A Simplified Method For Estimating The Peak Load In The Large Cold Stores

In this paper a simplified method for estimating the peak load in the large cold stores is found and its occurrence day by utilizing two new variables A & B. The first variable represents the number of days that is necessary for cooling the product (storage) up to the needed temperature according to the daily temperature differences only. The second variable represents the number of days required for cooling the total product (storage) depending upon the daily storage mass only without returning back to the details of distribution of the daily thermal load that is followed in calculating thermal load for cold stores. By the demonstrative and analysis of the thermal load in cold stores with detailing study for each effective factors in estimating the thermal load characteristic is observed that the factors daily temperature differences (


Introduction
Cold stores are vessels or rooms properly thermal insulation supplied by electrical and mechanical machines that decrease the temperature of the space and its contents to a proper temperature thus; this lowered proper temperature reduces or prevents the factors which help to corrupt the storage products.The suitable temperature for storing depends on two main factors; type of the storage and the duration of store [1].
These stores are frequently supplied with cooling equipment, which its capacity reaches thousands of tons of cooling, and other completion equipment that support in works of separate, transfer, pouch and so on.Also the initial cost of the cold stores is very expensive.The cooling system forms a very important part from the total cost, thus the equipment selection is very important and it is a real effective factor upon the total coast of the project.
Jing Xie et al.
[2] they used CFD technique to study a two-dimensional mathematical model for the inside a mini type construction cold store (4.5 m × 3.3 m × 2.5 m).The simulation results reflected the characteristics of airflow and temperature distribution, and a several design parameters (corner baffle, the stack mode of food stuffs, etc.) were analyzed.The results of calculation indicated that all these designed parameters influenced the flow and temperature fields.
S. Akdemir [3] studies the cooling load and choosing of cooling system elements for designing of the cold store.He calculated transmissions, infiltration, production, and other source of heat.In PDF created with pdfFactory Pro trial version www.pdffactory.comaddition the amount of the cooling fluid was also calculated.
[4] stated that the determination of total input cooling load would not be possible, for this reason there may be some deviation in the cooling load and the focus point must be minimizing the deviation.He explained that the ambient temperature of cold store, the situation of store product before entering the cold store, the daily working hours and the determination of which product will be stored are important for calculation of the cooling load.

Elements of thermal load in the cold stores
The thermal load per day in the cold stores can be divided into two main parts:

External thermal load
The external thermal load is due to transfer of the heat from external to internal space of the store.It consists of:

Heat transfer through wall, roof, and ground
Can be calculated using the following equation (Searle engineers hand book 1988) [5]:

Air change heat gain
It can be calculated using the following equation [5]:

Internal thermal load
This thermal load generated inside the cold store and it contains the following:

Lighting load
It is the thermal energy generated from lighting used inside a store.It can be calculated using the following equation [5]:

Equipment Load
The equipment load can be calculated using the following equation:

Respiration load
It is the amount of heat generates from product due to the vitality chemical reactions in any plan and it can calculate using the following equation [5]:

Factor affects thermal load
In the revision of the previous thermal loads which are used for design any cold store; it can be noticed that: • D Due to the probability of miss-match of all components loads in one day so it can not be able to find the peak load directly although the correct choice of the daily storing mass and the daily temperature difference was used.

Determination of the peak load and starting day
To simplifying the analysis of the calculation, it's important to redefine some variables: Figures (1) to ( 5) represent the distribution of the daily load during the first period of storing for cases in the Table-2.

Results and Discussion
The graphical analysis and study of the load component distribution of previous figures could build Table-3 which represents a summary of the obtained results and offers simple equations to calculate peak load for the most cases without returning back to the details of loads distribution or graphical analysis or backup figures that is required to calculate throughout the first days of storing reaching to last days with algebraic sum to determine the peak load and day of it.In general, these are the obtained simplified equations Peak load = Base Load + External oad + B×Q 7 + minimum of (A or B) ×Q 6 Peak load Day = maximum of (A or B) In order to verify the validity of the above equation, it can be applied on the case study example 45.3 kW/day Total load = 17.36 + 15.41 + 1718.5 +45.3 =1796.57(kW/day) and the peak load day is 20.

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The result of the total peak load above is about 1796 kW/day, whenever, the peak load calculated with traditional method listed in table-1 is 2454 kW/day.i.e. the actual peak load (suggested) is about 26% less than the other (traditional).
From Table-3 and figures (1) to ( 5), in some cases it appears that peak load duration might not be exceeded more than one day and then reduced gradually, that means the possibility to pass (avoid) this period of highly load in this day by changing the storing method.Table-3 enables the store manager or staff to determine previously the value and time of peak load to take the necessary technical arrangements.
The final peak load equation shows that the last term is the major factor affected on the result of the peak load calculation therefore the relation between 1/A or ∆ T/ ∆ T Tot and 1/B or m/m Tot can be plotted against the increasing in the load as a percentage of product load (Q 6 ), as shown in figure (6). Figure (6) shows the increasing of 1/A (increases the ratio of daily temperature difference to the total of temperature difference ( ∆T/∆T Tot )causes increasing the peak load while the value of 1/B is constant until it reaches its maximum value when A=B.As mentioned above the load ratio increases when 1/B is significantly increased with constant value of 1/A.In general, the peak load will decrease (practically is preferable) when the value of A and B is large; if there are no technical limitations.

Conclusions
The result of applying the present prediction method of peak load estimation reveals the significant difference between the two method of calculation and the probability of wrong estimation of the load in large cold stores which causes a loss of money by choosing over size machine to overcome the load due to the overestimate of the cooling load.PDF created with pdfFactory Pro trial version www.pdffactory.com

Eng)
doi.org/10.30684/etj.27.10.17 2412-0758/University of Technology-Iraq, Baghdad, Iraq This is an open access article under the CC BY 4.0 license http://creativecommons.org/licenses/by/4.0 of the internal air (inside cold store condition) (J/kg) K : Thermal conduction coefficient for materials forming the wall (W/m.C o ) m : Daily storing mass in aday (kg) m Tot : Total capacity mass storage( kg) between two sides of wall, roof and ground ( C o Is calculated from the knowledge of store's volume and the number of air change in a day as shown below ( And two new factors: * Factor -A : It represents number of days necessary for cooling the product (storage) up to the required temperature depending upon the daily temperature differences ( ∆ T) only i.e. : 20 × 1000 × 18 / 24 = 1500 W/day PDF created with pdfFactory Pro trial version www.pdffactory.comload distribution Curves A collection of different cases of storing were taken depending upon storing masses and the daily temperature difference which covered most of the expected states in the loads of cold stores.Table -2 shows those chosen cases and the two factors A and B.

Table ( 3) A summary of the result case Peak load day Nominal load starting day Peak load (W)
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