FORCED AIR COOLING. A Summary of Basic Principles. Compiled by Johan Strydom

Transcription

1 8.2.1C FORCED AIR COOLING A Summary f Basic Principles Cmpiled by Jhan Strydm 1. Why cl fruit? 1.1 Cntrl f the ripening prcess The ripening prcess f climacteric fruit (such as stne and pme fruit) cntinues after the fruit has been picked. Unless this prcess is cntrlled, the fruit will ripen within days, thereby reducing its marketability. The mst effective means f cntrlling fruit respiratin and thus ripening is by using refrigeratin t cl the strage air and, hence, the fruit. Reductin f the fruit pulp temperature als limits ethylene prductin, and thereby retards ripening. The ripening prcess can als be manipulated t extend the strage perid thrugh technlgies that are additive t refrigeratin like cntrlled atmsphere cld strage where xygen and carbn-dixide levels in the cld stre are cntrlled. In the case f certain plum varieties, the cld chain is adjusted t enhance the eating quality (ripeness) by raising and reducing the shipping temperature en rute. 1.2 Reduce misture lss String fruit at any relative humidity cnditins belw 100% will result in misture lss. The lss can be cntrlled by reducing the prduct temperature and keeping the relative humidity high (preferably abve 85%). Excessive misture lss can result in defects such as dry stems in grapes and shrivel in plums. 1.3 Cntrl f internal and external defects/diseases Fungal diseases may develp rapidly if fruit is nt cled sn after picking. Diseases like Btrytis develp expnentially abve 0 C. Different fruit kinds react differently t cld temperatures. Whereas -0.5 C may be ideal fr string grapes, citrus will shw skin disrders if stred at this temperature fr lng perids. Certain plums varieties (e.g. Sngld) will develp internal brwning if stred fr mre than 10 t 14 days cntinuusly at -0.5 C. In the case f Laetita plums, especially after a

2 heat wave, it is advisable nt t cl the plums t rapidly (say t +10 C in24 hurs) befre frced air cling t -0.5 C. 1.4 Cntrl f pests All fruit are expsed t pests that reside in rchards. Althugh cntrlled with pesticides and ther methds, pests may still be present during the packing prcess. The develpment f these pests and their resultant damage, can be cntrlled (t sme degree) by keeping the fruit at lw temperatures as insects tend t hibernate at lw temperatures. In the case f certain pests (e.g. Mediterranean Fruit Fly) the pest can be killed if expsed t extended perids f lw temperature. This cncept frms the base f the cld sterilisatin prcess stipulated by sme imprting cuntries. 2. Principles f Frced Air Cling The rate f cling f warm fruit depends n tw elements, namely heat transfer by cnductin and cnvectin, as well as temperature differential between the prduct and the cling medium (the greater this differential, the mre rapid the cling rate) Cnductin in fruit is nrmally a slw prcess invlving the heat transfer frm ne fruit t anther, as the cld frnt mves frm ne fruit t the next. Cnductin The cling prcess can, hwever, be speed up by ensuring that cld air reaches the fruit directly. This is where cnvectin plays a big rle as heat is transferred directly frm the warm fruit t the cld air. Cnvectin

3 If fruit is left in a cld stre with n r little air mvement the temperature f the fruit will eventually stabilise at the temperature f the cld rm. Hwever, t cl the fruit dwn t the target temperature within a shrt time, the cld air must mve ver the fruit. Cnsidering that the fruit is inside a cartn that is inside a pallet lad, the cling rate will be slw as air takes the path f least resistance and will nt find its way t the fruit n the inside f the pallet. Unless there is sufficient ventilatin in the pallet lad/cartns cnvective cling will nt take place. But even if the ventilatin is adequate, cld air will nt mve thrugh the cartns unless frced t d s. This is where frce air cling plays a vital rle. 3. Methdlgy The mst cmmn methd f frced air cling f fruit is t suck cld air thrugh the pallet lads and cartns. This is achieved by stacking tw rws f pallets leaving a tunnel (called the plenum) between the tw rws. By cvering the rf f the tunnel and tps f the pallets with a tarpaulin and placing a suctin fan at the end f the tunnel, a vacuum can be created in the tunnel. The cld air will mve thrugh the pallet lads thereby inducing cnvective cling and speeding up the cling prcess. In fixed pre-cling tunnels the exhaust fans are ften psitined abve the tunnel in the false ceiling f the rm. In these instances bth ends f the tunnel are then sealed. An effective methd t clse gaps n the utside f the tunnel, is t use vertical custmmade curtains. The latter cnsists f slid strips f tarpaulin t cver gaps between pallet lads and als the pallet base. Netting is attached t the strips t allw air t mve freely thrugh the pallet lads. Suctin Fan View frm the tp

4 Fixed Frced Air Cling Tunnel Tarpaulin cvers frnt and tp Extractin fan at the back Picture f a frced air cling tunnel 4. Factrs influencing Frced Air Cling Effectivity There are a number f factrs and sub factrs that play a rle in the effectivity f the frced air prcess. Sme f these factrs are bvius but in the case f sme technical issues, it is recmmended that the advice f a refrigeratin engineer r cling expert be sught fr guidance. 4.1 Delivery air Temperature (DAT) Target temperature Refrigeratin capacity Defrsting installatin design Defrst cycle time Cil surface Air vlume

5 Air vlume bypassing the cils Heat generated by fan mtrs Other surfaces including tunnel insulatin Ambient air infiltratin (e.g. pen drs) Fruit temperature (Difference between DAT and fruit temperature) Return Air Temperature (RAT) 4.2 Air Flw/Vacuum level Extractin Fans Number Size Speed Number f blades Blade pitch Psitin Cyclne design fr air intake Tunnel Length Width between legs Distance between tunnel leg and wall r next tunnel Design f curtains Sealing f gaps Tunnel insulatin Height number f pallets high e.g. 2 r 3 pallets 4.3 Packaging Cartn type Cartn ventilatin design Internal packaging Wrappers Trays Bags Punnets Other Palletisatin (neat and square) Orientatin f pallet stwed 1M face r 1.2M face Unifrmity f packaging 4.4 Fruit kind/variety Temperature sensitivity Misture lss risks Shape

6 Size 5. Frced Air Cling Guidelines The fllwing steps are recmmended t ensure successful frced air peratins: JGS/Oct Calibrate thermmeters prir t the seasn 5.2 Regularly cmpare thermmeters against each ther 5.3 Check thermmeters at least nce a week in 50:50 flaked ice/pure water slurry (the reading shuld be 0.0 C) 5.4 Ensure that wrkers put therm cuples in crrectly when palletising lads 5.5 Measure fruit temperature at receipt 5.6 Apply sund frced air cling principles (See Paragraph 4). It is recmmended that a refrigeratin engineer design the frced air cling tunnels. It is nt simply a case f placing pallets and installing a fan. 5.7 Avid different packs in ne tunnel where pssible Punnets tend t cl slwer Perfrated bags cl quicker than nn-perfrated bags Different pallet heights cause prblems Place slw cling packs clser t the fans 5.8 Check Delivery Air Temperature (DAT) regularly 5.9 Seal tunnels prperly 5.10 Check sealing effectivity with a manmeter (See Annexure 1) 5.11 Be careful with fruit with lw sugars as these fruit may freeze just belw zer. (Grape stems als freeze just belw zer due t lw sugar cntent, whereas the berries will nly freeze at lwer temperatures) 5.12 Regularly measure and recrd temperatures f: Therm cuples Air temperature Prbes (if any) 5.13 Get t knw the cld rm knw where ht spts are, cldest psitin, etc Keep in mind that the cling rate will be slwer the clser the fruit temperature gets t the target temperature. Fr example, with a delivery air f 0 C it may take 6 hurs t get the fruit dwn frm 22 C t 5 C, but it may take anther 10 hurs t reach 1 C Stp the frced air cling prcess as sn as the target temperature is attained, as FAC inevitably leads t misture lss in the fruit. This may require staff t wrk night shifts, but will pay in the lng run Measure fruit temperature at despatch. Descriptins f sme trubleshting cling prblems, are listed in Annexure 2

7 Annexure 1 End f tube psitined in tunnel Manmeter End f tube in cld rm envirnment Flexible tubing Glass U-Tube Manmeter Difference in levels indicates vacuum level Clured liquid (Anti-Freeze) (Actual size) The manmeter is a very simple tl that measures the pressure difference between spaces. When ne end is psitined (well-sealed) in the tunnel and the ther end is expsed t the cld rm envirnment, the liquid in the tube will mve twards the end psitined in the vacuum (due t lwer pressure in the tunnel, resulting frm the actin f the extractr fan). The difference in the height between the tw vertical tube psitins, is an indicatin f the level f the vacuum. A reading f abve 10mm (preferably 14mm) is indicative f a successfully sealed pre-cling tunnel. Frced air tunnels are designed t perate at specific pressure differentials (manmeter readings) and actual readings are an indicatin f the effectivity f the tunnel. Fr example, by using a manmeter a lt f pre-cling time can be saved if a lw reading, caused by gaps in the sealing f the tunnel, is fund. Crrective actin can take place immediately. It is needless t wait 24 hurs t find that the fruit has nt cled prperly because the tunnel was prly sealed.

8 Annexure 2 Trubleshting cling prblems Three cmmnly encuntered prblems in frced air cling systems are the fllwing: 1. Rm temperature rises as prducts are added t the cler. The mst likely cause f the prblem is insufficient refrigeratin capacity i.e. under-designed cler. The prblem als cmmnly ccurs when large rms with multiple cling tunnels are filled ver an extended perid f time. This culd lead t a decrease in the rate f cling, and in extreme cases even a warming f the first lad. A fairly straightfrward slutin is t divide the rm int separate cling bays, using curtains r uninsulated walls. This limits the influence that adjacent stacks have n each ther, and allws each evapratr t address the cling requirement f nly ne stack. 2. Prduct temperature in the uter cartns f the stack decreases slwly, despite DAT being n spec. The cause f the prblem is insufficient cntact between the cling air and the warm prduct. This may be due t inadequate ventilatin, nn-alignment f vents, packaging r prduct blcking vents, insufficient airflw capacity (perhaps due t excessive number f cartns placed n the cler), and shrt-circuiting f air past the cartns. 3. Prduct temperature in sme uter cartns decreases fairly rapidly, but thers cl slwly despite DAT being n spec. The cause f the prblem is nn-unifrmity f airflw thrugh the stack, due t prly designed supply r return air channels. If the supply channels are t narrw (i.e. stack is t clse t the wall r t clse t an adjacent stack), cartns at the base f the pallet receive inadequate airflw and cl less rapidly than cartns in the tp f the pallet. If the air return channel (plenum) is t narrw, the pallets furthest frm the fan cl slwly, due t inadequate airflw. This can easily be mnitred by measuring pulp temperatures and pressure drps acrss the stack at different heights and different distances frm the fan (large differences in pressure drp indicate large variatin in airflw, manifested as large pulp temperature differences). Thmpsn, J, et al., Cmmercial cling f fruits, vegetables and flwers. University f Califrnia, DANR Publicatin 21567