2-d Insulation Analysis of Clamp / Vacuum Vessel Insulation crushing

3-2-04

Problem Description

• What is the thermal impact of the “crushing” of insulation by the vessel ports.

• What should be the method to determine how much thermal gradient is allowable in the clamp.

• What steps can be taken to ensure low gradients within the clamp.

Base model and properties

•No convection at lower surface.

•Conductivities:

Kss = 16.2 W /m-K

Kcu = 385 W /m-K

Kmicrotherm = 0.03 W/m-K

• Gap exists between clamp and copper since the two do not exist in the same plane. (i.e. there is no copper under the clamp, only insulation.

• Clamp does not touch tee near insulation due to expected pour conductivity through Belleville washers (conservative approx.)

T = 350 C or 623 K (bake out)

T = 80 K from Cu chill plates (applied on a small strip near tubing connection.)

Insulation thickness = 1.5”

Color Scheme

Stainless steel = cyan

Copper chill plate = red

Microtherm© insulation = purpleNo convection

thickness = 1.5 in.(15 K rise across clamp top)

Temperature Scale (K)

Temperature contour plots for various insulation thicknesses

thickness = 0.5 in.(35 K rise across clamp top)

thickness = 0.25 in.(50 K rise across clamp top)

Temperatures greater than 300 K are gray in color

Effect of convection boundary condition from cryostat? (there is none)

Heat Convection of 15 W/m2-K, T = 80 K;

value from Goranson.

No ConvectionConvective BC applied along line

Slanted edge used to approximate port interference with clamp. (t = 1.5 in. near bottom, t = 0.5 in. near top.)

Approx 25 K rise across clamp for slanted

geometry

Example heat calculation: Heat through upper clamp in x direction: Q = 2.25 W; A = 0.75 in2, l = 3.4255 in, ΔT ≈ 25 K

Variation of insulation curves as a function of applied surface temperature

0

20

40

60

80

100

120

140

160

180

200

0 0.25 0.5 0.75 1 1.25 1.5

Insulation Thickness (in)

Te

mp

era

ture

(K

)

T = 350 C

T = 175 C

T = 90 C

T = 20 C Temperatures measured at edge of clamp (no copper)

Temperatures applied on front face of model

As the ports extend away from the vessel the temperature on the insulation face seen by the clamps will get cooler.

What if copper sheet is placed on the outer clamp surface in locations where insulation is “crushed”

1/32” Cu is placed on outer surface of clamp

Cu is linked to winding copper chill plates (lower portions of clamp are removed to provide a path for connection).

In reality, this connection would be fairly easy to make as a strip of copper could span from the top portion of the clamp to the winding chill plate.

Color Scheme

Stainless steel = cyan

Copper chill plate = red

Microtherm© insulation = purple

Copper over clamp = dark blueInsulation thickness = 1.5”

Copper sheet vs. plain stainless steel clamp Temperature Gradient Across Upper Clamp

0

10

20

30

40

50

60

0 0.25 0.5 0.75 1 1.25 1.5 1.75

Insulation thickness (inches)

Te

mp

era

ture

gra

die

nt

(K)

Without copper (default)

With copper

Temperature profile with copper on outer clamp surface (Insulation thickness = 0.25“)

T = 7.2 oC across clamp

Summary

• Lower Convective boundary not significant.• As Insulation degreases, gradient in upper clamp increases (i.e. 30 K

rise for 0.5 in. of insulation).• In cases where the insulation is “crushed” by the ports, copper can be

added to the outer surface of the clamp to alleviate any thermal gradient.

• Heat going into upper clamp is only (1- 4.5) Watts depending on the insulation thickness.

• Model is very conservative:– Clamp not connected to lower portion of tee (no conduction path) – Exterior insulation temperature is held at 350 C (this will decrease along

the length of the port. The further away from the vessel, the cooler it will be.

• Argument for success? if mod coil can experience a 40 oC rise in temperature, the clamp should be able to withstand the same gradient during bake out.