
steel

6.5 Evaporator accessories
The auxiliary system associated with an evaporator include:

a) Vapor condensation and the associated vacuum system
b) Condensate removal system to remove the condensate from each effect
The different common accessories are presented in Fig. 6.16.

6.5.1 Condensers
In surface condensers, the heat transfer surface (usual tubes) separates the condensing vapor and the
coolant. These are mostly shell and tube heat exchangers with vapor in the shell and cooling water
running in the tubes. A vacuum pump or steam ejector is required to remove air/noncondensable gases
from surface condensers operating below atmospheric pressure. Direct contact condensers allow
mixing of the coolant and vapor inside the condenser body. This is a cheaper option and commonly
employed with most aqueous systems. These may be classified as wet or dry, or barometric and low
level. A wet condenser removes noncondensed gases and cooling water by the same pump while
separate pumps are employed in a dry condenser. In practice, parallel flow condensers are almost
always wet condensers while counter-current condensers are always dry.