TFS contest!

Slay the P.E. · Jan 29, 2020

Hi all!

We are offering a special prize to the first 3 people who correctly answer the following problem. Please do not post the solution here. Send it to us (work it by hand and take a picture or scan to pdf) via private message.

PROBLEM:

An industrial ice-making machine operates on the ideal vapor-compression cycle, using ammonia. The refrigerant enters the compressor as saturated vapor at 30 psia and leaves the condenser as saturated liquid at 150 psia. Water enters the ice machine at 55°F and leaves as ice at 25°F. For an ice production rate of 500 pounds mass per hour, the power input (hp) to the ice machine is most nearly:
(A) 3.5
(B) 7.0
(C) 14.0
(D) 28.0
Note: 169 Btu of heat needs to be removed from each pound of water at 55°F to turn it into ice at 25°F.

The winners will receive a 20% discount for our recently published practice problems book for the TFS exam. The book contains over 300 problems, all solved in exquisite detail in over 640 pages. All relevant areas of the TFS exam are covered and every single problem can be solved using only the NCEES provided PE Mechanical Handbook. You can download sample pages of our book here

Dr. Barber · Jan 31, 2020

Wow.. tough crowd.

Hello? Is this thing on?

OldSquaw · Feb 1, 2020

Dr. Barber said:
Wow.. tough crowd.

Hello? Is thing on?

This sounds like a great deal for people studying for the exam. I would definitely be interested in this manual if I hadn't already bought all my study materials. Especially if Dr. Barber wrote the solution manual. He appears to know what he is doing

R Choquette · Feb 28, 2020

The irony is certainly not lost on me that I'm having quite a difficult time getting this one hammered out.

My answer is (A).

Based on the load I calculated Qout to be (169 btu/lbm)*(500lbm/hr) = 84500 BTU/hr. Then I did a state table using state one as the ammonia entering the compressor. I used constant pressure assumptions from 3 to 2 and 4 to 1, and constant entropy from 1 to 2. Then I pulled h values from the charts and the the h value for state 2 (from the curves; I'm lost as to a better way to find enthalpy for a super heated vapor) to be h=680 Btu/lbm-F. (I also don't know where that F ends up.)

Then I used Q(dot) = m(dot) * (h3-h2) to find mass flow of ammonia, then plugged it in to W(dot) = m(dot) * (h2-h1) and converted to hp.

Please help me find my error(s) and explain what I'm missing as far as the Fahrenheit element of the units for enthalpy and finding enthalpy for a superheated vapor from entropy.

Thanks!

Slay the P.E. · Feb 28, 2020

R Choquette said:
The irony is certainly not lost on me that I'm having quite a difficult time getting this one hammered out.

My answer is (A).

Based on the load I calculated Qout to be (169 btu/lbm)*(500lbm/hr) = 84500 BTU/hr. Then I did a state table using state one as the ammonia entering the compressor. I used constant pressure assumptions from 3 to 2 and 4 to 1, and constant entropy from 1 to 2. Then I pulled h values from the charts and the the h value for state 2 (from the curves; I'm lost as to a better way to find enthalpy for a super heated vapor) to be h=680 Btu/lbm-F. (I also don't know where that F ends up.)

Then I used Q(dot) = m(dot) * (h3-h2) to find mass flow of ammonia, then plugged it in to W(dot) = m(dot) * (h2-h1) and converted to hp.

Please help me find my error(s) and explain what I'm missing as far as the Fahrenheit element of the units for enthalpy and finding enthalpy for a superheated vapor from entropy.

Thanks!