quick freezing food
TRANSCRIPT
Performance evaluation indexes for quick-freezers
Zhongjie Huan*
Department of Refrigeration and Air-conditioning Engineering, Tianjin University of Commerce, Tianjin 300134, China
Received 21 November 2002; received in revised form 31 March 2003; accepted 1 April 2003
Abstract
The idea of evaluating refrigeration equipment and the practical performance evaluation indexes of quick-freezers,namely energy utilisation efficiency, efficiency of airflow organisation, and uniform coefficient of airflow organisation,
were suggested to compare the performances and to diagnose the existing problems for freezers. Based on themeasurement, the performance evaluation indexes were applied to the performance analysis and the improvement forLSJ1500 spiral quick-freezer, the problem with LSJ1500 spiral quick-freezer was detected and the improved spiral
quick-freezer LSJ1500A with the higher performance was achieved. It was proved that the suggested evaluation indexesare the effective tools in the food refrigeration industry.# 2003 Elsevier Ltd and IIR. All rights reserved.
Keywords: Quick-freezing; COP; Performance
Performance et coefficients d’efficacite des surgelateurs
Mots cles : Surgelation ; COP ; Performance
1. Introduction
Quick frozen foodstuffs are widely consumed by peo-
ple all over the world due to its favourable advantagessuch as high quality, nutrition, hygiene, and con-venience [1,2]. Nowadays, various kinds of freezers are
manufactured and their performances are of great dif-ference, whilst there are unfortunately no perfect eval-uation indexes to evaluate and to compare the
performance for freezers.
The Coefficient of Performance (COP) is adoptedgenerally to analyse the performance of the refrigerationsystem, the refrigeration system with higher refrigera-
tion and lower input power will have the higher COPvalue and vice verse. In industrial and commercialrefrigeration, the purpose of refrigeration produced is
for what to be processed. For example, the refrigerationproduced by the refrigeration system in quick-freezer isused to cool down and to freeze the foods. Therefore,
another important issue for the refrigeration equipment,whose importance is as same as that of how to producethe maximum refrigeration, is how to use the refrigera-
tion produced effectively. Even the higher COP of therefrigeration system is achieved, the equipment stillowns the low efficiency if the refrigeration cannot beutilised effectively. Consequently, both aspects of
refrigeration production and refrigeration utilisationshould be taken into account integrally when to evaluatethe refrigeration equipment.
0140-7007/03/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved.
doi:10.1016/S0140-7007(03)00072-0
International Journal of Refrigeration 26 (2003) 817–822
www.elsevier.com/locate/ijrefrig
* Corresponding author at current address: Technikon Free
State, Department of Mechanical Engineering, Private Bag
X20539, Bloemfontein, 9300, South Africa Fax: +27-51-507-
3254.
E-mail address: [email protected] (Z. Huan).
Harrison et al. [3] provided the performance coeffi-cient COPt for tunnel freezer, which was defined as:
COPt ¼Q
Wc þWfð1Þ
It is clear that such performance coefficient onlyreflects the performance of the refrigeration system butnot the total performance of the tunnel freezer.
The freezer is used to freeze food, the effective refrig-eration, which is used to cool down foods, is only onepart of the produced refrigeration. The factors influen-
cing the refrigeration utilisation or the performance ofthe freezing equipment include insulation performanceof the body, heat and mass transfer quantity throughthe entrance and exit of the freezer, usage ratio of the
freezing chamber, refrigeration consumed by the deliv-ery belts, heat generation of the motors of the blowingfans, airflow organisation, and so on. Therefore the
desired performance indexes should have the ability toreflect the performance of the refrigeration system andthe influence of all factors stated above.
For the blasting quick-freezer, the airflow organi-sation is the important factor affecting the energy utili-sation efficiency of the freezer and the difficult technical
issue in the process of freezer design. In addition, theuniform performance of the airflow organisation forsome specific blasting freezers is critical to the freezingquality. Therefore, it is necessary for industry to estab-
lish the evaluation indexes to indicate quality of thedesigned airflow organisation.
2. Performance evaluation indexes of quick-freezers
The following three evaluation indexes suggested canbe used to analyse the performance of the freezer; theyare energy utilisation efficiency �e, efficiency of airfloworganisation �f, and uniform coefficient of airflow
organisation k.
2.1. Energy utilisation efficiency of quick-freezers Ze
The energy utilisation efficiency can be defined as theratio of the refrigeration quantity absorbed by foods
and the consumed energy by the quick-freezer.The energy consumed by the delivery system is not
associated with the refrigeration production and usage,
therefore the sum of the energy consumed by the com-pressor and fans is taken as the total energy consumedby the quick-freezer. The refrigeration absorbed by thefrozen foods, which is used to drop the temperature of
the foods from the initial entrance temperature to thefinal exit temperature, is the enthalpy difference offoods, i.e. 3600.G.(Hi�Ho), therefore
Nomenclature
COP Coefficient of performance of therefrigeration system
COPt Coefficient of performance of the tunnelquick-freezer
D Characteristic dimension of food (m)G Production rate of the quick freezer
(kgh�1)Hi Enthalpy of food at the entrance of
freezer (kJ kg�1)
Ho Enthalpy of food at the exit of freezer (kJkg�1)
k Uniform coefficient of airflow
organisationn Number of the measurement points for
temperature and velocityN Shape factor
Nu Nusselt numberRe Reynolds numbert Freezing time (s)
ti Air temperature at the measurementpoint i (�C)
ui Air velocity at the measurement point i
(m s�1)Q Refrigeration capacity of refrigeration
system (kW)
Wc Input power of compressor (kW)Wf Input power of the motors of fans (kW)
Greek letters�0 Coefficient of convective heat transfer
corresponding to the blasting velocity of
the air cooler (kW m�2 �C�1)� Actual coefficient of convective heat
transfer (kW m�2 �C�1)
� Density of the frozen food (kg m�3)�e Energy utilisation efficiency�f Efficiency of airflow organisation
�� Temperature difference between thephase change point of the food and thecooling medium temperature (�C)
�H Enthalpy difference of food between the
initial temperature and final temperature(kJ kg�1)
l Thermal conductivity of frozen food (kW
m�2 �C�1)�a0 Evaporation temperature of evaporator
(�C)
�a Cooling medium temperature (�C)�f Phase change point of food (�C)
818 Z. Huan / International Journal of Refrigeration 26 (2003) 817–822
�e ¼3600 � G � Hi �Hoð Þ
Wc þWfð2Þ
Due to the refrigeration consumed by the insulation
body, entrance and exit, conveyor belt, and motors offans, the refrigeration absorbed by the frozen foods isless than the refrigeration produced by the refrigeration
system. i.e.
3600 � G � Hi �Hoð Þ < Q ¼ COP�Wc
therefore
�e <COP�Wc
Wc þWf< COP ð3Þ
�e is the overall evaluation index used to reflect thelevel of energy utilisation for all kinds of freezers. The
higher �e, the higher the overall coefficient of the energyutilisation is. This index is not only used to compare theenergy efficiency for the same kinds of freezers, but also
for different kinds of freezers.According to the definition, if the refrigeration system
in the quick-freezer doesn’t own the higher COP value,
the freezer will not have the higher energy utilisationefficiency. Whilst if the freezer has the higher energyutilisation, it ensures that the refrigeration system owns
the higher COP and at the same time the refrigerationproduced can be absorbed effectively by the foods to befrozen.The same idea can be applied to all kinds of refrig-
eration equipment.
2.2. Efficiency of airflow organisation Zf
Energy utilisation efficiency is the overall evaluationindex for energy utilisation, but it cannot indicate the
specific factor’s influence on value of �e. Among thefactors stated above, the difficult and important one isthe airflow organisation. It would not only affect theperformance of the freezer, but also the freezing quality
of foods.Based on Plank equation [4]:
t ¼DH � �
D��D
N
D
4lþ1
�
� �ð4Þ
The freezing time depends on the following twogroups of parameters, one is used to describe the foodcharacteristics, such as �H, �, l, D and shape, another
is to describe the freezing environment, such as �� and�. The freezing time can be divided into the followingtwo parts:
t1 ¼DHD�
��
N�D 2
4lð5Þ
t2 ¼DHD�
��
N�D
�ð6Þ
where t1 represents the influence of the thermal con-
ductivity of food on freezing time, t2 represents theinfluence of the convective heat transfer on freezingtime. The main factor affecting the convective heat
transfer is the airflow organisation; therefore, airfloworganisation can be evaluated by analysing t2.Efficiency of airflow organisation is defined as:
�f ¼t2ð Þ0
t2ð7Þ
where: (t2)0 is the partial freezing time of food corre-sponding to the ideal airflow organisation, it is thereference value for the calculation of airflow organi-sation. t2 is the partial freezing time corresponding to
the actual airflow organisation.For a specific quick-freezer without loads, the limi-
tation of the freezing air temperature can be assumed as
the evaporation temperature of the refrigeration system,which can be taken as the reference temperature. Theblasting velocity of the fans can be regarded as the
reference velocity of the airflow organisation.Consequently, the efficiency of airflow organisation
can be expressed as:
�f ¼
1
D��1
�
� �0
1
D��1
�
� � ¼D� � �ð Þ
D� � �ð Þ0ð8Þ
where: ��=�f��aPhase change point of food �f can be assumed as 0
�C,then:
D� ¼ ��a
therefore
�f ¼� � �a�0 � �a0
ð9Þ
The efficiency of airflow organisation will be identified
if the evaporation temperature of the refrigeration sys-tem, the freezing air temperature, and the convectiveheat transfer coefficients corresponding to the blasting
velocity and the freezing velocity.The efficiency of airflow organisation of the quick
freezer reflects the quality of the airflow organisation
designed. The higher the efficiency �f, the more reason-able the airflow organisation is. The range of �f is [0, 1].
2.3. Uniform coefficient of airflow organisation k
The uniform coefficient of airflow organisation is thecritical factor for the freezing quality in some freezers
Z. Huan / International Journal of Refrigeration 26 (2003) 817–822 819
such as the tunnel freezer and the patch freezing equip-ment, though it is not associated with the energy utili-sation efficiency apparently.Based on the following procedures, sufficient
measurement points for the temperature and velocity infreezing area are needed in order to identify the uniformcoefficient of airflow organisation
1. Arithmetic mean values:
t ¼
Ptin
u ¼
Puin
8><>: ð10Þ
2. Standard deviations:
�t ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPti � tð Þ
2
n
s
�u ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPui � uð Þ
2
n
r8>>><>>>:
ð11Þ
3. Uniform coefficient of airflow organisation:
kt ¼�tt
ku ¼�uu
8<: ð12Þ
It is obvious that the uniform performance is higher ifthe values of kt and ku are smaller, and vice versa.The energy utilisation efficiency is associated with the
efficiency of airflow organisation to some degree. If theenergy utilisation efficiency is higher, the efficiency ofairflow organisation is higher, if the efficiency of airflow
organisation is lower; the energy utilisation efficiency islower. When the efficiency of airflow organisation ishigh but the energy utilisation efficiency is low, it
ensures that there are some problems with the body,entrance, exit, or refrigeration system.
3. Application of the performance indexes of quick
freezers
The LSJ1500 model spiral quick-freezer made in acompany of China was taken as an example to illustratethe application of the evaluation indexes.
3.1. Measurement
In order to determine the evaluation indexes the fol-lowing parameters were measured.
1. The blasting and freezing velocities of the cooledair.
2. The evaporation and freezing temperatures.3. The energy consumed by compressors and fans.
30 probes of temperature and velocity were placedrespectively on three different level sections evenly inLSJ1500 model spiral quick-freezer. Multichannel Ane-
momaster (Model 1560 with precision of 0.15 m s�1
for 0.1–4.99 m s�1, 0.3 m s�1 for 5.0–9.99 m s�1,0.75 m s�1 for 10.0–24.9 m s�1 and 1.5 m s�1 for25.0–50.0 m s�1) was adopted to measure the velocity.
Hybrid Recorder (Model DR232 with precision of0.05%) was adopted to measure the temperature.
3.2. Result analysis
The mean temperature and velocity of the blasting air
is �40.0 �C and 12.0 m s�1 individually, the energyconsumed by the compressor and eight axial fans is132.0 kW and 38.0 kW.
3.2.1. Uniform coefficient of the freezing areaMeasurement results were shown in Tables 1 and 2.The measurement results show that when the steady
operation condition is reached for the unloaded freezer,the temperature distribution in the freezing area is even,almost all temperatures are kept in the range of �38.0–
38.3 �C. The average temperature of the freezing area is�38.0 �C, the biggest temperature difference is 1.8 �C,the standard deviation of the temperatures is 0.46 �C,
and the uniform coefficient of the airflow organisation is0.012. The average velocity of the freezing is 2.39 m s�1,the biggest difference is 4.0 m s�1, the standarddeviation is 1.30 m s�1, and the uniform coefficient of
airflow organisation is 0.544.
3.2.2. Efficiency of airflow organisation
The characteristic dimension of the frozen chicken of0.05 m, the kinetic viscosity of the cooling air of 10�10�6
m2 s�1 and its thermal conductivity of 0.02 W m�1 �C�1,
Table 1
Temperature results in freezing area of LSJ1500
T (�C)
1 2 3 4 5 6 7 8 9 10Level 1
�38.2 �38.2 �38.1 �38.1 �38.2 �38.0 �36.5 �38.2 �38.2 �38.2Level 2
�38.1 �38.2 �38.2 �38.1 �38.1 �38.1 �36.7 �38.2 �38.1 �38.2Level 3
�38.1 �38.1 �38.2 �38.2 �38.1 �38.0 �36.7 �38.2 �38.2 �38.3820 Z. Huan / International Journal of Refrigeration 26 (2003) 817–822
and the experimental co-relation of the convective heat
transfer of Nu=0.148 Re0.643 are adopted [5,6].Based on Reynolds number, Nusselt number rela-
tionship equations as well as the obtained results, the
coefficient of the convective heat transfer in the freezingarea is �=25.97 W m�2 �C�1, the coefficient of theconvective heat transfer corresponding to the blastingvelocity is �=73.89 W m�2 �C�1. Therefore the effi-
ciency of the airflow organisation is �f=33.4%.
3.2.3. Energy utilisation efficiency
The production rate for the fired chicken from theinitial temperature of 80 �C to the final temperature of�18 �C is 750 kg per hour, the enthalpy difference of the
chicken is 395 kJ kg�1 [7]. Combining with the mea-sured results of energy consumed by fans and com-pressor at full load, the energy utilisation efficiency of
the spiral freezer is 51.4%.The calculated results show that the airflow organi-
zation in LSJ1500 spiral quick-freezer has the eventemperature distribution, uneven airflow field, low effi-
ciency of airflow organisation, and the correspondinglow energy utilisation efficiency.
4. Effect of airflow blockage and guide technology on
performance evaluation indexes of quick-freezers
The measurement and analysis for LSJ1500 modelspiral freezer shows that there are some problems with
the airflow organisation. The airflow blockage andguide technology [8] was applied to the improved spiralquick-freezer named LSJ1500A.The temperature results (Table 3) show that the aver-
age air temperature in the freezing zone is �37.6 �C, thebiggest temperature difference is 1.5 �C, the standarddeviation of the temperatures is 0.41 �C, and the uni-
form coefficient of temperature is 0.011. It has the samedegree as LSJ1500 spiral quick-freezer.The velocity results (Table 4) show that the average
velocity of the freezing is 7.07 m s�1, the biggest differ-ence is 4.4 m s�1, the standard deviation is 1.33 m s�1,and the uniform coefficient of airflow organisation is
0.189, which is improved greatly compared to LSJ1500spiral quick-freezer.Based on the obtained results, the corresponding
coefficient of the convective heat transfer in the freezing
area is �=52.44 W m�2 �C�1, therefore the efficiency ofthe airflow organisation is �f=66.7%, which is abouttwice of LSJ1500 spiral quick-freezer.
The experimental results indicated that the averagevelocity in the freezing area was increased to 7.07 m s�1
after the airflow blockage and guide technologies were
adopted, correspondingly the production rate wasincreased to 962 kg h�1 due to the shortened freezingtime. The input electricity power is almost the same as
LSJ1500, therefore the energy utilisation efficiency is:
�e ¼G Hi �Hoð Þ
Wc þWf¼ 65:9%
the energy utilisation efficiency was increased by 28.3%.The evaluation indexes comparisons were summarised
in Table 5. Though freezers LSJ1500 and LSJ1500Ahave the same values of COP and refrigeration capacity,the values of energy utilization efficiency are different.
This is resulted from the different values of the efficiencyof airflow organisation. Therefore, it is evident that theperformance evaluation indexes can be used to evaluate
Table 2
Velocity results in freezing area of LSJ1500
V (m s�1)
1 2 3 4 5 6 7 8 9 10Level 1
2.6 3.1 0.9 4.4 3.6 1.1 3.3 3.9 1.0 0.6Level 2
2.8 2.9 1.0 4.5 3.2 1.2 3.0 3.7 0.9 0.5Level 3
2.7 2.7 1.0 4.5 3.3 1.2 2.9 3.6 1.0 0.6Table 4
Velocity results in freezing area of LSJ1500A
V (m s�1)
1 2 3 4 5 6 7 8 9 10Level 1
7.8 7.4 4.7 8.4 7.6 5.7 8.5 8.2 5.9 6.0Level 2
8.4 6.5 5.3 9.1 8.4 5.2 7.8 8.9 6.0 5.8Level 3
8.3 6.4 5.3 8.9 8.2 5.4 8.1 7.7 6.2 5.9Table 5
Comparison of the evaluation indexes for spiral quick-freezers
Model of freezer
�T �u �f (%) �e (%)LSJ1500
0.012 0.544 33.4 51.4LSJ1500A
0.011 0.189 66.7 65.9Table 3
Temperature results in freezing area of LSJ1500A
T (�C)
1 2 3 4 5 6 7 8 9 10Level 1
�38.1 �37.5 �37.4 �37.9 �37.3 �37.2 �37.1 �37.1 �37.7 �38.3Level 2
�38.4 �37.8 �37.6 �38.2 �37.7 �38.0 �37.0 �37.4 �37.9 �37.7Level 3
�37.7 �38.2 �37.2 �37.6 �37.9 �37.3 �37.7 �36.9 �36.9 �37.5Z. Huan / International Journal of Refrigeration 26 (2003) 817–822 821
the performance, to compare the performance amongdifferent types of freezers, and to diagnose the existingproblems in the freezers.The conventional evaluation indexes (COP and
refrigeration capacity) cannot be replaced by the newlypresented indexes. Their combination can be used toevaluate the refrigeration equipment comprehensively
from the aspects of the energy utilisation efficiency ofthe refrigeration equipment, refrigeration capacity, andthe efficiency of the refrigeration system (COP). �e indi-cates the overall efficiency (refrigeration production andutilisation) of the equipment, the refrigeration capacityindicates the maximum of the refrigeration production
of the refrigeration system, and COP shows the effi-ciency of the refrigeration system. The efficiency of air-flow organisation and uniform coefficient of airfloworganisation can be used to evaluate the performance of
the airflow organisation. The combination of the con-ventional and new indexes can be used to diagnose theexisting problems in the refrigeration equipment.
5. Conclusion
The new improvement idea for refrigeration equip-ment, which is to improve the efficiency of refrigeration
utilisation besides to enhance the COP of the refrigera-tion system, was suggested. Three performance eval-uation indexes were established to compare theperformance for different quick freezers and to diagnose
the existing problems for quick freezers, and wereproved to be the effective tools in practice.Energy utilisation efficiency is an important index
that can be applied to evaluating and comparing theperformance for all kinds of quick-freezers. Efficiency ofairflow organisation can be used to evaluate the quality
of the airflow organisation and the technology level of
design for blasting quick-freezers. The uniform coeffi-cient of airflow organisation can be used to judge whe-ther the airflow organisation is designed reasonably forsome special blasting freezers. The combination of the
three indexes can be used to diagnose the existing pro-blems for freezers.
Acknowledgements
The author thanks the Committee of Science andTechnology of Tianjin, China, for the financial support.
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