- Email: [email protected]
Heat pump dehumidifier drying of food
Review
Heat pump dehumidifier Although heat pumps have been used extensively in industry for many years, their use for drying, especially foods, has been limited. This article reviews the potential of heat pump dehumidifier (HPD) dryers for use in food drying. HPD dryers offer several advantages over conventional hot-air dryers for the drying of food products, including higher energy efficiency, better product quality, and the ability to operate independently of outside ambient weather conditions. In addition, this technology is environmentally friendly in that gases and fumes are not given off into the atmosphere. The condensate can be recovered and disposed of in an appropriate manner, and there is also the potential to recover valuable volatiles from the condensate. A large proportion of the energy required within the food industry is used to remove water from food products. Because drying processes are energy intensive, knowledge about their efficiency and optimum operating conditions is vital for the economical operation of dryers. Drying uses one or a combination of convection, conduction or radiation to conduct heat to the product that is to be dried. Much work has been done to increase the drying efficiency of convection drying, particularly by the application of heat pump dehumidifiers (HPDs). HPD dryers are finding increasing applications in the food industry for the drying of nuts, fruit, vegetables, herbs and fish products in several countries including Australia, New Zealand and Norway.
Mode of operation of HPD dryers The dehumidifier is an air-to-air heat pump that functions in a manner similar to the domestic refrigerator; it consists of a condenser (hot heat exchanger), a compressor, an evaporator (cold heat exchanger) and a fan to provide air movement. The heat pump is located along with the product in an enclosed chamber that has insulated walls. Dry, heated air is supplied continuously to the product to pick up moisture and is recirculated. Some of this humid air passes through the evaporator of the heat pump where it condenses, giving up its latent heat of vaporization, which is taken up by the refrigerant in the evaporator. This heat is used to reheat the cool dry air passing over the hot condenser of the heat pump. A schematic diagram of the operation of a typical HPD dryer is shown in Fig. I. In an HPD dryer, the source of the heat that is absorbed at the evaporator is the humid air that is drawn from a product during the drying process. As this moist air passes through the evaporator, it is rapidly cooled to a temperature below its dew point, resulting in water condensing out. The latent heat recovered in the process (-2255 kJ/kg of water condensed) is released at the condenser of the refrigeration circuit and used to reheat the Conrad O. Perera and M. Shafiur Rahmanare with the FoodScienceGroup,
HortResearch,PrivateBag 92 169, Auckland, New Zealandifax: +64-9-815 4243; [email protected]~z). Trends in Food Science & Technology March 1997 [Vol. 8]
drying of food Conrad O. Pereraand M. ShafiurRahman air within the dryer ~. The system is entirely recirculatory, leading to a thermal efficiency approaching 100% 1. Removal of water in its liquid state rather than its vapour state allows the latent heat of vaporization to be captured and only a small amount of sensible heat is lostL In practice, design modifications such as partial evaporator bypass systems and additional heat exchangers are used to maximize efficiency 2. Although most of the HPD dryers currently available recirculate all the air, a few non-recirculatory units are also now available 3'4. The three major advantages of HPD dryers are: • high energy efficiencies are achievable because both the sensible and the latent heat of vaporization are recovered; • drying can be carried out at relatively low temperatures; • drying can be conducted independent of the ambient weather conditions.
Drying efficiency Drying efficiency is a measure of the quantity of energy used in removing a unit mass of water from a product. Normally, it is measured in terms of kJ/kg, although when considering electrically operated HPD dryers, units of kW, h/kg are used. The efficiency of a heat pump can be expressed as its coefficient of performance (COP)~: COP = Qh/W where Qh is the energy absorbed at the evaporator plus the thermal equivalent of the total electrical energy input, W (in kW), required to bring about the compression: Q,= electrical input + water extraction rate × latent heat of vaporization Although it is usual to determine the efficiency of a heat pump by its COP value, in the case of a dehumidifier a more useful measure is the amount of water condensed per unit of electricity consumed. This is termed the specific moisture extraction rate or SMER: COP = 1 + SMER × hlg where SMER is given in kg/kW, h and hfg is the latent heat of vaporization. The SMER for a well-designed dehumidifier is in the range of l - 4 k g / k W . h , with an average value of -2.5 kg/kW- h. It is useful to compare this figure with the latent heat of vaporization of water, which is 2255 kJ/kg at 100°C or 1.596kg/kW.h. This demonstrates that a
Capvrigh~©19,!7. ElsevierS~ien~eLtd All rightsreserved.09242244/'97/$17.00 Pll: s0q24 2244(97)01013 3
75
3
/" /
4
z/
and 80% relative humidity. Thus, the prototype developed by ECNZ represents a significant breakthrough in the performance of HPD dryers.
Process efficiency
od
ertmsmoreutedtobtchopeato
than to continuous operation because batch systems 4[ ~ allow total recirculation with a very low air leakage rate, 1 ~ giving rise to high thermal efficiencies. However, con~ tinuous HPD dryer systems have been built for the dryi~ ~ ing of vegetables and gelatin 8. Strommen and Jonassen 9 5 J and Alves-Filho and Strommen ~° described the development of novel, counter-current HPD fluidized-bed dryers with high SMERs for the drying of heat-sensitive 7 £ products. Some of the earlier failures of HPD dryers l~ t were due to inadequate thermal insulation and gasI tightness of the seals of the chamber structure, resulting in loss of energy efficiency u. In addition to the electrical i , , energy required to drive the compressor, energy is also , required to pre-heat the product and chamber structure, to drive the fan for primary air flow over the product 6 that is to be dried, and to replace any heat loss through conduction and air leakages. The motors driving the fan Fig. 1 and the compressor can be located within the chamber A schematic diagram of the operation of a typical heat pump dehumidifierdryer, so that the residual heat produced by them is absorbed 1, Vapour-sealedand insulated structure; 2, humidifier;3, overheatvent; within the drying chamber instead of being lost to the 4, external condenser; 5, heat pump dehumidifier; 6, condensate; 7, product tray; atmosphere. Increasing the humidity in the drying air slows down 8, primaryair circulation fan: 9, air distributor, the drying process but improves the energy efficiency ~2. In general, the dehumidifier efficiency and capacity are proportional to increases in temperature and humidity. However, at high temperatures (e.g. 60°C), the increase in SMER with increase in relative humidity is compromised (Fig. 2). The SMER also decreases as the relative humidity of the air in the chamber decreases. Therefore, it is important to match the heat transfer area of the evaporator to the moisture load in the air. Ideally, HPD drying should be carried out at temperatures <60°C and average relative humidities >30%. These are ideal conditions Ior the drying of many food products including fruit and vegetables, meat, fish and several biologically active products. Most of these products, especially fruit, require drying at comparatively high relative humidities to prevent rapid drying resulting in case-hardening and sugar sweating, which yield a sticky Table 1. Generalcomparisonof heat pumpdehumidifier(HPD) with vacuumand hot-airdrying finished product. ........................... In addition to the relatively low drying Hot-air drying Vacuum drying HPD drying Refs temperatures (10-45°C) used in HPD systems, another major advantage of SMER(kg H20/kW,h) 0.12-1.28 0.72-1.2 1.0-4.0 3, 5 such systems is that they can be operated Drying efficiency(%) 35 40 _<70 95 6 independently of the ambient weather conditions. Thus, processes that require Operating temperaturerange (°C) 40 90 30-60 10-65 close control of drying conditions, such Operating % RH range Variable Low 10-65 as the drying of macadamia nuts, chestCapital cost Low High Moderate nuts, herbs, ginger, fish, etc., are well suited to the totally enclosed HPD sysRunning cost High Very high Low tem. HPD dryers are ideally suited for dehumidifier is able to remove water from a process at a SMER that is greater than that necessary for 100% efficiency of evaporation. The efficiency of hot-air dryers is generally <60% of the efficiency of evaporation, which translates to a SMER of only 0.95 kg/kW, h. Thus, HPD dryers are more energy efficient than conventional hot-air dryers. The comparative efficiencies and advantages of HPD, vacuum and hot-air dryers are shown in Table 1. The pilot-plant electric HPD dryer v developed by the Electricity Corporation of New Zealand (ECNZ) has a peak SMER of 7.94kg/kW.h at 50°C and 80% relative humidity. This is over twice the SMER reported by Hesse 3 for his HPD dryer operating with recuperator coils and air returning to the evaporator coils at 45°C
RH,Relativehumidity SMER,Specificmoistureextractionrate
76
the drying of foods and other products in those instances where heat-and-venttype dryers are inefficient because of
Trends in Food Science & Technology March 1997 [Vol. 8]
the high humidity of the ambient air, resuiting in longer drying times and higher energy requirements. They are ideal for drying in weather conditions where the relative humidity of the outside air is very high, such as in the tropics and on islands where high humidity prevails during most of the year.
45°C 60°C c"
5 30°C
v
Quality improvements
3
The quality of dried food products can be assessed by the degree of deg2 radation of their colour, flavour and to some extent texture. During drying, the 1 major factors that affect these attributes are the drying times and temperatures. 0 30 40 50 60 70 80 90 Major advantages of using an HPD 20 dryer for drying food products are the Relative humidity (%) potential improvements in the quality of resultant products. Usually, dried products have a low aroma volatile con- Fig. 2 tent, suffer loss of heat-labile vitamins Specific moisture extraction rate (SMER)as a function of humidity and dry-bulb temperature (data taken and have a high incidence of colour from Ref. 7). degradation. Ginger dried in an HPD dryer was found to retain over 26% of its gingerol, the principal volatile flavour component re- efficiency of the process and the quality of the dried sponsible for its pungency, compared with only ~20% products. Because of the deleterious effects of high retention for rotary-dried commercial samples ~3. This temperature on food products, optimization of product higher volatile retention in HPD-dried samples may be due quality is often at odds with the aims of maximizing to reduced degradation of gingerol at the lower drying efficiency and throughput. Mathematical modelling temperature used in the HPD system. The rate of loss of provides a tool to enable drying rate and efficiency to volatiles varies with the concentration; the greatest losses be predicted under a range of conditions ~8. Use of such occur during the early stages of drying, when the initial models to identify appropriate drying conditions should concentration of the volatile component in the drying limit the number of trials that may be necessary to determedium is low ~4. Because HPD drying is conducted in mine their effects on product quality. Jolly e t a l . 5 devela sealed chamber, any compound that volatilizes will oped a model for the continuous operation of an HPD remain within the chamber, and the partial pressure for dryer that predicts the following under given system that compound will gradually build up, retarding further specifications and operating conditions: volatilization from the product. • air conditions in the system, such as temperature, flow The development of a brown centre occurs in macrate and humidity; adamia nuts if high-moisture nuts are dried at elevated temperatures ~5. Van Blarcom and Mason ~6 have shown • heat transfer rate in the evaporator, condenser and external waste system; that HPD drying of macadamia nuts did not result in the above defect, even though they were dried at • COP of the HPD drying system; 50°C. This may be due to the faster drying rates associ• maximum efficiency achievable. ated with the HPD drying process. The losses in colour, The moisture loss from the product can be simulated flavour and nutritive value associated with dried products are attributed to non-enzymatic browning. It is by the characteristic drying curve for a set of operating recognized that the rate of reaction for non-enzymatic conditions of the dryer and physical characteristics of browning in dried products is highest at moisture levels the material. Two approaches are available in the literathat are commonly attained towards the end of the dry- ture to model drying curves: the relative drying rate ing cycle, when the drying rate is low and the product method ~2° and the constant drying rate method 2j-23. In temperature approaches that of the drying medium Iv. the relative drying rate method, a normalized drying However, the lower drying temperatures used through- rate is defined and correlated with the air-to-product out the drying cycle in HPD dryers reduce the extent of moisture ratio for the constant and falling-rate periods. Knowledge of the critical moisture content is necessary the non-enzymatic browning reactions ~6. for development of the model. In the constant drying rate method, a two-component model with two-exponent HPD dryer design The determination of optimum drying conditions re- terms has been used, and this is currently considered quires exhaustive testing and monitoring of both the to be the general form representing air-drying c u r v e s 22. Trends in Food Science & Technology March 1997 [Vol. 8]
77
Such models can be used to simulate drying behaviour over a range of conditions. Negligible information is available in the literature on the model constants for food materials in the case of the two methods discussed above. Equilibrium moisture isotherms are also necessary for both of the methods. The Guggenheim-Anderson-de Boer (GAB) isotherm is one of the semi-theoretical multi-layer sorption models and has been considered to be the best-fit model for most foods over a wide range of temperatures and water activities. Rahman 24 has discussed the GAB model and reviewed the model constants for a wide range of food products.
Constraints on HPD drying Although low-temperature drying is a potential advantage for drying foods, too low a temperature will limit drying rates, which has implications on throughput. Furthermore, slower drying rates at low temperatures may give rise to microbial growth problems. Some of the newer refrigerants now available will allow the use of higher drying temperatures. For example, use of the refrigerant 'R134a' should enable the operation of an HPD dryer up to -75°C without supplementary heating ~3. However, such high drying temperatures may compromise the benefits of low-temperature drying, especially for heat-labile materials. An HPD dryer is basically a convective dryer, where heat is transferred by the convection of warm air, and is more suitable for drying solid products than liquid or semi-solid products. Thus, its application at this stage is limited mainly to solid products. Like vacuum or freeze dryers, HPD dryers are more suited to batch drying because drying takes place in a hermetically sealed container; however, continuous HPD dryers have been developed for gelatin and vegetable drying s. The construction of continuous drying systems may require high engineering, modelling and design costs. Therefore, benefits need to be evaluated on the basis of cost rather than on energy efficiency alone.
Microbial safety Most vegetative cells of microorganisms will be destroyed by normal hot-air drying at 60-80°C, except a few species of heat-resistant bacteria, yeasts and moulds 2s. Although there is some concern about the potential for growth of microorganisms at the temperatures used in HPD dryers, in practice there have not been any reports of increased numbers of microorganisms in HPD-dried foods compared with those in foods dried by conventional means ~2. Serious microbiological problems may arise if the dryer is poorly designed. For example, if the refrigeration capacity of the refrigeration circuit is below that required to condense the moisture in the air, it will lead to high humidity in the chamber. Under normal circumstances, however, the rate of vaporization of moisture from the food and the rate of condensation of moisture from the air will be such that the water activity at the surface is maintained below the critical value of 0.6, thus avoiding microbial growth. 78
Future trends An application of HPD dryers in combination with fluidized-bed dryers has been investigated with respect to drying efficiency and improvements in product quality, especially in the case of biologically active material9.26 29. The ability to simulate freeze drying at atmospheric pressures and then carry out drying at temperatures above 0°C in the same dryer is an interesting concept, and one that needs further study. Such a process should offer the ability to regulate the physical properties such as apparent density and rehydration rate by increasing the porosity of the product owing to the formation of ice crystals in the frozen product. Biological activity, such as the viability of microbial cell cultures and enzyme activity, should also be possible to maintain without the use of expensive freezedrying techniques. The use of modified atmospheres for drying sensitive materials including food products is another important aspect of' HPD drying technology. Oxygen-sensitive materials such as flavour compounds and fatty acids can undergo oxidation during drying, giving rise to poor flavour, colour and rehydration properties. The use of modified atmospheres to replace air should permit new dry products to be developed without oxidative reactions occurring. There is also a need to obtain data on the physical properties of foods to provide the model constants required to develop suitable drying models for HPD dryers. Such information is scarce and is needed not only for improving the design and control of dryers but also for setting standards for different food products. Nesvadba 3° recently highlighted the initiatives that have been undertaken in this area in Europe.
References 1 Oliver, T.N. (1982I 'Process Drying with Dehumidifying Heat Pump' in International Symposium on the Industrial Application of Heat Pumps, pp. 73-88, BHRA Fluid Engineering, Cranfield, Bedford, UK 2 Jia, X., Jolly, P. and Clements, S. (1990) 'Heat Pump Assisted Continuous Drying. Part 2. Simulation Results' in Int. J. Energy Res. 14, 771 782 3 Hesse, B. (1994) 'Energy Efficient Electric Drying Systemsfor Industry' in Drying '94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 591-598, University of Queensland, Brisbane, Australia 4 Prasertsan,S., Saen-Saby,P., Prateepchaikul, G. and Ngamsritrakul, P. (19961 'Effects of Drying Rate and Ambient Conditions on the Operating Modes of Heat Pump Dryer' in Drying '96 - Proceedings of the 10th International Drying Symposium (Strumillo, C. and Pakowski, Z., eds), pp. 529-534, ~6d2 Technical University, Poland 5 Jolly, P., Jia, X. and Clements, S. (19901 'Heat Pump Assisted Continuous Drying. Part 1. Simulation Model' in Int. J. Energy Res. 14, 757-770 6 Strumil[o, C. and Lopez-Cacicedo, C. (1987) 'Energy Aspects in Drying' in Handbook of Industrial Drying (Mujumdar, A.S., ed.), pp. 823-862, Marcel Dekker 7 Barneveld, N., Bannister, P. and Carrington, C.G. (1996) 'Development of the ECNZ Electric Heat Pump Dehumidifier Drier Pi[ot-plant' in Proceedings of the Annual Conference ot the Institute of Professional Engineers of New Zealandl Vol. 2, Part l, pp. 68 71, Institute of Professional Engineers of New Zealand 8 Anon. (1988) 'Drying of Vegetables' in Industrial Drying by Electricity, Working Group 'Heat Recovery', pp. 52-53, 58-59, International Union for Electroheat, Paris, France 9 Strommen, I. and Jonassen,O. (1996) 'Performance Tests of a New 2-stage Counter-current Heat Pump Fluidized Bed Dryer' in Drying '96 - Proceedings of the 70th International Drying Symposium (Strumil]o, C. and Pakowski, Z., edsi, pp. 563-568, L6d2 Technical University, Poland 10 Alves-Filho, O. and Strommen, I. (1996} 'Performance and Improvements in
Trends in Food Science & Technology March 1997 [Vol. 8]
Heat Pump Dryers' in Drying '96 - Proceedings of the lOth International Drying Symposium (Strumillo, C. and Pakowski,Z., eds), pp. 405 416,"L6d;, Technical University, Poland 11 Bannister,P. and Carrington, C.G. (1995) 'Dehumidification Using Heat Pump Dryers' in Proceedings of the Annual Conference of the Institute ot Professional Engineers of New Zealand, VoL 1, pp. 327-333, Instituteof Professional Engineersof New Zealand 12 Britnell, P. et al. (1994) 'The Application of Heat Pump Dryers in the Australian Food Industry' in Drying '94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 897-904, University of Queensland, Brisbane,Australia 13 Mason, R.L. etal. {1994) 'Development and Application of Heat Pump Dryers to the Australian Food Industry' in FoodAust. 46(7), 319-322 14 Saravacos,G.D., Tsami, E. and Marines-Knurls, D. (1988)'Effect of Water Activity on the Volatile Componentsof Dried Fruits' in Mechanisms of Action offend Preservation Procedures (Gould, G.W., ed.), pp. 347-356, Elsevier 15 Priehavudhi,K. and Yamamoto,H.Y. (1965) 'Effectof Drying Temperatureon Chemical Composition and Quality of Macadamia Nuts' in Food TechnoL 19, 1153-1156 16 Van Blarcom, A. and Mason, R.L. (1988) 'Low Humidity Drying of Macadamia Nuts' in Proceedings of the Fourth Australasian Conference on Tree and Nut Crops (Batten,D., ed.), pp. 239 248, Exotic Fruit Growers Association, Lismore, NSW, Australia 17 Okos, M.R., Narsimhan,G., Singh, R.K. and Weitnauer, A.C (1992)'Food Dehydration' in Handbook of Food Engineering (Heldman, D.R. and Lund, D.B., eds), pp. 475-477, Marcel Dekker 18 Prasertsan,S., Saen-Saby,P., Ngamsritrakul,P. and Prateepchaikul,G. {1996) 'Heat Pump Dryer Part 1: Simulation of the Model' in Int. J. Energy Res. 20, 1067-1079 19 Keey, R.B. (1978) Introduction to Industrial Drying Operations, pp. 147-187, PergamonPress 20 Zhang, Q.J. and Keey, R.B. (1994) 'An ExperimentalTest of the Concept of the Characteristic Drying Curve Using the Thin-layer Method' in Drying '94
Proceedings of the 9th International Drying Symposium (Rudolph, V.,
Keey, R.B. and Muiumdar, A.S., eds), pp. 123-130, University of Queensland, Brisbane, Australia 21 Palipane,K.B. and Driscoll, R.H. (19941 'The Thin-layer Drying Characteristics of Macadamia In-shell Nuts and Kernels' in L Food Eng. 23, 129-144 22 Lebert,A. and Bimbenet,I. (1991) 'Drying Curves A General Processfor Their Representation'in Drying 91 (Mujumdar, A.S. and Eikova, I., eds), pp. 181-190, Elsevier 23 Rahman,M.S. and Perera,C.O. (1996) 'Effectof Pre-treatmenton Air Drying Rareand Thin LayerDrying Kineticsof FreshCherry' in Drying '96 Proceedings of the 10th International Drying Symposium iStrumillo, C. and Pakowski,Z., eds), pp. 1053-1060, k6d2 Technical Universib,, Poland 24 Rahman,M.S. (19951 Food Properties Handbook, CRC Press 25 Frazier,W.C. and Westhoff, D.C. (1978) Food Microbiology (3rd edn), McGraw-Hill 26 Strommen,I. and Kramer, K. (1994)'New Applications of Heat Pump Drying Process' in Drying TechnoL 12,889-901 27 Strommen,I., loseffsen, K. and Kramer,K. (1994)'Heat Pump Eluidised Bed Drying of Biologically Active Solutions' in Drying '94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mu)umdar, A.S., eds), pp. 1007-1014, University of Queensland, Brisbane,Australia 28 Jonassen,O. and Strommen, I. (1994) 'Non-adiabatic Heat Pump Fluidised Bed Drying of a Sticky, Biological Material' in Drying "94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 1015-1022, University of Queensland, Brisbane, Australia 29 Jonassen,O.K., Kramer, K., Strommen, I. and Vagle, E. (1994)'Non-adiabatic Two-stageCounter-currentFluidised Bed Drier with Heat Pump' in Drying "94 - Proceedings of the 9th International Drying Symposium {Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 511-517, University of Queensland, Brisbane, Australia 30 Nesvadba,P. (19961 'Thermal and Other Physical Propertiesof Foods:Needs for Data and for Standards'in Int. 1. Food Sci. Technol. 31,295-296
Review
ATP: A universal hygiene All food producers have a direct responsibility to ensure the safety and quality of their products. In order to evaluate a sanitation programme effectively, it is important to obtain results
monitor
rapidly. ATP (adenosine triphosphate) bioluminescence provides a reliable and rapid alternative to traditional microbiological techniques. The use of this technique for monitoring allows information to be provided
Jane-Marie Hawronskyjand JohnHolah
in time for corrective
action to be taken. This maintains control of the process, thus avoiding recall campaigns, adverse publicity or even food
aimed at protecting the health of consumers and facilitating international trade in foods) provides internaand/or profits. tional rules and guidelines: of particular relevance to the food industry are the General Principles of Food The introduction of the Food Safety Act I in the UK Hygiene guidelines 2. However, the concept of ensuring in 1990 placed a direct responsibility on all those in- food quality and safety is not a new one; it was in the volved in the production, distribution and sale of food 1970s that H. Bauman suggested a complete change in to ensure the safety and quality of their products. course with respect to ensuring microbiological safety 3. Furthermore, the Codex Alimentarius Commission (the Instead of relying on postmortem analysis inspections, international body responsible for the execution of the of doubtful significance, he advocated the introduction Joint FAOAVHO Food Standards Programme, which is of a forward control strategy. Bauman introduced the concept of hazard analysis and critical control points, Jane-Marie Hawronskyjand John Holah are at Campden & Chorleywood abbreviated to HACCP. The HACCP philosophy involves Food Research Association, Food Hygiene Department, Chipping Campden, the identification of specific hazards and the necessUK GL55 6LD Ifax: +44-1386-842100; e-mail: hawronskyj@campden, co.uk). ary measures for their prevention. The definition of a scares, all of which w o u l d ultimately end in reduced sales
Trends in Food Science & Technology March 1997 [Vol. 8]
Cop'~right,©1997 ElsevierS(ienceLtd. a,ll rightsreser',,ed,e924-2244/97/$17.00 PILS09242244(97)O1009-1
79
Heat pump dehumidifier Although heat pumps have been used extensively in industry for many years, their use for drying, especially foods, has been limited. This article reviews the potential of heat pump dehumidifier (HPD) dryers for use in food drying. HPD dryers offer several advantages over conventional hot-air dryers for the drying of food products, including higher energy efficiency, better product quality, and the ability to operate independently of outside ambient weather conditions. In addition, this technology is environmentally friendly in that gases and fumes are not given off into the atmosphere. The condensate can be recovered and disposed of in an appropriate manner, and there is also the potential to recover valuable volatiles from the condensate. A large proportion of the energy required within the food industry is used to remove water from food products. Because drying processes are energy intensive, knowledge about their efficiency and optimum operating conditions is vital for the economical operation of dryers. Drying uses one or a combination of convection, conduction or radiation to conduct heat to the product that is to be dried. Much work has been done to increase the drying efficiency of convection drying, particularly by the application of heat pump dehumidifiers (HPDs). HPD dryers are finding increasing applications in the food industry for the drying of nuts, fruit, vegetables, herbs and fish products in several countries including Australia, New Zealand and Norway.
Mode of operation of HPD dryers The dehumidifier is an air-to-air heat pump that functions in a manner similar to the domestic refrigerator; it consists of a condenser (hot heat exchanger), a compressor, an evaporator (cold heat exchanger) and a fan to provide air movement. The heat pump is located along with the product in an enclosed chamber that has insulated walls. Dry, heated air is supplied continuously to the product to pick up moisture and is recirculated. Some of this humid air passes through the evaporator of the heat pump where it condenses, giving up its latent heat of vaporization, which is taken up by the refrigerant in the evaporator. This heat is used to reheat the cool dry air passing over the hot condenser of the heat pump. A schematic diagram of the operation of a typical HPD dryer is shown in Fig. I. In an HPD dryer, the source of the heat that is absorbed at the evaporator is the humid air that is drawn from a product during the drying process. As this moist air passes through the evaporator, it is rapidly cooled to a temperature below its dew point, resulting in water condensing out. The latent heat recovered in the process (-2255 kJ/kg of water condensed) is released at the condenser of the refrigeration circuit and used to reheat the Conrad O. Perera and M. Shafiur Rahmanare with the FoodScienceGroup,
HortResearch,PrivateBag 92 169, Auckland, New Zealandifax: +64-9-815 4243; [email protected]~z). Trends in Food Science & Technology March 1997 [Vol. 8]
drying of food Conrad O. Pereraand M. ShafiurRahman air within the dryer ~. The system is entirely recirculatory, leading to a thermal efficiency approaching 100% 1. Removal of water in its liquid state rather than its vapour state allows the latent heat of vaporization to be captured and only a small amount of sensible heat is lostL In practice, design modifications such as partial evaporator bypass systems and additional heat exchangers are used to maximize efficiency 2. Although most of the HPD dryers currently available recirculate all the air, a few non-recirculatory units are also now available 3'4. The three major advantages of HPD dryers are: • high energy efficiencies are achievable because both the sensible and the latent heat of vaporization are recovered; • drying can be carried out at relatively low temperatures; • drying can be conducted independent of the ambient weather conditions.
Drying efficiency Drying efficiency is a measure of the quantity of energy used in removing a unit mass of water from a product. Normally, it is measured in terms of kJ/kg, although when considering electrically operated HPD dryers, units of kW, h/kg are used. The efficiency of a heat pump can be expressed as its coefficient of performance (COP)~: COP = Qh/W where Qh is the energy absorbed at the evaporator plus the thermal equivalent of the total electrical energy input, W (in kW), required to bring about the compression: Q,= electrical input + water extraction rate × latent heat of vaporization Although it is usual to determine the efficiency of a heat pump by its COP value, in the case of a dehumidifier a more useful measure is the amount of water condensed per unit of electricity consumed. This is termed the specific moisture extraction rate or SMER: COP = 1 + SMER × hlg where SMER is given in kg/kW, h and hfg is the latent heat of vaporization. The SMER for a well-designed dehumidifier is in the range of l - 4 k g / k W . h , with an average value of -2.5 kg/kW- h. It is useful to compare this figure with the latent heat of vaporization of water, which is 2255 kJ/kg at 100°C or 1.596kg/kW.h. This demonstrates that a
Capvrigh~©19,!7. ElsevierS~ien~eLtd All rightsreserved.09242244/'97/$17.00 Pll: s0q24 2244(97)01013 3
75
3
/" /
4
z/
and 80% relative humidity. Thus, the prototype developed by ECNZ represents a significant breakthrough in the performance of HPD dryers.
Process efficiency
od
ertmsmoreutedtobtchopeato
than to continuous operation because batch systems 4[ ~ allow total recirculation with a very low air leakage rate, 1 ~ giving rise to high thermal efficiencies. However, con~ tinuous HPD dryer systems have been built for the dryi~ ~ ing of vegetables and gelatin 8. Strommen and Jonassen 9 5 J and Alves-Filho and Strommen ~° described the development of novel, counter-current HPD fluidized-bed dryers with high SMERs for the drying of heat-sensitive 7 £ products. Some of the earlier failures of HPD dryers l~ t were due to inadequate thermal insulation and gasI tightness of the seals of the chamber structure, resulting in loss of energy efficiency u. In addition to the electrical i , , energy required to drive the compressor, energy is also , required to pre-heat the product and chamber structure, to drive the fan for primary air flow over the product 6 that is to be dried, and to replace any heat loss through conduction and air leakages. The motors driving the fan Fig. 1 and the compressor can be located within the chamber A schematic diagram of the operation of a typical heat pump dehumidifierdryer, so that the residual heat produced by them is absorbed 1, Vapour-sealedand insulated structure; 2, humidifier;3, overheatvent; within the drying chamber instead of being lost to the 4, external condenser; 5, heat pump dehumidifier; 6, condensate; 7, product tray; atmosphere. Increasing the humidity in the drying air slows down 8, primaryair circulation fan: 9, air distributor, the drying process but improves the energy efficiency ~2. In general, the dehumidifier efficiency and capacity are proportional to increases in temperature and humidity. However, at high temperatures (e.g. 60°C), the increase in SMER with increase in relative humidity is compromised (Fig. 2). The SMER also decreases as the relative humidity of the air in the chamber decreases. Therefore, it is important to match the heat transfer area of the evaporator to the moisture load in the air. Ideally, HPD drying should be carried out at temperatures <60°C and average relative humidities >30%. These are ideal conditions Ior the drying of many food products including fruit and vegetables, meat, fish and several biologically active products. Most of these products, especially fruit, require drying at comparatively high relative humidities to prevent rapid drying resulting in case-hardening and sugar sweating, which yield a sticky Table 1. Generalcomparisonof heat pumpdehumidifier(HPD) with vacuumand hot-airdrying finished product. ........................... In addition to the relatively low drying Hot-air drying Vacuum drying HPD drying Refs temperatures (10-45°C) used in HPD systems, another major advantage of SMER(kg H20/kW,h) 0.12-1.28 0.72-1.2 1.0-4.0 3, 5 such systems is that they can be operated Drying efficiency(%) 35 40 _<70 95 6 independently of the ambient weather conditions. Thus, processes that require Operating temperaturerange (°C) 40 90 30-60 10-65 close control of drying conditions, such Operating % RH range Variable Low 10-65 as the drying of macadamia nuts, chestCapital cost Low High Moderate nuts, herbs, ginger, fish, etc., are well suited to the totally enclosed HPD sysRunning cost High Very high Low tem. HPD dryers are ideally suited for dehumidifier is able to remove water from a process at a SMER that is greater than that necessary for 100% efficiency of evaporation. The efficiency of hot-air dryers is generally <60% of the efficiency of evaporation, which translates to a SMER of only 0.95 kg/kW, h. Thus, HPD dryers are more energy efficient than conventional hot-air dryers. The comparative efficiencies and advantages of HPD, vacuum and hot-air dryers are shown in Table 1. The pilot-plant electric HPD dryer v developed by the Electricity Corporation of New Zealand (ECNZ) has a peak SMER of 7.94kg/kW.h at 50°C and 80% relative humidity. This is over twice the SMER reported by Hesse 3 for his HPD dryer operating with recuperator coils and air returning to the evaporator coils at 45°C
RH,Relativehumidity SMER,Specificmoistureextractionrate
76
the drying of foods and other products in those instances where heat-and-venttype dryers are inefficient because of
Trends in Food Science & Technology March 1997 [Vol. 8]
the high humidity of the ambient air, resuiting in longer drying times and higher energy requirements. They are ideal for drying in weather conditions where the relative humidity of the outside air is very high, such as in the tropics and on islands where high humidity prevails during most of the year.
45°C 60°C c"
5 30°C
v
Quality improvements
3
The quality of dried food products can be assessed by the degree of deg2 radation of their colour, flavour and to some extent texture. During drying, the 1 major factors that affect these attributes are the drying times and temperatures. 0 30 40 50 60 70 80 90 Major advantages of using an HPD 20 dryer for drying food products are the Relative humidity (%) potential improvements in the quality of resultant products. Usually, dried products have a low aroma volatile con- Fig. 2 tent, suffer loss of heat-labile vitamins Specific moisture extraction rate (SMER)as a function of humidity and dry-bulb temperature (data taken and have a high incidence of colour from Ref. 7). degradation. Ginger dried in an HPD dryer was found to retain over 26% of its gingerol, the principal volatile flavour component re- efficiency of the process and the quality of the dried sponsible for its pungency, compared with only ~20% products. Because of the deleterious effects of high retention for rotary-dried commercial samples ~3. This temperature on food products, optimization of product higher volatile retention in HPD-dried samples may be due quality is often at odds with the aims of maximizing to reduced degradation of gingerol at the lower drying efficiency and throughput. Mathematical modelling temperature used in the HPD system. The rate of loss of provides a tool to enable drying rate and efficiency to volatiles varies with the concentration; the greatest losses be predicted under a range of conditions ~8. Use of such occur during the early stages of drying, when the initial models to identify appropriate drying conditions should concentration of the volatile component in the drying limit the number of trials that may be necessary to determedium is low ~4. Because HPD drying is conducted in mine their effects on product quality. Jolly e t a l . 5 devela sealed chamber, any compound that volatilizes will oped a model for the continuous operation of an HPD remain within the chamber, and the partial pressure for dryer that predicts the following under given system that compound will gradually build up, retarding further specifications and operating conditions: volatilization from the product. • air conditions in the system, such as temperature, flow The development of a brown centre occurs in macrate and humidity; adamia nuts if high-moisture nuts are dried at elevated temperatures ~5. Van Blarcom and Mason ~6 have shown • heat transfer rate in the evaporator, condenser and external waste system; that HPD drying of macadamia nuts did not result in the above defect, even though they were dried at • COP of the HPD drying system; 50°C. This may be due to the faster drying rates associ• maximum efficiency achievable. ated with the HPD drying process. The losses in colour, The moisture loss from the product can be simulated flavour and nutritive value associated with dried products are attributed to non-enzymatic browning. It is by the characteristic drying curve for a set of operating recognized that the rate of reaction for non-enzymatic conditions of the dryer and physical characteristics of browning in dried products is highest at moisture levels the material. Two approaches are available in the literathat are commonly attained towards the end of the dry- ture to model drying curves: the relative drying rate ing cycle, when the drying rate is low and the product method ~2° and the constant drying rate method 2j-23. In temperature approaches that of the drying medium Iv. the relative drying rate method, a normalized drying However, the lower drying temperatures used through- rate is defined and correlated with the air-to-product out the drying cycle in HPD dryers reduce the extent of moisture ratio for the constant and falling-rate periods. Knowledge of the critical moisture content is necessary the non-enzymatic browning reactions ~6. for development of the model. In the constant drying rate method, a two-component model with two-exponent HPD dryer design The determination of optimum drying conditions re- terms has been used, and this is currently considered quires exhaustive testing and monitoring of both the to be the general form representing air-drying c u r v e s 22. Trends in Food Science & Technology March 1997 [Vol. 8]
77
Such models can be used to simulate drying behaviour over a range of conditions. Negligible information is available in the literature on the model constants for food materials in the case of the two methods discussed above. Equilibrium moisture isotherms are also necessary for both of the methods. The Guggenheim-Anderson-de Boer (GAB) isotherm is one of the semi-theoretical multi-layer sorption models and has been considered to be the best-fit model for most foods over a wide range of temperatures and water activities. Rahman 24 has discussed the GAB model and reviewed the model constants for a wide range of food products.
Constraints on HPD drying Although low-temperature drying is a potential advantage for drying foods, too low a temperature will limit drying rates, which has implications on throughput. Furthermore, slower drying rates at low temperatures may give rise to microbial growth problems. Some of the newer refrigerants now available will allow the use of higher drying temperatures. For example, use of the refrigerant 'R134a' should enable the operation of an HPD dryer up to -75°C without supplementary heating ~3. However, such high drying temperatures may compromise the benefits of low-temperature drying, especially for heat-labile materials. An HPD dryer is basically a convective dryer, where heat is transferred by the convection of warm air, and is more suitable for drying solid products than liquid or semi-solid products. Thus, its application at this stage is limited mainly to solid products. Like vacuum or freeze dryers, HPD dryers are more suited to batch drying because drying takes place in a hermetically sealed container; however, continuous HPD dryers have been developed for gelatin and vegetable drying s. The construction of continuous drying systems may require high engineering, modelling and design costs. Therefore, benefits need to be evaluated on the basis of cost rather than on energy efficiency alone.
Microbial safety Most vegetative cells of microorganisms will be destroyed by normal hot-air drying at 60-80°C, except a few species of heat-resistant bacteria, yeasts and moulds 2s. Although there is some concern about the potential for growth of microorganisms at the temperatures used in HPD dryers, in practice there have not been any reports of increased numbers of microorganisms in HPD-dried foods compared with those in foods dried by conventional means ~2. Serious microbiological problems may arise if the dryer is poorly designed. For example, if the refrigeration capacity of the refrigeration circuit is below that required to condense the moisture in the air, it will lead to high humidity in the chamber. Under normal circumstances, however, the rate of vaporization of moisture from the food and the rate of condensation of moisture from the air will be such that the water activity at the surface is maintained below the critical value of 0.6, thus avoiding microbial growth. 78
Future trends An application of HPD dryers in combination with fluidized-bed dryers has been investigated with respect to drying efficiency and improvements in product quality, especially in the case of biologically active material9.26 29. The ability to simulate freeze drying at atmospheric pressures and then carry out drying at temperatures above 0°C in the same dryer is an interesting concept, and one that needs further study. Such a process should offer the ability to regulate the physical properties such as apparent density and rehydration rate by increasing the porosity of the product owing to the formation of ice crystals in the frozen product. Biological activity, such as the viability of microbial cell cultures and enzyme activity, should also be possible to maintain without the use of expensive freezedrying techniques. The use of modified atmospheres for drying sensitive materials including food products is another important aspect of' HPD drying technology. Oxygen-sensitive materials such as flavour compounds and fatty acids can undergo oxidation during drying, giving rise to poor flavour, colour and rehydration properties. The use of modified atmospheres to replace air should permit new dry products to be developed without oxidative reactions occurring. There is also a need to obtain data on the physical properties of foods to provide the model constants required to develop suitable drying models for HPD dryers. Such information is scarce and is needed not only for improving the design and control of dryers but also for setting standards for different food products. Nesvadba 3° recently highlighted the initiatives that have been undertaken in this area in Europe.
References 1 Oliver, T.N. (1982I 'Process Drying with Dehumidifying Heat Pump' in International Symposium on the Industrial Application of Heat Pumps, pp. 73-88, BHRA Fluid Engineering, Cranfield, Bedford, UK 2 Jia, X., Jolly, P. and Clements, S. (1990) 'Heat Pump Assisted Continuous Drying. Part 2. Simulation Results' in Int. J. Energy Res. 14, 771 782 3 Hesse, B. (1994) 'Energy Efficient Electric Drying Systemsfor Industry' in Drying '94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 591-598, University of Queensland, Brisbane, Australia 4 Prasertsan,S., Saen-Saby,P., Prateepchaikul, G. and Ngamsritrakul, P. (19961 'Effects of Drying Rate and Ambient Conditions on the Operating Modes of Heat Pump Dryer' in Drying '96 - Proceedings of the 10th International Drying Symposium (Strumillo, C. and Pakowski, Z., eds), pp. 529-534, ~6d2 Technical University, Poland 5 Jolly, P., Jia, X. and Clements, S. (19901 'Heat Pump Assisted Continuous Drying. Part 1. Simulation Model' in Int. J. Energy Res. 14, 757-770 6 Strumil[o, C. and Lopez-Cacicedo, C. (1987) 'Energy Aspects in Drying' in Handbook of Industrial Drying (Mujumdar, A.S., ed.), pp. 823-862, Marcel Dekker 7 Barneveld, N., Bannister, P. and Carrington, C.G. (1996) 'Development of the ECNZ Electric Heat Pump Dehumidifier Drier Pi[ot-plant' in Proceedings of the Annual Conference ot the Institute of Professional Engineers of New Zealandl Vol. 2, Part l, pp. 68 71, Institute of Professional Engineers of New Zealand 8 Anon. (1988) 'Drying of Vegetables' in Industrial Drying by Electricity, Working Group 'Heat Recovery', pp. 52-53, 58-59, International Union for Electroheat, Paris, France 9 Strommen, I. and Jonassen,O. (1996) 'Performance Tests of a New 2-stage Counter-current Heat Pump Fluidized Bed Dryer' in Drying '96 - Proceedings of the 70th International Drying Symposium (Strumil]o, C. and Pakowski, Z., edsi, pp. 563-568, L6d2 Technical University, Poland 10 Alves-Filho, O. and Strommen, I. (1996} 'Performance and Improvements in
Trends in Food Science & Technology March 1997 [Vol. 8]
Heat Pump Dryers' in Drying '96 - Proceedings of the lOth International Drying Symposium (Strumillo, C. and Pakowski,Z., eds), pp. 405 416,"L6d;, Technical University, Poland 11 Bannister,P. and Carrington, C.G. (1995) 'Dehumidification Using Heat Pump Dryers' in Proceedings of the Annual Conference of the Institute ot Professional Engineers of New Zealand, VoL 1, pp. 327-333, Instituteof Professional Engineersof New Zealand 12 Britnell, P. et al. (1994) 'The Application of Heat Pump Dryers in the Australian Food Industry' in Drying '94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 897-904, University of Queensland, Brisbane,Australia 13 Mason, R.L. etal. {1994) 'Development and Application of Heat Pump Dryers to the Australian Food Industry' in FoodAust. 46(7), 319-322 14 Saravacos,G.D., Tsami, E. and Marines-Knurls, D. (1988)'Effect of Water Activity on the Volatile Componentsof Dried Fruits' in Mechanisms of Action offend Preservation Procedures (Gould, G.W., ed.), pp. 347-356, Elsevier 15 Priehavudhi,K. and Yamamoto,H.Y. (1965) 'Effectof Drying Temperatureon Chemical Composition and Quality of Macadamia Nuts' in Food TechnoL 19, 1153-1156 16 Van Blarcom, A. and Mason, R.L. (1988) 'Low Humidity Drying of Macadamia Nuts' in Proceedings of the Fourth Australasian Conference on Tree and Nut Crops (Batten,D., ed.), pp. 239 248, Exotic Fruit Growers Association, Lismore, NSW, Australia 17 Okos, M.R., Narsimhan,G., Singh, R.K. and Weitnauer, A.C (1992)'Food Dehydration' in Handbook of Food Engineering (Heldman, D.R. and Lund, D.B., eds), pp. 475-477, Marcel Dekker 18 Prasertsan,S., Saen-Saby,P., Ngamsritrakul,P. and Prateepchaikul,G. {1996) 'Heat Pump Dryer Part 1: Simulation of the Model' in Int. J. Energy Res. 20, 1067-1079 19 Keey, R.B. (1978) Introduction to Industrial Drying Operations, pp. 147-187, PergamonPress 20 Zhang, Q.J. and Keey, R.B. (1994) 'An ExperimentalTest of the Concept of the Characteristic Drying Curve Using the Thin-layer Method' in Drying '94
Proceedings of the 9th International Drying Symposium (Rudolph, V.,
Keey, R.B. and Muiumdar, A.S., eds), pp. 123-130, University of Queensland, Brisbane, Australia 21 Palipane,K.B. and Driscoll, R.H. (19941 'The Thin-layer Drying Characteristics of Macadamia In-shell Nuts and Kernels' in L Food Eng. 23, 129-144 22 Lebert,A. and Bimbenet,I. (1991) 'Drying Curves A General Processfor Their Representation'in Drying 91 (Mujumdar, A.S. and Eikova, I., eds), pp. 181-190, Elsevier 23 Rahman,M.S. and Perera,C.O. (1996) 'Effectof Pre-treatmenton Air Drying Rareand Thin LayerDrying Kineticsof FreshCherry' in Drying '96 Proceedings of the 10th International Drying Symposium iStrumillo, C. and Pakowski,Z., eds), pp. 1053-1060, k6d2 Technical Universib,, Poland 24 Rahman,M.S. (19951 Food Properties Handbook, CRC Press 25 Frazier,W.C. and Westhoff, D.C. (1978) Food Microbiology (3rd edn), McGraw-Hill 26 Strommen,I. and Kramer, K. (1994)'New Applications of Heat Pump Drying Process' in Drying TechnoL 12,889-901 27 Strommen,I., loseffsen, K. and Kramer,K. (1994)'Heat Pump Eluidised Bed Drying of Biologically Active Solutions' in Drying '94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mu)umdar, A.S., eds), pp. 1007-1014, University of Queensland, Brisbane,Australia 28 Jonassen,O. and Strommen, I. (1994) 'Non-adiabatic Heat Pump Fluidised Bed Drying of a Sticky, Biological Material' in Drying "94 - Proceedings of the 9th International Drying Symposium (Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 1015-1022, University of Queensland, Brisbane, Australia 29 Jonassen,O.K., Kramer, K., Strommen, I. and Vagle, E. (1994)'Non-adiabatic Two-stageCounter-currentFluidised Bed Drier with Heat Pump' in Drying "94 - Proceedings of the 9th International Drying Symposium {Rudolph, V., Keey, R.B. and Mujumdar, A.S., eds), pp. 511-517, University of Queensland, Brisbane, Australia 30 Nesvadba,P. (19961 'Thermal and Other Physical Propertiesof Foods:Needs for Data and for Standards'in Int. 1. Food Sci. Technol. 31,295-296
Review
ATP: A universal hygiene All food producers have a direct responsibility to ensure the safety and quality of their products. In order to evaluate a sanitation programme effectively, it is important to obtain results
monitor
rapidly. ATP (adenosine triphosphate) bioluminescence provides a reliable and rapid alternative to traditional microbiological techniques. The use of this technique for monitoring allows information to be provided
Jane-Marie Hawronskyjand JohnHolah
in time for corrective
action to be taken. This maintains control of the process, thus avoiding recall campaigns, adverse publicity or even food
aimed at protecting the health of consumers and facilitating international trade in foods) provides internaand/or profits. tional rules and guidelines: of particular relevance to the food industry are the General Principles of Food The introduction of the Food Safety Act I in the UK Hygiene guidelines 2. However, the concept of ensuring in 1990 placed a direct responsibility on all those in- food quality and safety is not a new one; it was in the volved in the production, distribution and sale of food 1970s that H. Bauman suggested a complete change in to ensure the safety and quality of their products. course with respect to ensuring microbiological safety 3. Furthermore, the Codex Alimentarius Commission (the Instead of relying on postmortem analysis inspections, international body responsible for the execution of the of doubtful significance, he advocated the introduction Joint FAOAVHO Food Standards Programme, which is of a forward control strategy. Bauman introduced the concept of hazard analysis and critical control points, Jane-Marie Hawronskyjand John Holah are at Campden & Chorleywood abbreviated to HACCP. The HACCP philosophy involves Food Research Association, Food Hygiene Department, Chipping Campden, the identification of specific hazards and the necessUK GL55 6LD Ifax: +44-1386-842100; e-mail: hawronskyj@campden, co.uk). ary measures for their prevention. The definition of a scares, all of which w o u l d ultimately end in reduced sales
Trends in Food Science & Technology March 1997 [Vol. 8]
Cop'~right,©1997 ElsevierS(ienceLtd. a,ll rightsreser',,ed,e924-2244/97/$17.00 PILS09242244(97)O1009-1
79