Use Manipulate. Here is a partial example :
Manipulate[
Module[
{tan, ll, lr, ur, r, pt, t},
tan = Tan[30 Degree];
ll = {-w/2, -d/2};
lr = {w/2, -d/2};
ur = {w/2, d/2};
r = Sqrt[(20 + d/2)^2 + (w/2)^2];
pt = {x, y} /. FindRoot[
{tan == (x - w/2)/(y - d/2),
x^2 + y^2 == r^2}, {x, 12}, {y, 20}];
t = ArcTan @@ Reverse[pt];
Graphics[
{Thin, Circle[{0, 0}, 7],
Line[{{-7, -4}, {-7, 4}}],
Circle[{0, 0}, r, -90 Degree + {-t, t}],
{AbsoluteDashing[{6, 6, 72, 6}],
Line[{{0, 9}, {0, -22 - d/2}}]},
{White, EdgeForm[Thick],
Rectangle[ll, ur]},
{AbsoluteDashing[{8, 4}],
Table[Line[lr + # & /@
{{0, 0}, {0, -x}, {x*tan, -x}, {0, 0}}],
{x, 10, 20, 5}],
Line[{ll, -pt}]},
Text["High- \ntemperature zone",
{-4.5, 4}, {-1, 0}],
Text["B=0.5774*A\nC=1.1547*A", {8, 3.5}, {-1, 0}],
Text["SI units: 1 in = 25.4 mm; 1 ft = 0.31 m",
{-w/2 - 15*tan, -23 - d/2}, {-1, 0}],
Text["B\n5ft 9-5/16in", {w/2 + 5*tan, -10 - d/2}],
Text["B\n8ft 7-7/8in", {w/2 + 7.5*tan, -15 - d/2}],
Text["B\n11ft 6-11/16in", {w/2 + 10*tan, -20 - d/2}],
Text["Intermediate-temperature\nzone",
{-w/4 - 7.5*tan, -r*Cos[t]}],
Text["Airflow", {-5*Tan[t], -9.5 - d/2}],
Arrow[{{-5*Tan[t], -10 - d/2},
{-5*Tan[t], -13 - d/2}}]}]],
{{w, 4., "Rectangle\nWidth (ft)"}, 1, 10, .01,
Appearance -> "Labeled"},
{{d, 2., "Rectangle\nDepth (ft)"}, 1, 10, .01,
Appearance -> "Labeled"}]
Bob Hanlon