Chemical Engineering February 2011 - 6

Letters
How efficiency is calculated
September 2010, Recover Waste Heat From Fluegas, pp.
37-40: I have performed a basic energy balance for the
examples in the article and am baffled that their Orangic
Rankine Cycle (ORC) efficiency is over 100%.
Dannis Jen
Houston
Circle 7 on p. 54 or go to adlinks.che.com/35062-07
Author replies
The focus of the article is on demonstrating a means of
cooling fluegas that produces net electrical energy output.
The reader's assumption that efficiency is equal to
actual work divided by heat input is correct. However, his
calculation is incorrect. In attempting a heat balance to
determine the fluegas exchanger duty, the reader has used
electrical consumptions of the pump and condenser motors,
evidently mistaking those values for thermal duties.
The actual work, simply, is equal to the net power produced
(calculated by subtracting the power consumed by
the pump and the condenser motors from the power produced
by the generator attached to the turbine). In other
words, the tables in the article reflect electrical energy to
and from the system, not thermal energy. The heat input
cannot actually be calculated from the information given
in the article; the specific heat of the fluegas is needed. Assuming
that the composition of the fluegas stream is 100%
CO2, the specific heat value can be calculated, from which
a heat balance can be performed.
It is a good question. The article did not address the efficiency
of the ORC cycle. This dialogue will shed more light
on the ORC application and add value to all interested parties.
An energy conversion efficiency calculation has been
added to the online version of the article at www.che.com.
Ali Bourji
WorleyParsons, Bellaire, Tex.
Steam thermodynamics
Brad Buecker's article, Steam Generation Thermodynamics
(November 2010, pp. 44-47), was excellent - good
information without getting bogged down with too much
theory and formulae. I never had formal education in
thermodynamics so some of the examples he stated were
over my head. Still his explanations enabled me to refresh
my knowledge at a level that I could understand. I spent
my career sailing in ship's engine rooms for the U.S. Coast
Guard and Chevron Shipping Co. Along the way I also was
a boiler operator at a small power plant (1,060,000 lb/h).
I have had much experience working on boilers from tube
repair and water chemistry to soot blower maintenance
and air casing repair. Although all the systems were in
place and working well at my plants, I enjoyed reading
about the theory surrounding steam generation and why
the designers put in a certain piece of equipment. Marine
steam-power plants obviously are in a limited space,
which makes it easier to trace out steam, water and fuel
systems. On one ship I could light off the whole steam
plant by myself. I will save the magazine in my file and
continue to review it. Thank you.
Circle 21 on p. 54 or go to adlinks.che.com/35062-21
6 CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2011
Brian T. Bender
Napa, Calif.
http://adlinks.che.com/35062-07 http://www.che.com http://adlinks.che.com/35062-21 http://WWW.CHE.COM
Chemical Engineering February 2011

Table of Contents for the Digital Edition of Chemical Engineering February 2011
