Analysis of refrigeration load in potato cold storage under practical operating conditions

Introduction

The foundation for planning and running any kind of cold storage is based on a precise assessment of the refrigeration loads under the prevailing practical conditions. In the older potato cold storage units, the design and selection of refrigeration system is based on the common refrigeration load calculation practice wherein a constant daily drop in product temperature under the fixed product loading rate had been considered. The potato cold storage units set up before the year 2010 used to follow this approach for design and selection of refrigeration system components. Subsequently, the refrigeration load has been estimated as per criteria prescribed in National Horticulture Board (NHB) standards NHB -CStype 01 -2010 (Anonymous, 2010) [1] . In NHB standards, a constant daily loading rate (4 -5%) and temperature drop of 10 °C in the first day and thereafter 1 o C per day is prescribed. However, these conditions of loading and cooling rates are being rarely followed by commercial potato cold storages. The prevalent practical loading pattern is quite uneven and hence the cooling rates also varied widely from the suggested one. The daily loading rate of potatoes in cold storage directly correlates with the arrival of the goods, which also fluctuates widely throughout the loading period. Because of this erratic loading pattern, energy demand fluctuates sharply, storage air temperature is uncontrolled, and cooling rates are unequal (Chourasia et al. 2004a) [6] . Many methods are available in the literature for calculation of refrigeration load for various types of refrigerated facilities. The different aspects of cooling load estimation of a potato cold storage and presented the ideas about possible efficiency improvements in existing cold storages (Islam et al., 2013) [7] . Using energy simulation, the refrigeration load assessed in cold storage facilities and used phase change materials to build energy-efficient plant operations (Joybari et al., 2015) [8] . Sari and Pratami (2018) [12] analysed the overall cooling load by using the concept of cooling load temperature difference (CLTD). Using a CFD tool, Tasiya, and Gajdhar (2021) [14] concentrated on the temperature distribution of fresh fruit and vegetable cold rooms and computed heat load calculations. Therefore, the refrigeration load [11] . Concentrating on the relevant work for the Indian potato cold storage system, the entire refrigeration demand in potato cold storage during a brief cooling period was explicitly (Chourasia et al., 2004b) [4] . estimated the effects of loading patterns and cooling rates on moisture loss and refrigeration load requirements using the CFD technique (Chourasia and Goswami, 2008) [5] . Paranjpey (2014) [9] presented the refrigeration load calculation for potato cold storage as per NHB standards -CStype 01 -2010. Chaudhary and Sharma (2015) [3] presented a thermal system design of a potato cold storage based on common cooling load calculation practice. However, none of these cited refrigeration load calculation approaches considered the conditions of variable daily loading rates that prevailing in Indian potato cold storage industry. Therefore, attempts have been made in the present study to estimate the actual refrigeration load under the prevalent operating conditions. The transient refrigeration load was estimated based on actual loading patterns collected from the potato cold storages during the loading season and compared with that of calculated as per criteria prescribed in NHB standards as well as common calculation practice.

Materials and Methods

This chapter outlines the theoretical framework and methodology used to study the effect of various practical operating parameters on the transient refrigeration loads in the cold storage. For this purpose, a commercial potato cold storage unit of 18600 metric ton was selected to collect relevant data on operational, design and constructional features to use as input parameters for calculation of the refrigeration load.

Theory

The relevant formulae that incorporate variable loading and cooling rates, along with their associated attributes to calculate the transient refrigeration load for a potato cold storage facility are described below.

Structural heat leakage load

The heat transferred through the cold storage structure is calculated using the following relationship (Prasad, 2003) [

where Q s (kW) is total structure heat leakage load; q x (kW) is heat leakage through the x th surface (walls, ceiling and floor) of the cold storage; U x (Wm -2 K -1 ) and A x (m 2 ) are the overall heat transfer coefficient and exposed surface area of the x th surface, respectively; ∆T x ( o C) is temperature difference across the x th heat transfer surface; T s,x ( o C) is temperature increment for solar radiation exposure of the x th surface.

Product load

The product is made up of sensible heat, which lowers the product's temperature from its starting point to its final storage temperature, and heat of respiration, which uses less heat to evaporate the product's moisture. Chourasia et al.

(2004a) [4] evaluated the product load on a specific day (t).

where Q p is product load (kW); m p is mass of the product loaded on a particular day (kg); C P is specific heat of product (kJ /kg ˚C); ∆T p is temperature of the product (˚C); (q res ) t is metabolic heat of respiration (kW); m l is rate of moisture loss from the product (kg / kg day); (h eva ) t the latent heat of vaporization (kJ).

Infiltration load

The infiltration heat load was calculated from (Stoecker, 1998) [13] .

where Q inf is infiltration load (kW); C inf is infiltration coefficient (√ms -1 ); A d is surface area of the door (m 2 ); H is height of door (m); ρ i is density of inside air (kg m -3 ); ρ o is density of outside air (kgm -3 ); H o is enthalpy of corridor air (kJkg -1 dry air); H i is enthalpy of inside air (kJkg -1 dry air); t is door opening time (hour).

Ventilation load

The calculation of ventilation load can be done by using following equation Prasad (2003) [10] .

where Q v is ventilation load (kW); n is number of air exchange in a day; V is volume of cold storage (m 3 ).

Load due to human

The calculation of load due to human can be estimated (Stoecker, 1998) [13] .

where Q h is load due to human being (kW); n p is number of persons; h G is heat generated per person (kW); t w is working time in a day (hour).

Load due to electrical appliances

The load caused by electrical appliances, which includes lights, fans, and other electrical devices occurred only during the loading time in a day. Nonetheless, because the fans run constantly, the heat is released throughout the day.

Using the relationship below, the cooling load from lights, fans was computed (ASHRAE, 1989) [2] .

where, Q e is load due to electrical appliances (kW); I is required light intensity (Wm -2 ); A f is floor area (m 2 ); F n is number of fans in a row, r n is number of rows; W F is fan wattage. The total refrigeration load (Q T,t in kW) due to all components on a particular day (t) can be computed from the summation of all the individual components as given by equation 1 to 6. The total refrigeration load,

Methodology 2.2.1. Data collection

A commercial potato cold storage unit was visited in Hooghly district of West-Bengal state to collect the relevant inputs for calculation of the transient refrigeration load using the equations 1 to 7. All the necessary data like structural details, daily loading rate, prevalent temperature and relative humidity of air, details of electric appliances (like, number of fans, lights, etc and their wattage) were collected from cold storage for the calculation of refrigeration load under the prevalent operating conditions. A summary of the collected data from the cold storage unit are presented in table 1.

Estimation of transient refrigeration load under the variable loading and cooling rates

The actual loading pattern is quite irregular in nature which causes the drift in refrigeration demand from the estimated one (Chourasia et al., 2004b) [6] . Therefore, daily loading rate data were collected from a cold storage to estimate the total refrigeration load during transient cooling under the various combinations of loading pattens and cooling rates as given in table 2.

Results and Discussion

Loading pattern of cold storage unit 3.1.1. Total loading in the cold storage

Structural details and other relevant design parameters of cold storage unit selected to analyse the actual refrigeration load under the prevalent operating conditions are shown in table 1. The actually stored capacity for the cold storage unit was found to be different from the designed capacity as can be seen in table 1.

The actual daily product loading rate in cold storage unit is presented in fig 1 . It can be observed that the loading pattern is quite irregular and daily loading rate varied abruptly from the constant loading rate. Also, the loading duration was found to vary from 26 to 31 days in comparison to prescribed period of 20 to 25 days (Paranjpey, 2014) [9] . This, irregular loading pattern is one of the main causes behind the fluctuations in storage air temperature and poor performance of the cold storage in term of product quality.

Cumulative loading in the cold storage

The cumulative percentage quantity of product loaded with storage period is shown in fig 2 . During the loading period of 31 days, three distinct phases (initial, middle and last) of loading durations were observed. In the initial phase of 5 days loading, the loading rates increased slowly and speed up during the middle phase between the 5-20 days of time span. In the last phase (20-31 days), the loading rate was again found to decrease. wherein three distinct phases can be observed. In the initial phase, the refrigeration was found to increase sharply for first 9 days of the loading period, thereafter, increased slowly and attained the peak load value on last day of loading similar to constant loading and NHB cooling pattern. Thereafter, from 25 th day, it decreased sharply to a stable value of steady state refrigeration load. The observed loading patterns in the cold storage unit was specific and quite irregular in nature, in which, the daily loading rate varied abruptly from the constant loading rate. Also, the loading duration was found to vary from 26 to 31 days in comparison to prescribed period of 20 to 25 days and the loading of product completed in a maximum of three distinct phases. Following the trend of loading pattern, the estimated daily transient refrigeration load was also quite uneven for the cold storage unit. This indicated the dominancy of loading pattern, in comparison to cooling rates, on the refrigeration load. The refrigeration load has shown a tendency to increase with time up to middle of storage period and then declining to a steady value. For this cold storage unit, the peak refrigeration load value was about 20% higher for actual loading rate in comparison to constant loading rate.

Total transient refrigeration load in cold storage for different cooling and loading patterns

Acknowledgement

AcknowledgmentsSincere thanks are due to the Santa Cold Storage (P) Limited, Pandua, district Hooghly in West Bengal state, India for allowing us to collect data and providing all the possible help during the data collection in the cold storage.

References

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