Page 419 - Mechanical Engineers' Handbook (Volume 4)

408 Cooling Electronic Equipment

(f) V DWH (volume)
(g) A aV,m 2 (total surface)
5. Heat balance
(a) Assume average fluid specific heat, c , J/kg C.
p
(b)
t t t q/Wc , C
2
1
p
(c) t t
t, C
1
2
1
(d) t av ⁄2(t t )
1
2
(e) Check assumed value of c . Make another assumption if necessary.
p
6. Fluid properties at t av
(a) c (already known), J/kg C
p
(b) , N/sec m 2
(c) k, W/m C
(d) (Pr) 2/3 (c /k) 2/3
p
7. Heat-transfer coefficient
(a) G W/A , kg/sec m 2
c
(b) Re d G/
e
(c) Obtain j from curve (see item 3).
(d) Obtain ƒ from curve (see item 3).
(e) h jGc /(Pr) , W/m C
2/3
2
p
8. Fin efficiency
1/2
(a) m (2h/k ) ,m 1
(b) mb/2 is a computation
(c) (tanh mb/2)/mb/2
f
9. Overall passage efficiency
(a) Use Eq. (105).
10. Effectiveness
(a) Required (t t )/(T T )
1
s
2
1
(b) Form NTU from Eq. (104).
(c) Actual available 1 e NTU
(d) Compare required and actual and begin again with step 1 if the comparison
fails. If comparison is satisfactory go on to pressure loss calculation.
11. Pressure loss
3
(a) Establish v (specific volume), m /k.
1
3
(b) Establish v ,m /kg.
2
(c) v ⁄2(v v ), m /kg
1
3
2
m
1
(d) Form v /v .
m
1
(e) Form v /v .
2
1
(f) Obtain K and K e 41
c
(g) Determine
P, cm.