a.) The exit temperature of the oil.
b.) The rate of entropy generation in the heat exchanger.
c.) The rate at which work is lost due to the irreversible nature of heat transfer in this process in kW. Assume the surroundings are at 20oC.
Monday, May 14, 2007
HW #8, P5 - Lost Work in a Heat Exchanger - 4 pts
A well-insulated shell-and-tube heat exchanger is used to heat water (CP = 4.18 kJ/kg-K) in the tubes from 20oC to 70oC at a rate of 4.5 kg/s. Heat is supplied by hot oil (CP = 2.30 kJ/kg-K) that enters the shell side at 170oC at a rate of 10 kg/s. Disregarding any heat loss from the heat exchanger, determine...
a.) The exit temperature of the oil.
b.) The rate of entropy generation in the heat exchanger.
c.) The rate at which work is lost due to the irreversible nature of heat transfer in this process in kW. Assume the surroundings are at 20oC.
a.) The exit temperature of the oil.
b.) The rate of entropy generation in the heat exchanger.
c.) The rate at which work is lost due to the irreversible nature of heat transfer in this process in kW. Assume the surroundings are at 20oC.
Subscribe to:
Post Comments (Atom)
2 comments:
I have some insecurities about temperature and rounding. In this case I obtained a different temperature than the key states (by a degree, ~129ish). This affects the calculation of Sgentot significantly. This difference is further exacerbated when you determine lost work (for example, being off 0.01 in Sgentot can mean being off by upto 3 J in lost work). Whats the general rule of thumb on such things? I've been having these inconsistencies for quite a few problems this homework, even ones that are not dependent on the source (TFT vs. NIST).
Hi Parwiz:
The numbers I provided in the answers have been rounded to the number of digits presented. But when I did the calculations, I carried 15 digits because i used Excel for the calculations. I never round-off until the final answer.
I use TFT when thermo data is needed for water, ammonia and R-134a. I expect you to get slightly different results depending on where/how you get your thermo data.
I use the ideal gas property tables for ideal gases and you should too. We should get the same results unless you round-off too early.
I use 0 degC = 273.15 K. It is surprising how much the 0.15 sometimes matters.
But, in general, the important part is to know how to solve the problem and not to get too focused on small differences. If you are off 3 J out of 10J, it is a big problem. But 3 J out of 1000 J is not.
Does this help ?
Post a Comment