L9- Heat transfer

Question 1

Medium of heat transfer

Answer 1

Heat of vaporisation: Steam (or Dowtherm)

Cooling: Cooling water (or brine)

Question 2

What is a steam trap?

Answer 2

Device that lets condensate out, keeps steam in and allows for variations in condensate flow

Steam condenses in device and returned to boiler via condensate return system

Question 3

Types of steam traps

Answer 3

Thermostatic (temp difference)
Mechanical (density difference)
Thermodynamic (velocity difference)
Miscellaneous

Question 4

Factors affecting overall heat transfer coefficient between vessel to jacket

Answer 4

Film heat transfer coefficients, hw, at the inside vessel wall and hj at the outer surface of the vessel wall

Thermal resistances, sum(xi/ki), of vessel wall
(Negligible for metal-walled vessel, but significant for rubber/glass lined)

Larger vessel ignored due to large thickness-> no problem with unequal internal and external areas, ignoring fouling

Question 5

Effect of material on thermal conductivity

Answer 5

Compromise of U and cost:

Steel U=1040, cost=1
Al/Cu slight better U for more cost
Rubber and glass poor U

Question 6

Condensing steam film coefficient

Answer 6

High value (8000) can be used as not offering limiting resistance to heat transfer

Question 7

Film coefficient between jacket fluid and wall & how to improve?

Answer 7

Film coefficient between fluid in jacket and wall is small and determined by natural convection (as jacket velocity is small)

Improve by adding direction and velocity in jacket fluid

Question 8

Jacket choice

Answer 8

Cost: simple (no baffles)<agitation nozzles<spiral baffles<dimple<half-pipe

Heat transfer rate: baffles or half-pipe jacket give higher rates

Pressure: Jackets up to 10 bar; dimpled jacket up to 20 bar; half-pipe up to 70 bar

Question 9

Film coefficient at vessel wall (equation -derive terms)

Answer 9

[hwDv/k] = f[((D^2)Np/mu)^a][(Cpmu/k)^1/3]

hw = inside wall film heat coefficient
Dv = vessel diamater
k = thermal conductivity of vessel
D = agitator diameter
N - agitator revs
p = density of vessel contents
Cp = specific heat of vessel contents
f,a = constants specific to agitator design and geometry

Question 10

Sensible heat medium: Heat balance for flowing heat transfer medium

Answer 10

mCdt = U(T-t)dA

Question 11

Sensible heat medium: Dynamic heat balance

Answer 11

[sum(mCp)]all(dT/dt) = -UADelta LM

Question 12

Constant jacket T: dynamic heat balance

Answer 12

[sum(mCp)all](dT/dt) = UA(Tj-T)

Question 13

For constant jacket T: heating and cooling order of temp variables

Answer 13

Heating: Tj>T2>T1

Cooling: T1>T2>Tj

Question 14

Constant jacket T: Heating time

Answer 14

tH = [(sum(mCp))/UA]ln[(Tj-T1)/(Tj-T2)]

Question 15

Equation for Delta T LM

Answer 15

Delta LM = [(T-t1)-(T-t2)]/[ln[(T-t1)/(T-t2)]]