

Capitolo 5
Il bilancio entropico
64
7XWWDYLD FRPH LQGLFDWR VFKHPDWLFDPHQWH LQ )LJXUD QHOOH WUDVIRUPD]LRQL ULJXDUGDQWL
VLVWHPL FKLXVL SUHYDOH O¶DVSHWWR WHPSRUDOH FLRq HVVH VDUDQQR FDUDWWHUL]]DWH GD XQ
prima
e un
dopo
RYYHUR GD XQ
inizio
e una
¿QH
GHOOD WUDVIRUPD]LRQH DQFKH VH PDJDUL HVVD FRPSRUWD XQR
VSRVWDPHQWR QHOOR VSD]LR
Stato
i
Stato
f
Tempo
Figura 5.1
,QYHFH FRPH LQGLFDWR LQ )LJXUD QHO FDVR GL WUDVIRUPD]LRQL ULJXDUGDQWL VLVWHPL DSHUWL
LQ UHJLPH SHUPDQHQWH SUHYDOH O¶DVSHWWR VSD]LDOH H FL VDUj XQ
qui
e un
lì
FLRq XQ¶
entrata
e una
uscita
D SUHVFLQGHUH GDO IDWWR FKH LO WUDQVLWR GDOO¶LQJUHVVR DOO¶XVFLWD ULFKLHGD XQ FHUWR WHPSR
,Q HQWUDPEL L FDVL VL SRWUj FRPXQTXH SHQVDUH D
trasformazioni chiuse
GHWWH DQFKH
tra-
sformazioni cicliche
FDUDWWHUL]]DWH GDO IDWWR FKH DOOD ¿QH GHO SURFHVVR VL ULWRUQD QHOOH FRQ
-
GL]LRQL LQL]LDOL 7DOL WUDVIRUPD]LRQL ULYHVWRQR XQ¶LPSRUWDQ]D WDOH GD PHULWDUH XQD WUDWWD]LRQH
SL DSSURIRQGLWD
Spazio
Entrata
Uscita
dm
e
dm
u
Figura 5.2
5.2
LE TRASFORMAZIONI CICLICHE
,Q EDVH D TXDQWR DSSHQD GHWWR VL SRWUj UHDOL]]DUH XQD
trasformazione chiusa
o
trasforma-
zione ciclica
VLD PHGLDQWH XQ VLVWHPD FKLXVR FLRq SULYR GL VFDPEL GL PDVVD FKH FRPSLH XQD
VHULH GL WUDVIRUPD]LRQL SDU]LDOL FKH OR ULSRUWDQR QHOOH FRQGL]LRQL GL SDUWHQ]D )LJXUD
VLD
PHGLDQWH XQD VXFFHVVLRQH GL VLVWHPL DSHUWL LQ UHJLPH VWD]LRQDULR FKH IDQQR Vu FKH OD VRVWDQ]D
FKH OL DWWUDYHUVD DVVXPD QXRYDPHQWH OH FRQGL]LRQL LQL]LDOL )LJXUD
,Q HQWUDPEL L FDVL VL
WUDWWHUj GL PRGXODUH RSSRUWXQDPHQWH JOL VFDPEL GL FDORUH H GL ODYRUR DL TXDOL YLHQH DVVRJJHW
-
WDWR LO VLVWHPD FKLXVR R OD VXFFHVVLRQH GL VLVWHPL DSHUWL LQ PRGR GD FKLXGHUH LO FLFOR
1HO FDVR GL XQ VLVWHPD FKLXVR SHU FLDVFXQD WUDVIRUPD]LRQH SDU]LDOH LO ELODQFLR HQHUJHWLFR
FRPSRUWD FKH
Q
if
= (
U
T
,
f
–
U
T
,
i
) +
L
if
GRYH DO VROLWR FRQ
Q
si indicano gli scambi termici, con
L
JOL VFDPEL GL ODYRUR H FRQ
U
T
la
YDULD]LRQH GL HQHUJLD WRWDOH GHO VLVWHPD 0D QHO FDVR GL XQD VHULH GL WUDVIRUPD]LRQL SDU]LDOL
FKH FRPSOHVVLYDPHQWH VL FKLXGRQR VX Vp VWHVVH GDQGR OXRJR DG XQD WUDVIRUPD]LRQH FLFOLFD VH
VL VRPPDQR PHPEUR D PHPEUR OH VLQJROH HTXD]LRQL
VL RWWLHQH
Q
if
L
if
(5.2)