• Card 7 / 35: A countercurrent heat exchanger operates with the following temperatures: cold fluid inlet 20°C, hot fluid inlet 90°C. The heat capacity of the hot fluid is 2.2 kJ/kg K. The heat capacity of the cold fluid is 1 kJ/kg K. The flow rate of the hot fluid is 5 kg/s. The flow rate of the cold fluid is 66 kg/s. The area for heat transfer is 10 m2. The overall heat transfer coefficient is 2418 W/m2 K. What are the outlet temperatures?
    A) Thoto = 29.6° C, Tcoldo = 30.1°C
    B) Thoto = 29.6° C, Tcoldo = 40.1°C
    C) Thoto = 29.6 °C, Tcoldo = 50.1°C
    D) Thoto = 19.6° C, Tcoldo = 20.1°C
    E) Thoto = 39.6 °C, Tcoldo = 30.1°C

    Answer:
    A) Thoto = 29.6° C, Tcoldo = 30.1°C

  • Keyboard Shortcuts

    Previous Card ← Previous Card Button
    Next Card → Next Card Button
    Flip Card Space-Bar
<< First < Previous Next > Last >>

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now
Explanation:

This is an interesting calculation. We know the inlet temperatures of both fluids (Tcoldi = 20 C, and Thoti = 90 C); we know their flow rates and heat capacities (66 kg/s, 1 kJ/kg K) and (5 kg/s, 2.2 kJ/kg K); we know the area for heat exchange (10 m2); and we know the overall heat transfer coefficient (2418 W/m2 K). We wish to determine the outlet temperatures of each fluid. We have three unknowns: the two outlet temperatures and the heat transferred. We can write three expressions: (1) Q = (66 kg/s)(1 kJ/kg K) (Tcoldo - 20) (2) Q = (5 kg/s) (2.2 kJ/kg K) (90-Thoto) (3) Q = (2418 W/m2 K) (10 m2) ΔTlm The first two of these can be rearranged to give Tcoldo = 35 - 0.166Thoto We can then expand 3 to read Q = 24.18ΔT1 - ΔT2 / ln (ΔT1/ΔT2) where Δ T1= 90 - Tcoldo and ΔT2=Thoto-20. We may then solve the system iteratively by guessing a value of Thoto, calculating Tcoldo from equation 4, and then computing Q from equations 2 and equations 5. We continue to guess Thoto until we find one for which the two values of Q (from equations 2 and 5) agree. A spreadsheet is an easy way to implement such a calculation. Note that the outlet temperature of the hot fluid does not change by much for this situation.

Hide Choices Interactive Question Quiz Home Page
https://www.jobilize.com/thermal-fluid-systems-mcq-quiz-by-dr-steve-gibbs

Thermal-Fluid Systems ME303

Author:

Access: Public Instant Grading

Attribution:  Dr. Steve Gibbs. Thermal-Fluid Systems. The Saylor Academy 2014, http://www.saylor.org/courses/me303/
Flash Cards plugin by Curtis Blackwell github.com/curtisblackwell/flash_cards
Google Play and the Google Play logo are trademarks of Google Inc.
Ask
Vanessa Soledad
Start Exam
Michael Sag
Start Exam
Copy and paste the following HTML code into your website or blog.
<iframe src="https://www.jobilize.com/embed/thermal-fluid-systems-mcq-quiz-by-dr-steve-gibbs" width="600" height="600" frameborder="0" marginwidth="0" marginheight="0" scrolling="yes" style="border:1px solid #CCC; border-width:1px 1px 0; margin-bottom:5px" allowfullscreen webkitallowfullscreen mozallowfullscreen> </iframe>