The art of air blast freezing: Design and efficiency considerations
tải về 1.05 Mb. Chế độ xem pdf
|
LEO PHAM, Document, Email sample
| m ¼
u $PD 60$v suc 1 þ C C P dis P suc 1=n (3) where piston displacement (PD) is the volume actually swept during one cycle. The clearance volume C is the refrigerant volume left in the cylinder after completion of the discharge process is calculated by: C ¼ V dis V suc V dis (4) The compressor work is calculated by: _ W ¼ _ m 2 4 n n 1 P suc v suc 0 @ P dis P suc n 1 n 1 1 A 3 5 (5) The polytropic exponent n, in Eq. (5) , is a function of the discharge specific volume (a model output), and is calculated from experimental data. n ¼ logðP dis =P suc Þ logðv dis =v suc Þ (6) The compressor model determines the LSC flow rate _ m 1 and the HSC flow rate _m 2 . The intercooler flow rate is calculated as the difference between the HSC and LSC flow rates. 10.2. Condenser model The condenser was modelled by dividing into zones corre- sponding to the refrigerant state (superheated, two-phase, sub- cooled). The condenser consisted of 46 individual tubes (N tubes ) and 6 tube passes (N passes ). The ε-NTU method is used to evaluate the heat exchanger performance. The conductance, UA, for each zone is determined by calculating UA for a single tube (i.e. 1 of the 46 condenser tubes), then multiplying the UA single tube by the total number of tubes, i.e.: UA SH;2Ph;SC ¼ N tubes $UA single tube;SH;2Ph;SC (7) The model sequence starts by guessing the fraction of the tube length required to de-superheat the refrigerant, FSH (F SH (F SH < 1). To calculate the resistance over the superheated zone, each full resistance, that is the each of the five resistances over the full tube length (R full ), is divided by the fraction of the tube length (F SH ) used for that particular section, The total resistance is the sum of the individual resistances. The conductance of the superheat zone, as calculated by the model, is given by: UA SH;model ¼ N tubes $ 1 R total;SH (8) where R total,SH is the sum of the superheat resistances. In order to verify the initial guess of the superheat fraction F SH . The actual rate of heat transfer over the superheat section ( _q SH ) is given by: _q SH ¼ _ m 3 h in h sat;x¼1 (9) The NTU conductance is calculated by: UA SH;NTU ¼ NTU SH $ _ C min (10) The model conductance UA SH,model is checked against the NTU conductance UA tải về 1.05 Mb. Chia sẻ với bạn bè của bạn:
|