Heat exchanger doubt in design criteria

[agiraias]

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

 

[davefitz]

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.

 

[davefitz]

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.

 

[rmw]

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