×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Heat exchanger doubt in design criteria

Heat exchanger doubt in design criteria

Heat exchanger doubt in design criteria

(OP)
Hi everybody!
I´m project engineer from Spain. I´m working in a new Rankine cycle project and I´m looking for a heat exchanger (condenser).
The conditions are different depending on the season, that is;  
Summer conditions: P= 2 bar, Temperature from 50ºC (inlet) to 33ºC (outlet)
Winter conditions: P= 1 bar, temperature from 40ºC (inlet) to 14.5ºC (outlet)
The working fluid is (R245fa).
The refrigerant is water = summer conditions 27ºC
The refrigerant is water = winter conditions 8ºC
Mass flow R245fa = 9kg/s in summer
Mass flow R245fa = 8.2kg/s in winter
Heat power( from the rankine cycle to the water) in summer = 1817kw
Heat power( from the rankine cycle to the water) in summer = 1560kw
I have some doubts in the design criteria. I have received an offer from a manufacturer. They offer us a heat exchanger working in summer conditions. My question is, can that machine work in winter conditions? The most important thing, what is design criteria used in heat exchangers? (Temperature, power…) Could someone explain me that? I will be so grateful for your answer.Thanks

RE: Heat exchanger doubt in design criteria

The subject is extensive, in the case of a real commercial heat exchanger.

Technical design criteria include heat transfer effectiveness, pressure drop, metallurgical compatibility, corrosion, fatigue, code compliance, cost, warranty-equipment life expectation, etc.

Considering only heat transfer effectiveness, as defined by the theory of compact heat exchangers ( Kays and London):

a) determine whihc fluid has the highest value of W* Cp- that fluid defines the definition of HX effectiveness, e.

b) e = (Ti-To)/ ( Ti- Ti*), where Ti and To  are the inlet and outlet temps of the governing fluid, and Ti* is the inlet temp of the other fluid, for the case of a counter flow HX. e cannot exceed 1.0. Typically, an e= 0.65 is a cheap design, e= 0.85 is an efficient design, e= 0.95 is a very expensive design.

c) See Kays and London for the simplified means of prediction e for a HX , using the e- NTU method ( NTU = number of trnasfer units , NTU = UA/ WCp ).

You have enough info to compute your required e- see if it is a cheap, or eficient, or expensive design.

RE: Heat exchanger doubt in design criteria

error- the governing fluid is the one with the minium value of W*Cp. Likelwise, the definition of e is based on Ti, To of the governing fluid, whihc has the smaller value of W*Cp.

sorry.

RE: Heat exchanger doubt in design criteria

To answer your question, yes it will work in your winter conditions.  If you don't regulate the water flow to your condenser, your condensing back pressure will be lower in the winter.  If this is a problem, then just regulate the refrigerant water flow so that the back pressure you want in the condenser is maintained.

Your vendor is wise to offer his condenser based on summer conditions because to do it conversely would mean that it would have a high back pressure in the summer time.

rmw

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources