30079_63b.pdf

Fig. 63.26 End correction for regenerator heat transfer calculation using symmetrical

cycle theory 2 7 (courtesy Plenum Press):

A = 4HS(T + r,) = reducedlengt h

U C 1 C + ^W 1 W

12H 0 (7" C -f T w )

5 ^

— = reduced period

TT =

Cp 5 C/

1 [1

0.1dl

U ° = 4U + — J

where T w , T 0 = switching times of warm and cold streams, respectively, hr

S = regenerator surface area, m 2

U 0 = overall heat transfer coefficient uncorrected for hysteresis, kcal/m 2 • hr • 0 C

U = overall heat transfer coefficient

C w , C 0 = heat capacity of warm and cold stream, respectively, kcal/hr • 0 C

c = specific heat of packing, kcal/kg • 0 C

of = particle diameter, m

p s = density of solid, kg/m 3

phases are well distributed in the flow stream approaching the distribution point. Streams that cool

during passage through an exchanger are likely to be modestly self-compensating in that the viscosity

of a cold gas is lower than that of a warmer gas. Thus a stream that is relatively high in temperature

(as would be the case if that passage received more than its share of fluid) will have a greater flow

resistance than a cooler system, so flow will be reduced. The opposite effect occurs for streams being

warmed, so that these streams must be carefully balanced at the exchanger entrance.

63.4 INSULATIONSYSTEMS

Successful cryogenic processing requires high-efficiency insulation. Sometimes this is a processing

necessity, as in the Joule-Thomson liquefier, and sometimes it is primarily an economic requirement,

as in the storage and transportation of cryogens. For large-scale cryogenic processes, especially those

operating at liquid nitrogen temperatures and above, thick blankets of fiber or powder insulation, air

Fig. 63.27 A T limitation for contaminant cleanup in a regenerator.

or N 2 filled, have generally been used. For lower temperatures and for smaller units, vacuum insulation

has been enhanced by adding one or many radiation shields, sometimes in the form of fibers or

pellets, but often as reflective metal barriers. The use of many radiation barriers in the form of metal-

coated plastic sheets wrapped around the processing vessel within the vacuum space has been used

for most applications at temperatures approaching absolute zero.

63.4.1 Vacuum Insulation

Heat transfer occurs by convection, conduction, and radiation mechanisms. A vacuum space ideally

eliminates convective and conductive heat transfer but does not interrupt radiative transfer. Thus heat

transfer through a vacuum space can be calculated from the classic equation:

q = 0-AF 12 (T 4 , - T 4 )

(63.10)

where q = rate of heat transfer, J/sec

cr - Stefan-Boltzmann constant, 5.73 X 10~ 8 J/sec • m 2 • K

F 1 2 = combined emissivity and geometry factor

T 19 T 2 = temperature (K) of radiating and receiving body, respectively

In this formulation of the Stefan-Boltzmann equation it is assumed that both radiator and receiver

are gray bodies, that is, emissivity e and absorptivity are equal and independent of temperature. It is

also assumed that the radiating body loses energy to a totally uniform surroundings and receives

energy from this same environment.

The form of the Stefan-Boltzmann equation shows that the rate of radiant energy transfer is

controlled by the temperature of the hot surface. If the vacuum space is interrupted by a shielding

surface, the temperature of that surface will become T 5 , so that

q/A = F 1 , (T 4 - T 4 } = F s2 (T 4 S - T 4 )

(63.11)

Since qlA will be the same through each region of this vacuum space, and assuming F ls = F s2 =

< 63 - 12 )

T 4 4- T4

T 3 = ^p

For two infinite parallel plates or concentric cylinders or spheres with diffuse radiation transfer

from one to the other,

< 63 - 13 )

F 12 = I /- + T(-~ I }

/ C 1

A 2 Ve 2

IfA 1 is a small body in a large enclosure, F 1 2 = C 1 -If radiator or receiver has an emissivity that

varies with temperature, or if radiation is spectral, F 1 2 must be found from a detailed statistical

analysis of the various possible radiant beams. 3 0

Table 63.5 lists emissivities for several surfaces of low emissivity that are useful in vacuum

insulation. 3 1

It is often desirable to control the temperature of the shield. This may be done by arranging for

heat transfer between escaping vapors and the shield, or by using a double-walled shield in which is

contained a boiling cryogen.

It is possible to use more than one radiation shield in an evacuated space. The temperature of

intermediate streams can be determined as noted above, although the algebra becomes clumsy. How-

ever, mechanical complexities usually outweigh the insulating advantages.

63.4.2 Superinsulation

The advantages of radiation shields in an evacuated space have been extended to their logical con-

clusion in superinsulation, where a very large number of radiation shields are used. A thin, low

emissivity material is wrapped around the cold surface so that the radiation train is interrupted often.

The material is usually aluminum foil or aluminum-coated Mylar. Since the conductivity path must

also be blocked, the individual layers must be separated. This may be done with glass fibers, perlite

bits, or even with wrinkles in the insulating material; 25 surfaces/in. of thickness is quite common.

Usually the wrapping does not fill in the insulating space. Table 63.6 gives properties of some

available superinsulations.

Superinsulation has enormous advantages over other available insulation systems as can be seen

from Table 63.6. In this table insulation performance is given in terms of effective thermal

conductivity

*.-$

where k e = effective, or apparent, thermal conductivity

L = thickness of the insulation

T = T 1 - T 2

This insulating advantage translates into thin insulation space for a given rate of heat transfer, and

into low weight. Hence designers have favored the use of superinsulation for most cryogen containers

Table 63.5 Emissivities of Materials Used for Cryogenic

Radiation Shields

Emissivity at

Material

300 K

77.8 K

4.33 K

Aluminum plate

0.08

0.03

Aluminum foil (bright

finish)

0.03

0.018

0.011

Copper (commercial polish)

0.03

0.019

0.015

Monel

0.17

0.11

304 stainless steel

0.15

0.061

Silver

0.022

Titanium

0.1

Table 63.6 Properties of Various Multilayer Insulations (Warm Wall at 300 K)

Sample

Shields

Cold

Conductivity

Thickness

per

Density

Wall

(/tW/cm •

Material 3

(g/cm 3 )

7"(K)

(cm)

Centimeter

3.7

0.12

0.7

3.7

0.12

0.5

2.5

0.09

2.3

1.5

0.76

5.2

4.5

0.03

3.9

2.2

0.03

3.0

3.2

0.045

0.85

1.3

0.09

1.8

a 1. Al foil with glass fiber mat separator.

2. Al foil with nylon net spacer.

3. Al foil with glass fabric spacer.

4. Al foil with glass fiber, unbonded spacer.

5. Aluminized Mylar, no spacer.

built for transport, especially where liquid H 2 or liquid He is involved, and for extraterrestrial space

applications.

On the other hand, superinsulation must usually be installed in the field, and hence uniformity is

difficult to achieve. Connections, tees in lines, and bends are especially difficult to wrap effectively.

Present practice requires that layers of insulation be overlapped at a joint to ensure continuous

coverage. Some configurations are shown in Fig. 63.28. Also, it has been found that the effectiveness

Fig. 63.28 Superinsulation coverage at joints and nozzles: (a) Lapped joint at corner. Also usa-

ble for nozzle or for pipe bend, (b) Rolled joint used at surface discontinuity, diameter change,

or for jointure of insulation sections, (c) Multilayer insulation at a nozzle.

of superinsulation drops rapidly as the pressure increases. Pressures must be kept below 10 3 torr;

evacuation is slow; a getter is required in the evacuated space; and all joints must be absolutely

vacuum tight. Thus the total system cost is high.

63.4.3 Insulating Powders and Fibers

Fibers and powders have been used as insulating materials since the earliest of insulation needs. They

retain the enormous advantage of ease of installation, especially when used in air, and low cost. Table

63.7 lists common insulating powders and fibers along with values of effective thermal conductivity. 3 2

Since the actual thermal conductivity is a function of temperature, these values may only be used

for the temperature ranges shown.

For cryogenic processes of modest size and at temperatures down to liquid nitrogen temperature,

it is usual practice to immerse the process equipment to be insulated in a cold box, a box filled with

powder or fiber insulation. Insulation thickness must be large, and the coldest units must have the

thickest insulation layer. This determines the placing of the process units within the cold box. Such

a cold box may be assembled in the plant and shipped as a unit, or it can be constructed in the field.

It is important to prevent moisture from migrating into the insulation and forming ice layers. Hence

the box is usually operated at a positive gauge pressure using a dry gas, such as dry nitrogen. If rock

wool or another such fiber is used, repairs can be made by tunneling through the insulation to the

process unit. If an equivalent insulating powder, perlite, is used, the insulation will flow from the

box through an opening into a retaining bag. After repairs are made, the insulation may be poured

back into the box.

Polymer foams have also been used as cryogenic insulators. Foam-in-placed insulations have

proven difficult to use because as the foaming takes place cavities are likely to develop behind process

units. However, where the shape is simple and assembly can be done in the shop, good insulating

characteristics can be obtained.

In some applications powders or fibers have been used in evacuated spaces. The absence of gas

in the insulation pores reduces heat transfer by convection and conduction. Figure 63.29 shows the

effect on a powder insulation of reducing pressure in the insulating space. Note that the pressures

may be somewhat greater than that needed in a superinsulation system.

63.5 MATERIALS FOR CRYOGENIC SERVICE

Materials to be used in cryogenic service must operate satisfactorily in both ambient and cryogenic

temperatures. The repeated temperature cycling that comes from starting up, operating, and shutting

down this equipment is particularly destructive because of expansion and contraction that occur at

every boundary and jointure.

63.5.1 Materials of Construction

Metals

Many of the normal metals used in equipment construction become brittle at low temperatures and

fail with none of the prewarning of strain and deformation usually expected. Sometimes failure occurs

at very low stress levels. The mechanism of brittle failure is still a topic for research. However, those

metals that exhibit face-centered-cubic crystal lattice structure do not usually become brittle. The

austenitic stainless steels, aluminum, copper, and nickel alloys are materials of this type. On the other

hand, materials with body-centered-cubic crystal lattice forms or close-packed-hexagonal lattices are

usually subject to a brittle transformation as the temperature is lowered. Such materials include the

low-carbon steels and certain titanium and magensium alloys. Figure 63.30 shows these crystal forms

and gives examples of notch toughness at room temperature and at liquid N 2 temperature for several

example metals. In general carbon acts to raise the brittle transition temperature, and nickel lowers

Table 63.7 Effective Thermal Conductivity of Various Common

Cryogenic Insulating Materials (300 to 76 K)

Gas Pressure

(g/cm 2 )

(W/cm • K)

Material

(mm Hg)

20.8 X 10~ 6

<1(T 4

0.096

Silica aerogel (25OA)

N 2 at 628

195.5 X IQ- 6

0.096

18.2 X 10~ 6

<10~ 5

0.096

Perlite (+30 mesh)

N 2 at 628

X 10~ 6

0.096

334

X 10" 6

Polystyrene foam

Air, 1 atm

0.046

259

X 10~ 6

Polyurethane foam

Air, 1 atm

0.128

328

X 10~ 6

Foamglas

Air, 1 atm

0.144

346

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