Encyclopedia of Environmental Science and Engineering, Volume I and II

PCBs AND ASSOCIATED AROMATICS 911

where

X t  mass desorbed at time, t

X   mass desorbed at infinite time.

a

1 

X

X

X

X

∞

∞

s

s

where ( X s ) is the total solute in the system.

( X s ) and ( X  ) are experimental quantities and therefore

( a ) can be obtained directly.

A function of time and diffusivity can be defined as:

bD

t

d

⋅ 2.

(16)

Knowing ( a ), a series of curves can be drawn relating ( X t / X  )

with ( b 1/2 ).

The experimental value of ( a ) is matched to the corre-

sponding theoretical curve of ( X t / X  ) vs. b 1/2 and since b  D

· t / d^2 a value can be obtained for the diffusion constant, D.

Measurements of the PCB content of impregnated dielec-

tric papers show that a typical paper will contain 58 3% by

weight of an askarel such as Inerteen 70/30. Then, to calculate

the total quantity of askarel absorbed in the paper one needs

to approximate the amount of paper in a typical design.

For a 500 KVA transformer, the approximate quantities

of the different types of materials are as follows:

Paper 
 27 lb

Core steel 
 947 in^2

Oil 
 85 gal.

Thus, the amount of askarel impregnating the paper

900

800

700

600

500

400

300

200

100

0 50 100 150 200 250

Time After Conventional Retrofill (days)

90

PCB Concentration(ppm PCB)

Approximately

6 ppm/d slope

Approximately

1 ppm/d slope

the retrofilled transformer

returns to PCB

status in about 90 days

Conventional Freon Retrofill Technology

FIGURE 41 Conventional retrofill processes allow continued PCB leaching from the core/coil assembly.

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