Applied Energy 89 (2012) 81–88

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Applied Energy

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Modelling of energy flows in potato crisp frying processes

H. Wu, H. Jouhara, S.A. Tassou ⇑, T.G. KarayiannisSchool of Engineering and Design, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom

a r t i c l e i n f o

Article history:Received 23 July 2010Received in revised form 15 December 2010Accepted 4 January 2011Available online 4 February 2011

Keywords:Frying processesEnergy consumptionHeat recoveryPotato crispModelling of frying processes

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.01.008

⇑ Corresponding author. Tel.: +44 (0)1895 266865;E-mail address: savvas.tassou@brunel.ac.uk (S.A. T

a b s t r a c t

Food frying is very energy intensive and in industrial potato crisp production lines frying is responsiblefor more than 90% of the total energy consumption of the process. This paper considers the energy flowsin crisp frying using a First Law of Thermodynamics modelling approach which was verified against datafrom a potato crisp production line. The results indicate that for the frying process considered, most of theenergy used is associated with the evaporation of water present in the potato and on the surface of potatoslices. The remainder is from evaporation of frying oil and air of the ventilation system and heat lossesfrom the fryer wall surfaces by convection and radiation. The frying oil is heated by an industrial gas fur-nace and the efficiency of this process was calculated to be 84%. The efficiency of the overall frying pro-cess which was found to be of the order of 70% can be improved by employing exhaust heat recovery andoptimising other operating and control parameters such as exhaust gas recirculation.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Energy consumption is a significant contributor to greenhousegas (GHG) emissions and global warming. In its efforts to reduceemissions of greenhouse gases and dependence on imported fossilfuels the European Union has set an indicative objective to reduceits primary energy consumption by 20% by 2020 compared to pro-jected 2020 energy consumption [1]. The UK Government has seteven more ambitious targets. The Climate Change Act 2008 enactsa legal obligation for Government to reduce GHG emissions by 34%compared to 1990 levels by 2012 and by at least 80% by 2050 [2].The Energy White Paper published in 2007 sets out a framework ofmeasures to address these challenging targets and energy effi-ciency is one of them [3]. Energy efficiency is of particular impor-tance to the process industries due to the rapid rises in energycosts in the last few years and the volatility of energy prices. En-ergy costs may also represent a significant proportion of the overallproduction costs in various process sectors and energy efficiencycan offer one of the best approaches to increasing profitability evenin what are perceived as mature and energy efficient processes.

Energy efficiency can be achieved in a number of ways includ-ing improving the efficiency of equipment and unit operations,heat recovery and process integration. Over the last 30 years con-siderable research and development effort has been devoted tothese fields. Process integration and optimisation involves a num-ber of techniques and methodologies that can be applied in a sys-tematic manner to facilitate the selection or modification of

ll rights reserved.

fax: +44 (0)1895 269803.assou).

processing steps in large processing plants to minimise energyconsumption and resource use. Amongst the most common pro-cess integration techniques are pinch analysis [4–6], exergy andthermodynamic analysis of specific equipment and processes,and mathematical programming techniques, which can be usedto simulate and optimise the process or part of the process usingtechniques such as neural networks and genetic algorithms [7,8].Pinch analysis is a structured approach which can be used to iden-tify inefficiencies in energy use in industrial processes. It has beenapplied to large plants particularly in the petrochemical industryand a number of case studies on its application exist both in thepetrochemical and food industries [9]. Successful application ofthe technique, however, requires considerable expertise that veryoften is not available within the organisation and so, to date, appli-cation of pinch analysis has not been applied widely outside thepetrochemical industry.

Exergy analysis which is based on the Second Law of Thermody-namics, allows the consideration of losses not only in the energybut also the energy quality in complex systems and can thus pro-vide useful information where energy can be saved [10]. An exten-sion of exergy analysis is thermoeconomics which includes costsand the resources needed to effect energy savings. Thermoeconom-ic analysis has been gaining in popularity in recent years but itsapplication for plant monitoring and optimisation presents diffi-culties in the identification of the independent variables that char-acterise the behaviour of the process. For this reason, where themain interest is the practical analysis of the behaviour of thermalsystems, the application of models based on the thermodynamicdescription of the system is the preferred method with manyresearchers. This is because this approach relies on the use ofproperties and performance variables that are well known and

Nomenclature

A surface area of fryer casing, m2

cp specific heat, kJ/kg KCV calorific value, kJ/m3

_E energy, kWhfg latent heat, kJ/kg_m mass flow rate, kg/s_Qloss heat loss, kWT temperature, KU heat transfer coefficient, kW/m2 KV volume, m3

X composition percentage, %

Greek lettersg efficiency, %q density, kg/m3

Subscriptsa airact actualamb ambienta,2 air in foul gasb boiling waterc combustion productsCHE combustor and heat exchangerfo frying oilHE heat exchangerin inleto oilout outletov oil evaporation

o,2 oil in foul gaso,13 oil in potato crispps potato slice solidpw water in potato slicess potato solids,9 potato solid in raw potato slicess,13 potato solid in potato crispSY systemtotal total energy inputtw transmission through external wall of fryerv vapourv,2 water vapour in foul gasw waterw,9 water in raw potato slicesw,13 water in potato crisp1 fuel2 foul gas3 combustion air4 recycling exhaust gas5 combustion product6 exhaust gas7 oil outlet8 air flow9 raw potato slices10 surface water of the raw potato slices11 oil return12 fines removal13 potato crisp14 oil inlet

82 H. Wu et al. / Applied Energy 89 (2012) 81–88

accepted by industry and are widely used for the monitoring ofindustrial processes. This paper uses thermodynamic modellingbased on the First Law as a first step in the analysis of the energyflows in crisp frying.

The food and drink industry is the single largest manufacturingsector in the EU in turnover and employment [11]. It is also thelargest manufacturing sector in the UK, contributing 14.2% of man-ufacturing’s gross value added (GVA) and employing some 470,000people. The greenhouse gas footprint of the UK food chain is in theregion of 160 MtCO2e and food manufacturing is responsible foraround 13 MtCO2e and primary energy consumption of 42 TWh[2]. In food manufacturing approximately 68% of the energy is usedby fuel fired boilers and direct heating systems for process andspace heating. From the remainder, 16% is electrical energy usedby electric motors, 8% is used by electric heating, 8% by refrigera-tion equipment and the remainder 2% by air compressors [12].

Potato product manufacturing and preservation is one of thelarger contributors to food manufacturing emissions at 0.71MtCO2e or 5.5% of total [13]. Crisp manufacture in the UK is esti-mated to be around 220,000 tonnes per year and is responsiblefor around 0.18 MtCO2e. This was estimated from a recent studyby a major crisp manufacturer in the UK which indicated thatapproximately 34% of the emissions from the production of crispsis from the manufacturing process and that 1 ton of potato crisps isresponsible for 2.3 tons of CO2e emissions [14].

In the production of potato crisps, frying consumes more than80% of the total processing energy requirement so the greatest po-tential for energy savings is offered by design and control optimi-sation to minimise heat input to the potato slices and reducethermal losses [15]. Frying is a complex processing operation andoptimisation entails short frying times, high product quality andreasonable costs [16,17]. Numerous research investigations on

deep-fat frying modelling have been carried out [18–25]. Manyof these have considered and combined heat and mass transferprinciples to describe the temperature and moisture content pro-files of the product [21,22] whilst others have concentrated onempirical [18,20] and semi-empirical [23] relationships for heatand mass transfer. The majority of the reported research on fryingprocesses to date, however, has concentrated mainly on the phys-ical and chemical changes occurring in the food item under theinfluence of high temperature and prolonged heating. Very limitedwork has been done to include energy requirements and energyefficiency in industrial frying processes even though this is of sig-nificant importance both in terms of GHG emissions and economiccompetitiveness [24,25]. To this end, this paper presents a simpli-fied analysis of the key processes using energy balance equations.The primary aim is to provide a first approximation of the energyflows and efficiency of the frying process in order to identifyopportunities for energy conservation measures.

2. Potato crisp frying

A typical potato crisp production line is illustrated schemati-cally in Fig. 1. As can be seen, the bulk hopper holds the potatoesfor the line while a conveyor feeds potatoes from the hopper, ata controlled rate to correspond to changes in feed rates in the fry-ing line. From the hopper, the potatoes enter the de-stoning andwash unit where they are washed and any stones present removedby a store removal conveyor. The washed potatoes are then trans-ferred by another conveyor to the peeler. The peeler which is nor-mally a disc with gritted surface peels the potatoes as it rotates.The peelings are washed down by a water jet into a drainage chan-nel below the peeler where they are removed by a drum washer.

Fig. 1. Flow diagram of industrial fried potato crisp production line.

H. Wu et al. / Applied Energy 89 (2012) 81–88 83

The potatoes are then inspected and sorted, by size, before enteringthe slicer where they are cut into slices of 2–3 mm in thickness.The diameter of the slices can vary but normally fall within therange 30–40 mm.

The potato slices are then transported to the cold wash rinsingunit where they are rinsed with cold water to remove starch solids.The cold water is subsequently destarched and re-used on the pro-cess line for potato cleaning. Following the cold wash, the potatoslices enter the hotwash stage, where hot water removes sugarsand other water soluble solids from the slices. The washed potatoslices then enter a dewatering stage where surface water is re-moved, normally with warm air blown over the potatoes, beforethe slices enter the fryer. Removal of water from the slices ensuresthat the frying process is faster and more energy efficient since lessenergy will be required to evaporate the water from the potatoslice surface.

On entering the fryer, the slices are submerged in hot oil flowwhich enters the fryer at temperatures in the range 170–190 �C.The rate of flow of potato slices through the fryer is controlled bythe moisture content of the potato after the fryer which is normallykept below 2% by weight. The crisps then travel along cooling andtransportation belts, where they are salted, flavoured and vacuum-packed.

Foul Gas (Frying Vapour)

Recycling Exhaust Gas (

HEAT EXCHANGER

COMBUSTOR

Fuel

Combus

FRYERAir

Raw potatoSurface water of the

raw potato

1

2

4

7

89

10

11

12

1

3

Combustion Air

Fig. 2. Schematic diagram of flow through the comb

3. Frying system

The frying system is designed to transfer energy, produced bycombustion, to heat oil (sunflower oil) which is circulated throughthe fryer. The system can be divided into three main parts, com-bustor, heat exchanger and fryer as illustrated in Fig. 2. In the com-bustor, a gas burner burns natural gas with fresh air and foul gas(vapours from the fryer) to produce products of combustion atapproximately 702 �C. In some cases exhaust gas recirculationcan be used to increase turbulence, provide surface cooling and re-duce emissions. The products of combustion then flow through aheat exchanger to heat up the frying oil that is re-circulatedthrough the fryer.

3.1. Combustor

For complete combustion in the chamber, air–fuel ratio of16.5:1 was assumed giving 20% excess air. The combustor is wellinsulated and this minimizes heat loss to the ambient to an insig-nificant amount compared to the total energy input. Referring toFig. 3, the energy balance in the combustor is given by:

_E1 þ _E2 þ _E3 þ _E4 ¼ _E5 ð1Þ

where _E1 is energy input by the combustion of fuel given by:

_E1 ¼ _m1 � CV1=q1 ð2Þ

where _m1, CV1 and q1 are the mass flow rate, calorific value anddensity of fuel, respectively._E2 is energy input to combustor by the foul gas given by:

_E2 ¼ cp2 � _m2 � T2 ð3Þ

_E3 is energy input by combustion air calculated from:

_E3 ¼ cp3 � _m3 � T3 ð4Þ

_E4 is energy input to combustor by re-circulated exhaust gas deter-mined from:

_E4 ¼ cp4 � _m4 � T4 ð5Þ

_E5 is energy in the products of combustion before the heat exchan-ger given by:

Exhaust Gas

40%)

tion Products

Oil Outlet

Oil Inlet

Crisp Product

Fines Removal

5

6

14

3

ustor, heat exchanger and fryer (frying system).

Foul Gas

Recycling Exhaust Gas (40%)

COMBUSTOR

Fuel

Combustion Products

1

2

4

5

3

Combustion Air

Fig. 3. Schematic diagram of flow through the combustor.

Exhaust Gas

Recycling Exhaust Gas (40%)

HEAT EXCHANGER

Combustion Products

Oil Outlet

Oil Inlet

4

5

6

14

7

Fig. 4. Schematic diagram of flow through the heat exchanger.

84 H. Wu et al. / Applied Energy 89 (2012) 81–88

_E5 ¼ cp5 � _m5 � T5 ð6Þ

3.2. Heat exchanger

The heat exchanger transfers heat from the exhaust gases to thefrying oil. The heat exchanger in the current study is of the U-tubecross-counterflow type. The energy flows in the heat exchanger areillustrated in Fig. 4. Energy balance on the heat exchanger gives:

ð _E5 þ _E14Þ ¼ ð _E7 þ _E4 þ _E6Þ ð7Þ

where _E6 is energy in the exhaust gas given by:

_E6 ¼ cp6 � _m6 � T6 ð8Þ

_E7 is energy in oil leaving the heat exchanger:

_E7 ¼ cpo � _m7 � T7 ð9Þ

_E14 is energy of oil at inlet to the heat exchanger:

_E14 ¼ cpo � _m14 � T14 ð10Þ

The efficiency for the heat exchanger can be written as follows:

gHE ¼_E7 � _E14

_E5 � ð _E4 þ _E6Þð11Þ

where gHE is the energy efficiency of heat exchanger.

The combined combustor and heat exchanger energy efficiencycan be calculated from:

gCHE ¼_E7 � _E14

_E1

ð12Þ

3.3. Fryer

Fig. 5 shows a schematic diagram of the various mass flows inthe fryer. The energy analysis of the fryer assumed:

(i) A steady flow process.(ii) The heat required for chemical reactions is small compared

to the heat required to evaporate the water.(iii) Constant water and air specific heats.

The general equation for the conservation of mass in the fryingprocess is given by:X

_min ¼X

_mout or _m7 þ _m8 þ _m9 þ _m10

¼ _m2 þ _m11 þ _m12 þ _m13 ð13Þ

where _min and _mout represent the total inlet and outlet mass flows ofthe fryer respectively; _m2 is the mass flow rate of foul gas, _m7 is themass flow rate of supplied oil, _m8 is the mass flow rate of air, _m9 isthe mass flow rate of raw potato slices, _m10 is the mass flow rate ofsurface water associated with the raw potato slices, _m11 is the mass

10

Fig. 5. Mass flow balance in the industrial crisp frying process.

H. Wu et al. / Applied Energy 89 (2012) 81–88 85

flow rate of oil return, _m12 is the mass flow rate of fines removal and_m13 is the mass flow rate of crisp product.

In addition, the mass conservation equations of each species canbe determined as follows:

3.3.1. Frying oil

_m7 ¼ _m11 þ _m12 þ _mo;2 þ _mo;13 ð14Þ

where

_mo;2 ¼ _m2 � Xo;2 ð15Þ

_mo;13 ¼ _m13 � Xo;13 ð16Þ

_mo;2 and _mo;13 are the mass flow rates of oil in foul gas and potatocrisp, respectively. Xo,2 is the percentage of oil in the foul gas bymass and Xo,13 is the percentage of oil in potato crisp by mass.

3.3.2. Potato solid

_ms;9 ¼ _ms;13 ð17Þ

where

_ms;9 ¼ _m9 � Xs;9 ð18Þ

_ms;13 ¼ _m13 � Xs;13 ð19Þ

_ms;9 and _ms;13 are the mass flow rates of potato solid in raw potatoslices and potato crisp, respectively. Xs,9 is the percentage of potatosolid in raw potato slices by mass and Xs,13 is the percentage of po-tato solid in potato crisp by mass.

3.3.3. Water

_mw;9 þ _m10 ¼ _mv;2 þ _mw;13 ð20Þ

where

_mw;9 ¼ _m9 � Xw;9 ð21Þ

_mv;2 ¼ _m2 � Xv;2 ð22Þ

_mw;13 ¼ _m13 � Xw;13 ð23Þ

_mw;9 is the mass of water in the raw potato slices, _mv;2 is the mass ofwater vapour in the foul gas, and _mw;13 is the mass of water in thecrisp product respectively. Xw,9 is the percentage of water in raw po-tato slices by mass (wet basis), Xv,2 is the percentage of water va-pour in foul gas by mass and Xw,13 is the percentage of water inpotato crisp by mass.

3.3.4. Air

_m8 ¼ _ma;2 ð24Þ

where

_ma;2 ¼ _m2 � Xa;2 ð25Þ

_ma;2 is the mass flow rate of air in the foul gas and Xa,2 is the per-centage of air in the foul gas by mass.

The general form of the energy equation of the fryer can be ex-pressed in a rate form as

_E ¼ _Eps þ _Epw þ _Eov þ _Ea þ _Etw ð26Þ

where _E is total energy input to the fryer, _Eps is energy needed forraw potato slice solid heating during the whole frying process givenby:

_Eps ¼ cps � _ms;9 � ðT13 � T9Þ ð27Þ

where cps is the specific heat of the potato slice solid, T13 is the tem-perature of the potato crisp and T9 is the temperature of the raw po-tato slice solid.

_Epw is energy needed for heating and evaporation of the watercontained in the raw potato slices and can be determined from:

_Epw ¼ ½cpw � ðTb � T9Þ þ hfgw� � ð _mw;9 þ _m10 � _mw;13Þ ð28Þ

where cpw is the specific heat of water, Tb the boiling temperature ofwater and hfgw

the latent heat vaporization of free water._Eov is energy needed for oil evaporation given by:

_Eov ¼ hfgo� ð _m7 � _m11 � _m12 � _mo;13Þ ð29Þ

where hfgois the latent heat of oil.

_Ea is energy needed for heating the air entering the fryer calcu-lated from:

_Ea ¼ cpa � _m8 � ðT2 � T8Þ ð30Þ

where cpa is the specific heat of air and T8 is the temperature of air._Etw is energy transmitted through the external wall of the fryer

to the environment determined from:

_Etw ¼ U � A � ðTfo � TambÞ ð31Þ

where U is the overall heat transfer coefficient of the casing of thefryer, A is surface area of the casing, subscript fo denotes the fryingoil. Tamb is the average ambient temperature for the location atwhich the system under consideration operates. In this analysisTamb was assumed to be 25 �C.

2.4

2.6

2.8

3.0

cons

umpt

ion

of f

ryer

(M

W) Predicted

Measured

86 H. Wu et al. / Applied Energy 89 (2012) 81–88

Substituting Eqs. (27)–(31) into Eq. (26), gives:

_E ¼ cps � _ms;9 � ðT13 � T9Þ þ ½cpw � ðTb � T9Þ þ hfgw� � ð _mw;9

þ _m10 � _mw;13Þ þ cpa � _m8 � ðT2 � T8Þ þ hfgo� ð _m7 � _m11

� _m12 � _mo;13Þ þ U � A � ðTfo � TambÞ ð32Þ

To solve Eq. (32), several parameters need to be defined. Theseparameters were directly measured or estimated from a commer-cial crisp frying system in service and are listed in Table 1. The datarelate to the period December 2009 to March 2010.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

2.2Ene

rgy

Dates: 01/12/2009--29/03/2010

Fig. 6. Comparison between measured and predicted energy consumption of thefryer.

4. Results and discussion

4.1. Analysis of fryer

Fig. 6 shows a comparison between predicted daily energy con-sumption from the model and data obtained from the plant overthe 4 month period December 2009 to March 2010. The averagequantity of surface water during a sampling period was found tobe 5.4% by weight (wet basis) of the potato slice and the initialwater content of the potatoes was assumed to be 80% by weight.The surface water on the potato slices was determined by ran-domly sampling potato slices just before entry to the fryer andweighing them. The slices were then dried with a water absorbingtowel before they were re-weighted. The percentage surface waterwas then determined as the difference between the average weightof wet and dried slices divided by the average weight of wet slices.

The model assumes steady state conditions during the periodwhereas in the real process there are fluctuations in the through-put and energy consumption which depend on a large number ofoperational and control factors. Despite this, it can be seen thatin general there is a good agreement between the predicted and ac-tual energy consumption. The maximum error is around 13%.

The average energy requirement of the fryer was found to be2574 kW. Using the data in Table 1, and a final moisture contentfor the crisps of 1.5% by weight it was determined from the modelthat approximately 90% of this energy was used for the evaporationof water from the potato slices. The other energy losses are quite

Table 1Parameters for the developed model.

Symbol Value or expressions Source

A 45 m2 Measuredcpa 1.006 kJ/kg Kcps 1.3 kJ/kg K Measuredcpw 4.18 kJ/kg Khfgw

2256.7 kJ/kghfgo

300.0 kJ/kg MeasuredU 1.4 � 10�3 kW/m2 K [21]_m2 1.07 kg/s Assumed_m7 62.73 kg/s Measured_m8 0.04 kg/s Assumed_m9 1.1 kg/s Measured_m10 0.054 _m9 kg/s Measured_m11 62.57 kg/s Assumed_m12 0.02 kg/s Assumed_m13 _m9:Xs;9=Xs;13 ðkg=sÞ Calculated

Tamb 298.0 K MeasuredTfo 446.0 K MeasuredT2 375.9 K MeasuredT8 T8 = Tamb

T9 333.0 K MeasuredT13 423.0 K AssumedXs,9 20.0% MeasuredXo,13 30.3% MeasuredXo,2 0.13% CalculatedXv,2 98.8% CalculatedXw,13 1.6% Measured

small by comparison with energy input to the ventilation air rep-resenting 2% of total, energy input to potato solid 0.9% and energylost through the external walls by convection 0.4%.

The moisture content of potatoes will depend, among others, ontheir variety and growing conditions. To investigate the effect ofmoisture content on the energy consumption, three different initialmoisture contents, 60%, 70% and 80% were considered. Fig. 7 showsthe variation of the energy consumption of the fryer as a functionof raw material flow rate and moisture content. It can be seen, asexpected, that the energy consumption is a linear function of theraw material flow rate. It can also be deduced that the energy con-sumption is also a linear function of the potato moisture content.At a raw material flow rate of 1.0 kg/s, increasing the moisture con-tent from 70% to 80% increased the energy input to the processfrom 2322 kW to 2574 kW which represents a 10% increase.

Even though before entering the fryer the potato slices are nor-mally dewatered, using one of a variety of methods, not all thewater is removed and this contributes to the energy consumptionof the fryer. The impact of surface water on energy consumption isshown in Fig. 8 for surface water percentages of between 4% and10% by weight. It can be seen that surface water increases the en-ergy input to the process almost linearly. At a raw material massflow rate of 1.0 kg/s, and surface water of 4% the energy input to

0.80 0.85 0.90 0.95 1.00 1.05

1800

2000

2200

2400

2600

2800

Pred

icte

d en

ergy

con

sum

ptio

n of

fry

er (

kW)

Mass flow rate of raw material entering the fryer (kg/s)

60% 70% 80%

Initial moisture content

Fig. 7. The effect of initial moisture content of raw material on the predicted energyconsumption.

0.80 0.85 0.90 0.95 1.00 1.05

2200

2400

2600

2800

Pred

icte

d en

ergy

con

sum

ptio

n of

fry

er (

kW)

Mass flow rate of raw material entering the fryer (kg/s)

4% 6% 8% 10%

Percentage of surface water

Fig. 8. Predicted energy consumption at different percentages of raw materialssurface water.

Table 2Combustor and heat exchanger operating data.

Symbol Value or expressions

cpo 2.34 kJ/kg Kcp4 1.08 kJ/kg Kcp5 1.54 kJ/kg KCV1 40,120 kJ/m3

_m1 0.061 kg/s_m3 1.21 kg/s_m4 1.19 m3/s_m5 3.28 m3/s

T3 298 KT4 = T6 473 KT5 975 KT7 445.8 KT14 427.9 Kq1 0.8 kg/m3

H. Wu et al. / Applied Energy 89 (2012) 81–88 87

the fryer is 2541 kW. Increasing the surface water to 10% increasesthe energy input to 2686 kW which represents a 145 kW (6%)increase.

The energy required by the fryer is provided by the combustorand oil heat exchanger. To optimise the whole process it is essen-tial to appreciate the efficiency of the various sub-processes in thesystem.

4.2. Analysis of heat exchanger

The heat lost to the ambient through the exhaust gases:

_Q loss ¼ _E6 ¼ ð _E5 � _E4Þ � ð _E7 � _E14Þ

Based on the fact that _m7 ¼ _m14, cp4 = cp6 and T4 = T6

_Q loss ¼ _m5 � ðcp5 � T5 � cp4 � T4Þ � _m7 � cpo � ðT7 � T14Þ¼ 622 kW ð33Þ

The efficiency of the heat exchanger can be calculated from Eq.(11):

gHE ¼_E7 � _E14

_E5 � ð _E4 þ _E6Þ¼ 2597

3249¼ 79:9% ð34Þ

The energy efficiency of the combined combustor and heat ex-changer can be determined from Eq. (12):

gCHE ¼_E7 � _E14

_E1

¼ cpo � _m7 � ðT7 � T14Þ_m1 �CV1

q1

¼ 25973076

¼ 84% ð35Þ

The above compares well with studies on industrial steam boilersreported by Saidur et al. [26] which indicated efficiencies of be-tween 72% and 94%.

4.3. Overall frying system efficiency

The energy efficiency of the overall frying system (gSY) can bedetermined from:

gSY ¼energy in productstotal energy input

¼_Eps þ _Epw þ _Eo;13

_Etotal

ð36Þ

where _Eps, _Epw and _Eo;13 are the quantities of energy needed for rawpotato heating, water heating and evaporation, and oil absorption,respectively. Etotal is the total energy input of the whole system.Using the data in Tables 1 and 2,

_Eo;13 ¼ cpo � _mo;13 � T13 ¼ 116 kW ð37Þ

Etotal ¼ E1 þ E3 ¼_m1 � CV1

q1þ cp3 � _m3 � T3 ¼ 3440 kW ð38Þ

therefore,

gSY ¼_Eps þ _Epw þ _Eo;13

_Etotal

¼ 25þ 2264þ 1163440

¼ 69:9% ð39Þ

The above indicates that with an overall efficiency of only around70%, there are opportunities to reduce the energy consumption offrying processes. Despite the fact that the overall efficiency of com-bustor and heat exchanger was determined to be 84%, 622 kW wererejected to the ambient with the exhaust gases. Some of this heatcan be recovered and used in a number of ways to reduce the en-ergy input to the crisp production line or the fryer itself.

5. Conclusions

This paper presented a thermodynamic analysis of potato crispfrying. The analysis is based on data from a crisp production lineand a First Law modelling of the frying process. The results indi-cated that the overall efficiency of the frying system investigatedwas of the order of 70%. The major energy requirement of the pro-cess is for the evaporation of water contained in the potatoes andon the surface of the slices which represents over 90% of the energyinput in the fryer. The energy in the exhaust gases to the atmo-sphere is also significant, of the order of 622 kW compared to3440 kW energy input to the combustor. Some of this heat canbe recovered and used in a number of ways such as preheatingthe combustion air to the combustor or to heat water in the hotwash system of the production line.

Exhaust gas recirculation to the combustor has an impact on theeffectiveness of the heat exchanger and the energy available in theexhaust for heat recovery. Future investigations will address in de-tail the influence of this control parameter as well as effective waysof recovering and utilising energy in the exhaust gases.

Acknowledgements

The authors would like to acknowledge the financial supportfrom the RCUK’s Energy programme and contributions from theindustrial partners and academic collaborators from the Universi-ties of Newcastle and Northumbria. The Energy Programme is aResearch Councils UK cross council initiative led by EPSRC andcontributed to by ESRC, NERC, BBSRC and STFC.

88 H. Wu et al. / Applied Energy 89 (2012) 81–88

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