Page 330 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:48 PM Page 289

Chapter

11

Refrigeration Systems

Process refrigeration is used at many different tempera- The most common light hydrocarbon refrigerant cooling
ture levels to condense or cool gases, vapors, or liquids. temperature ranges are (evaporation temperature):
Refrigeration is necessary when the process requires cooling
to a temperature not reliably available from the usual water Methane 200° to 300°F
service or other coolant source, including Joule-Thompson, Ethylene and ethane 75° to 175°F
or polytropic expansion of natural gas or process system Propylene and propane 40° to 50°F
vapors.
In general, auxiliary refrigeration is used for temperature Mehra 8—11 has developed a valuable series of working
requirements from 80—85°F to as near absolute zero as the charts for the common industrial refrigerants along with
process demands. The usual petrochemical and chemical application examples for ethylene, propylene, ethane, and
range does not go much below 200°F. This section does propane.
not include low-temperature air separation for oxygen,
nitrogen, argon, etc., or the separation of process gases at Terminology
liquid air temperatures.
A valuable technical presentation of refrigeration is given Ton of refrigeration: The heat equivalent to melting 2,000 lb
in the ASHRAE handbook. 2 (one ton) of ice in 24 hours. One ton equals 12,000 Btu/hr
or 200 Btu/min. To be comparative, refrigeration equip-
Types of Refrigeration Systems
ment must have the refrigerant level (or evaporation tem-
The three most used systems are as follows: perature) specified.
Approx.
Temperature Selection of a Refrigeration System for
System Coolant Range, °F Refrigerant a Given Temperature Level and Heat Load
Steam jet† 35 to 70°F Water
In general, the simplest system is selected for any specific
Absorption
Water-Lithium 40 to 70°F Lithium Bromide refrigeration requirement, because it should be the least
Bromide Solution (water*) expensive from first cost and operating cost viewpoints. Fac-
Ammonia 40 to 30°F Ammonia*-water tors that are weighed in arriving at the process-directed
Mechanical refrigeration system include the following, in addition to the
compression purchase and operating costs:
(Reciprocating
centrifugal or
1. Temperature Level of Evaporating Refrigerant
rotary screw) 200° to 40°F Ammonia,
halogenated hydrocar-
Refrigerant temperatures greater than 32°F suggest the
bons, propane
steam jet or lithium bromide absorption system. Between
ethylene, and others
30°F and 40°F, the ammonia-water absorption or a
Plus:
mechanical compression system is indicated. At less than
Cryogenics 150° to 200°F Liquefaction of gases,
40°F, a mechanical compression is used, except in special
and power/temperature
recovery from natural desiccant situations. The economics of temperature level
gas selection will depend on utility (steam, power) costs at the
point of installation and the type of pay-out required,
* refrigerant
†Vacuum system, discussed in detail, Chapter 6, Vol. 1, 3 Ed., this text because in some tonnage ranges, the various systems are
rd
series. competitive based on first costs.
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