Question

A counter-flow double-piped heat exchange is to heat water from 20oC to 80oC at a rate of 1.2 kg/s. The heating is to be accomplished by geothermal available at 160oC at a mass flow rate of 2 kg/s. The inner tube is thin-walled and has a diameter of 1.5 cm. If the overall heat transfer coefficient of the heat exchanger is 640 W/m2.oC, determine the length of the heat exchanger required to achieve the desired heating using the effectiveness-NTU method.

Answer 1

110 m or 11,000 cm

Step-by-step explanation:

• let mass flow rate for cold and hot fluid = Mc and Mh respectively
• let specific heat for cold and hot fluid = Cpc and Cph respectively
• let heat capacity rate for cold and hot fluid = Cc and Ch respectively

Mc = 1.2 kg/s and Mh = 2 kg/s

Cpc = 4.18 kj/kg °c and Cph = 4.31 kj/kg °c

Using effectiveness-NUT method

1. First, we need to determine heat capacity rate for cold and hot fluid, and determine the dimensionless heat capacity rate

Cc = Mc × Cpc = 1.2 kg/s × 4.18 kj/kg °c = 5.016 kW/°c

Ch = Mh × Cph = 2 kg/s × 4.31 kj/kg °c = 8.62 kW/°c

From the result above cold fluid heat capacity rate is smaller

Dimensionless heat capacity rate, C = minimum capacity/maximum capacity

C= Cmin/Cmax

C = 5.016/8.62 = 0.582

.2 Second, we determine the maximum heat transfer rate, Qmax

Qmax = Cmin (Inlet Temp. of hot fluid - Inlet Temp. of cold fluid)

Qmax = (5.016 kW/°c)(160 - 20) °c

Qmax = (5.016 kW/°c)(140) °c = 702.24 kW

.3 Third, we determine the actual heat transfer rate, Q

Q = Cmin (outlet Temp. of cold fluid - inlet Temp. of cold fluid)

Q = (5.016 kW/°c)(80 - 20) °c

Qmax = (5.016 kW/°c)(60) °c = 303.66 kW

.4 Fourth, we determine Effectiveness of the heat exchanger, ε

ε = Q/Qmax

ε = 303.66 kW/702.24 kW

ε = 0.432

.5 Fifth, using appropriate effective relation for double pipe counter flow to determine NTU for the heat exchanger

NTU =
`\\ (1)/(C-1) ln((ε-1)/(εc -1) )`

NTU =
`(1)/(0.582-1) ln((0.432 -1)/(0.432 X 0.582   -1) )`

NTU = 0.661

.6 sixth, we determine Heat Exchanger surface area, As

From the question, the overall heat transfer coefficient U = 640 W/m²

As =
`\frac{NTU C{min} }{U}`

As =
`(0.661 x 5016 W. °c )/(640 W/m²)`

As = 5.18 m²

.7 Finally, we determine the length of the heat exchanger, L

L =
`(As)/(\pi D)`

L =
`(5.18 m² )/(\pi (0.015 m))`

L= 109.91 m

L ≅ 110 m = 11,000 cm