The art of air blast freezing: Design and efficiency considerations
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(17) The NTU over the sub-cooling section is calculated by: NTU SC ¼ UA model;SC _C min (18) The rate of heat transfer over the sub-cooling section is deter- mined by: _q SC ¼ ε SC _C min ð T Ref ;sat T air;in Þ (19) The enthalpy of the refrigerant leaving the condenser is deter- mined by an energy balance across on the refrigerant side: h out ¼ h sat;x¼0 _q SC _ m 3 (20) Finally, the total rate of heat transfer across the condenser is determined by: _q cond;total ¼ _q SH q 2Ph þ _q SC (21) If F SC (from Eq. (15) ) is less than one, the refrigerant leaves the condenser in a saturated state. Under these conditions the refrig- erant enthalpy at the condenser outlet is calculated in the same manner as described for the sub-cooling zone. 10.3. Evaporator model The two-stage blast freezer has two identical wavy fin-and-tube evaporators with a staggered tube layout. Each evaporator has three TXV’s with a distributor further dividing the flow into 15 tubes, i.e. 15 tubes per TXV. This equates to 45 tubes per evaporator. Each tube has four passes. The evaporator is modelled in the same manner as the condenser. The evaporator is divided into zones corresponding to the refrigerant state. The refrigerant enters the evaporator in two- phase state and leaves superheated. The air-side heat transfer coefficient is calculated for a six row heat exchanger, same as the condenser. The remaining resistances are calculated for four rows. 10.4. Expansion valves model The six evaporator expansion valves and the intercooler expansion valve are all of the mechanically controlled thermostatic type, TXV. The throttling of the liquid refrigerant is achieved by assuming the expansion is enthalpic. 10.5. Intercooler model The intercooler could not be modelled physically like the heat exchangers because the geometric data was unavailable. Therefore, a thermodynamic model is used to simulate the desuperheating and sub-cooling processes of the intercooler. Fig. 6 a shows the log P-h diagram of the two-stage system with the state points used for modelling. State points 1, 2, 4, 5, 6 and the mass flow rates _m evap _ m cond and _ m evap are calculated from the compressor, condenser and expansion valve models coupled with the evaporating and condensing pressures given as model inputs. The intermediate pressure is calculated as the geometric mean of the condensing and evaporating pressures given by: P int ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P evap $P cond q (22) State point 3 is determined from the fraction of superheat x, given by h 3 ¼ x h 2 h sat;P int (23) where 0 < x 1 (24) The fraction of superheat x is determined from the experimental data and can be adjusted for parametric analysis. The enthalpy at state point 7 is determined from an energy balance across the intercooler: tải về 1.05 Mb. Chia sẻ với bạn bè của bạn:
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