Page 255 - Chemical Process Equipment - Selection and Design

224 HEAT TRANSFER AND HEAT EXCHANGERS

TABLE 8.22. Specifications of Thicknesses of Pipe Insulation expanding to a low pressure at which the solution becomes cold
for Moderate and High Temperatures, in Single and flashed.
or Double Strength as May Be Needed 3. Steam jet action in which water is chilled by evaporation in a
(a) Insulation of 85% Magnesia or Equivalent up to 600°F chamber maintained at low pressure by means of a steam jet
ejector. A temperature is 55°F or so is commonly attained, but
Pipe Size Standard Thick Double Standard down to 40°F may be feasible. Brines also can be chilled by
(in.) (in.) Thick (in.) evaporation to below 32°F.
1-112 or less 718 1-1 511 6 The unit of refrigeration is the ton which is approximately the
2 1-1/32 2-5/32
2- 112 1-1/32 2-5/32 removal of the heat of fusion of a ton of ice in one day, or
3 1-1/32 2-5/32 288,000 Btu/day, 12,000 Btu/hr, 200 Btu/min. The reciprocal of the
4 1-118 2- 1 /4 efficiency, called the coefficient of performance (COP) is the term
5 1-118 2-5/16 employed to characterize the performances of refrigerating
6 1-118 2-5/16 processes:
8 1-114 2-112
10 1-114 2-112 COP
12-33 1-112 3
- energy absorbed by the refrigerant at the low temperature
-
(b) Molded Diatomaceous Earth Base Insulation, to 1900°F. energy input to the refrigerant
Single or Double Thickness as Needed
A commonly used unit of COP is (tons of refrigeration)/
Pipe Size
(in.) Thickness (in.) (horsepower input). Some of the refrigerants suited to particular
temperature ranges are listed in Tables 1.10, 8.23, and 8.24.
~~
1-112 2 2
2 1-114 2- 1 /8 COMPRESSION REFRIGERATION
2-112 1-5/16 1-13/16
3 1-9/16 2-1/16 A basic circuit of vapor compression refrigeration is in Figure
4 1-9/16 2-1/16 8.24(a). After compression, vapor is condensed with water cooling
5 1-112 2 and then expanded to a low temperature through a valve in which
6 1-112 2-1/16 the process is essentially at constant enthalpy. In large scale
8 1-112 2
10 1-9/16 2-118 installations or when the objective is liquefaction of the
12 1-9/16 2-118 “permanent” gases, expansion to lower temperatures is achieved in
14-33 1-112 2 turboexpanders from which power is recovered; such expansions are
approximately isentropic. The process with expansion through a
(c) Combination Insulation, inner Layer of Diatomaceous Earth valve is represented on a pressure-enthalpy diagram in Figure
Base, and Outer of 85% Magnesia or Equivalent, for High Level
Insulation to 19OOOF 8.24(b).
A process employing a circulating brine is illustrated in Figure
Inner Layer Outer Layer 8.24(c); it is employed when cooling is required at several points
distant from the refrigeration unit because of the lower cost of
Pipe Size Thickness Nominal Pipe Thickness circulation of the brine, and when leakage between refrigerant and
(in.) (in.) Size (in.) (in.) process fluids is harmful.
1-112 or less 2 no outer layer - For an overall compression ratio much in excess of four or so,
2 1-114 4- 1 /2 1-112 multistage compression is more economic. Figure 8.24(d) shows two
2- 112 1-5/16 5 1-112 stages with intercooling to improve the capacity and efficiency of the
3 1-9/16 6 1-112 process.
4 1-9/16 7 1-112 Many variations of the simple circuits are employed in the
5 1-112 8 2 interest of better performance. The case of Example 8.17 has two
6 1-112 9 2 stages of compression but also two stages of expansion, a scheme
8 1-112 11 2 due originally to Windhausen (in 1901). The flashed vapor of the
10 1-9/16 14 2
12 1-9/16 16 2 intermediate stage is recycled to the high pressure compressor. The
14-33 1-112 17-36 2 numerical example shows that an improved COP is attained with
the modified circuit. In the circuit with a centrifugal compressor of
Data of an engineering contractor. Figure 8.25, the functions of several intermediate expansion valves
and flash drums are combined in a single vessel with appropriate
8.13. REFRIGERATION internals called an economizer. This refrigeration unit is used with a
fractionating unit for recovering ethane and ethylene from a
Process temperatures below those attainable with cooling water or mixture with lighter substances.
air are attained through refrigerants whose low temperatures are Low temperatures with the possibility of still using water for
obtained by several means: final condensation are attained with cascade systems employing
coupled circuits with different refrigerants. Refrigerants with higher
1. Vapor compression refrigeration in which a vapor is compressed, vapor pressures effect condensation of those with lower vapor
then condensed with water or air, and expanded to a low pressures. Figure 8.26 employs ethylene and propylene in a cascade
pressure and correspondingly low temperature through a valve for servicing the condenser of a demethanizer which must be cooled
or an engine with power takeoff. to -145°F. A similar process is represented on a flowsketch in the
2. Absorption refrigeration in which condensation is effected by book of Ludwig (1983, Vol. 1, p. 249). A three element cascade
absorption of vapor in a liquid at high pressure, then cooling and with methane, ethylene and propylene refrigerants is calculated by