# Experimental Analysis of a Spray Hydrocooler with Cold Energy Storage for Litchi

^{1}

^{2}

^{*}

## Abstract

**:**

^{−1}), and litchi loads (8–28 kg) with one-third TES capacity. Results showed that: (1) the spray hydrocooler allows for the rapid and effective precooling of litchis within 15 min after harvest; (2) the hydrocooler can precool 299 kg litchis with one-third TES storage, meeting the precooling requirements; (3) the effective TES capacity achieved 1.25 × 10

^{8}J at the maximum TES capacity of the hydrocooler, while the energy efficiency ratio (EER) is 2; (4) the precooling capacity was maximum and the average power consumption was minimum when the litchi load was 23 kg and the spray flow rate was 30 L min

^{−1}. Longer charging time is the most important factor in increasing the precooling capacity and reducing the average power consumption. It provides feasible precooling equipment for rapid precooling in litchi-production regions.

## 1. Introduction

## 2. Mathematical Model of the Hydrocooler

#### 2.1. Mathematical Model of TES System

#### 2.2. Thermal Resistance of Ice-on-Coil

_{ice}increases and R

_{siw}decreases with increasing ice thickness when the coil radius remains unchanged.

#### 2.3. Mathematical Model of Refrigeration System

#### 2.4. Solution of Mathematical Model

^{7}J. Solving Equations (2)–(7), the TES capacity ${Q}_{chr}$ of the TES tank was 2.4 × 10

^{8}J, and the ice volume reached 256 kg.

^{−2}K

^{−1}, and the total thermal resistance is 0.056 m

^{2}K W

^{−1}when ice is not formed. The heat transfer coefficient decreases rapidly with ice thickness when the ice thickness is less than 40 mm. The heat transfer coefficient decreases significantly with ice thickness when the thickness is greater than 40 mm. The total thermal resistance is 0.143 m

^{2}K W

^{−1}, and the heat transfer coefficient is 6.995 W m

^{−2}K

^{−1}, which is 60.9% lower than that without ice when the ice thickness is 40 mm. Once the ice thickness exceeds 120 mm, the heat transfer coefficient reduces slowly.

^{2}for a single refrigeration unit. Solving for Equation (19) determines that the evaporation heat transfer area of the evaporator should be greater than 0.84 m

^{2}for a single refrigeration unit. The length of the evaporation coil was 19.75 m of a single refrigeration unit, so the outer surface area of the evaporation coil of a single refrigeration unit was 1 m

^{2}, which met the requirements of evaporation heat transfer area. The solutions of equations and determination of ice thickness are summarised in Table 2.

## 3. Design and Fabrication of Hydrocooler

#### 3.1. Overall Structure

^{2}heat transfer area each were used. According to Section 2, the heat transfer area, A

_{eva}, of the evaporator exceeded 1 m

^{2}, and the coil length, L, was not less than 39.6 m. Here, the length of the evaporation coil of the two refrigeration units was 43.4 m. The circulating water pipe with a diameter of 32 mm and a Weller SD-750 variable frequency water pump was selected, and the maximum flow reached 200 L min

^{−1}. The spray precooling method was adopted to realize the rapid precooling of the fruit. The sprinkler was selected as the spray structure, which had high uniformity and pressure regulation characteristics. The size of a single sprinkler was 0.25 × 0.25 × 0.006 m

^{3}. The diameter of nozzle was 1.5 mm, which was made of silica gel. The spray structure was made up of six sprinklers. The size of the spray precooling box was 0.75 × 0.52 × 0.45 m

^{3}; two precooling baskets could be placed in it at the same time. The size of the cold storage tank was 1.7 × 0.85 × 1 m

^{3}, and the maximum water capacity was 1.2 m

^{3}. The parameters of the spray hydrocooler are shown in Table 3.

#### 3.2. Structure of TES System

#### 3.3. Structure of Spray Hydrocooling System

^{3}, and each basket could hold 11.5 kg of litchi. Figure 6 shows the schematic diagram and photograph of the spray precooling box. Sprinklers were mounted at the top and a perforated plate was used at the bottom of the spray hydrocooling box. The outside of the box was covered with 1 cm thick insulation cotton. Six square stainless-steel sprinklers had nozzles made of silica gel. The nozzles were 1.5 mm in diameter under zero-pressure conditions and could enlarge with water pressure. The spray flow rate was controlled by the variable frequency water pump. Litchi packed in perforated baskets was kept in the spray precooling box for precooling. The cold water sprayed over the litchi initiates rapid heat exchange. The water then flows through the perforated bottom plate and enters the TES tank.

## 4. Charging Test and Litchi Spray Precooling Tests

#### 4.1. Experimental Setup

#### 4.2. Charging Test

#### 4.3. Litchi Spray Precooling Tests

^{−1}) was conducted at the central location, and then the ranges of the three parameters in both directions were explored. The spray precooling test was to study different test conditions and identify the best operating parameters of the spray hydrocooling box. Five litchis were selected from the middle layer of the litchi basket for monitoring the temperature of the pumps, and PT100 temperature sensors were inserted in litchi pulp, respectively. The litchi was taken out when the pulp temperature dropped to 8 °C and was replaced with the litchi to be precooled. This was repeated until the water temperature of the TES tank reached 7 °C, ending the precooling test.

#### 4.4. Evaluation of Spray Hydrocooler Performance

_{wat}) was measured at three different layers (upper, middle, and lower) in the TES tank with nine temperature sensors in each layer. In the charging process, the water temperature of each layer was considered the average value. During litchi precooling, the water temperature was represented by the mean value of these 27 temperature sensors.

_{ice}, as shown in Figure 2, was the average value of the ice thickness.

_{chr}) indicated the sensible heat released by the water cooling and latent heat released by the water freezing. It could be expressed as:

_{eff}) refers to the thermal energy storage that can be used by litchi precooling. It is the part that uses the TES capacity to subtract the part that cannot be used, and that is the part where the water temperature of the TES tank decreases from the initial temperature to the end of precooling temperature. It can be expressed as:

_{tal}) including water pump power consumption in the spray precooling test and compressor power consumption in the charging process can be expressed as:

_{spr}) refers to the total amount of litchi precooled until finishing precooling in one test.

_{spr}) refers to the time from the beginning of precooling to the end of precooling in one test, when the water reaches 7 °C.

_{spr}) is the ratio of precooling capacity to precooling time in one test.

_{kg}) is the power consumption per kilogram of litchi precooled. It is the most intuitive method to evaluate the cost of litchi precooling and an important index to evaluate the economy of precooling equipment. The average power consumption is given by:

## 5. Results and Discussion

#### 5.1. Experiment I: Charging of TES Tank

#### 5.1.1. Variation of Water Temperature and Ice Thickness

#### 5.1.2. Variation of TES Capacity and EER

^{8}J. The TES capacity was 2.47 × 10

^{8}J, which was twice that of the effective TES capacity. It can be seen from Section 2.4. that the design TES capacity of the device was 2.4 × 10

^{8}J, and the test value meets the design requirements. The remaining 1/2 TES capacity that cannot be utilized was used to reduce the water temperature from its initial state to 7 °C. If the remaining 1/2 TES capacity for reducing the water temperature of the TES tank can be reused, the power consumed by charging will be greatly reduced.

#### 5.2. Experiment II: Litchi Spray Hydrocooling Performance

#### 5.2.1. The Temperature of Litchi and Water

#### 5.2.2. Precooling Capacity and Precooling Rate

^{−}

^{1}was higher than other spray flow rates. When the spray flow rate was 50 L min

^{−}

^{1}, it had the minimum precooling capacity. This is because the cooling capacity stored in the storage box remains constant, and when the spray flow rate is higher, the power of the water pump increases, and the water circulation becomes faster, resulting in an increase in cooling capacity loss. The precooling rate at 50 L min

^{−}

^{1}was higher than other spray flow rates, while at 30 L min

^{−}

^{1}it had the minimum precooling rate. The precooling capacity and precooling rate for different spray flow rates had a small difference. The higher the flow rate, the more sufficient the contact between cold water and fruits/vegetables, thus accelerating the precooling speed. Due to the small temperature difference in the cold water, the precooling rate does not increase significantly. Overall, the precooling capacity clearly increased with an increase in charging time, and the precooling rate increased along with the litchi load until it reached less than 23 kg.

#### 5.2.3. Total Power Consumption and Average Power Consumption

^{−}

^{1}, 349 kJ kg

^{−}

^{1}, 250 kJ kg

^{−}

^{1}, and 193 kJ kg

^{−}

^{1}for charging times of 3 h, 4 h, 5 h, and 6 h, respectively. The average power consumption can be reduced by increasing the TES capacity within the limits of the hydrocooler capacity. Figure 13b,c shows a few examples of power consumption difference during 4 h of charging in different tests. There was likely an environmental effect under natural conditions. The maximum difference in energy consumption during the charging process was 2.52 × 10

^{6}J.

^{−1}. The minimum average power consumption is 250 kJ kg

^{−1}when the litchi load is 23 kg. As seen in Figure 13c, the minimum average power consumption of litchi was at a spray flow rate of 30 L min

^{−1}, and a 40 L min

^{−1}spray flow rate was a little higher. The average power consumption of 30 L min

^{−1}and 40 L min

^{−1}spray flow rates was 243 kJ kg

^{−1}and 253 kJ kg

^{−1}, respectively. The optimal spray flow rate is 30 L min

^{−1}in litchi spray precooling, followed by 40 L min

^{−1}.

## 6. Conclusions

^{−}

^{1}), and litchi loads (8–28 kg) with one-third TES capacity, and the following conclusions were drawn:

^{−1}, the spray hydrocooler achieves the largest precooling capacity and the smallest average power consumption in litchi spray precooling.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Abbreviation

Nomenclature | |

A | Area, m^{2} |

c | Specific heat, J kg^{−1} K^{−1} |

C | Latent heat, J kg^{−1} |

d | Diameter, m |

E | Energy consumption, J |

EER | Energy efficiency ratio |

E_{kg} | Average energy consumption of precooling 1 kg of litchi, kJ kg^{−1} |

E_{tal} | Power consumption of charging and precooling, J |

h | Heat transfer coefficient, W m^{−2} K^{−1} |

l | Length, m |

m | Weight, kg |

n | Number of precooling batches of litchi |

N_{0} | Theoretical energy consumption, W |

P | Heat load, W |

P_{0} | Refrigerating capacity of refrigeration system, W |

q | Heat load per unit area, W m^{−2} |

q_{0} | Refrigeration capacity per unit mass, kJ kg^{−1} |

q_{l} | Density of heat flow rate, W m^{−2} |

q_{m} | Mass flow, kg s^{−1} |

q_{V} | Refrigeration capacity per unit volume, kJ m^{−3} |

Q | Quantity of heat, J |

Q_{max} | The maximum TES capacity of TES tank, J |

Q_{chr} | TES capacity of charging, J |

r | Radius, m |

R | Heat conduction resistance, m^{2} K W^{−1} |

t | Time, s or h |

T | Temperature, °C |

TES | Thermal energy storage, J |

u | Precooling rate, kg h^{−1} |

v_{in} | Inspiratory specific volume, m^{3} kg^{−1} |

V_{R} | Volume flow rate, m^{3} s^{−1} |

V_{th} | Theoretical gas transfer volume, m^{3} |

w_{0} | Unit theoretical work, kJ kg^{−1} |

ΔT | Temperature difference, °C |

Greek symbols | |

α | Power coefficient of water pump |

β | Volumetric efficiency of compressor |

λ | Thermal conductivity, W m^{−1} K^{−1} |

δ | Thickness, m |

ρ | Density, kg m^{−3} |

Subscripts | |

amb | Ambient |

chr | Charging |

cic | The contact surface between the ice and coil |

coi | Evaporation coil |

com | Comprehensive |

con | Condenser |

ctp | The end of charging to the end of precooling |

cwp | Circulating water pipe |

eff | Effective TES |

eli | Energy lost through insulation |

eva | Evaporator |

itw | Initial temperature of water |

itl | Initial temperature of litchi |

iw | Ice and water |

ilt | Insulation layer of TES tank |

ils | Insulation layer of spray precooling box |

ilc | Insulation layer of circulating water pipe |

isc | Inner surface of coil |

iic | Inner surface of insulation layer of circulating water pipe |

lit | Litchi |

load | Load of litchi precooling |

lod | Cooling loss rate caused by opening the door |

oic | Outer surface of insulation layer of circulating water pipe |

osc | Outer surface of coil |

osi | Outer surface of ice |

otp | Operation of water pump |

siw | Contact surface between insulation layer and water |

spr | Spray precooling |

spb | Spray precooling box |

src | Contact surface between refrigerant and coil |

spa | Contact surface between spray precooling box and air |

sta | Contact surface between TES tank and air |

tal | Total |

thb | Thermal bridge |

tst | TES tank |

ttl | Termination temperature of litchi precooling |

ttw | Termination precooling temperature of water |

wat | Water |

wpp | Water pump power |

wpw | Water pump works on water |

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**Figure 3.**The variation of the total thermal resistance and heat transfer coefficient with the ice thickness.

**Figure 5.**Structure diagram of TES tank and refrigeration system. (

**a**) The main view and. (

**b**) The left view.

**Figure 6.**Schematic diagram and photograph of spray precooling box and cold-water circulation system.

**Figure 8.**Plots showing variation of (

**a**) water temperature and (

**b**) ice thickness of different layers with charging.

**Figure 12.**Variation of precooling capacity and precooling rate under different (

**a**) charging times, (

**b**) litchi loads, and (

**c**) spray flow rates.

**Figure 13.**Variation of total power consumption and average power consumption of spray hydrocooler under different (

**a**) charging times, (

**b**) litchi loads, and (

**c**) spray flow rates.

Parameters | Property | Values |
---|---|---|

A_{cwp} | Area of contact surface between circulating water pipe and air | 1.1 m^{2} (Calculated) |

A_{spb} | Area of contact surface between spray precooling box and air | 4 m^{2} (Assumed) |

A_{tst} | Area of contact surface between thermal storage tank and air | 10 m^{2} (Assumed) |

c_{lit} | Specific heat of litchi | 3.704 × 10^{3} J kg^{−1} K^{−1} [35] |

c_{wat} | Specific heat of water | 4.2 × 10^{3} J kg^{−1} K^{−1} [36] |

C_{ice} | Latent heat of water freezing | 3.35 × 10^{5} J kg^{−1} [36] |

h_{cic}h _{isc} | Thermal conductivity coefficient between the ice and coil Boiling coefficient of heat transfer on the refrigerant in the coil | 2300 W m^{−2} K^{−1} [36]4000 W m ^{−2} K^{−1} [36] |

h_{osi} | Convective heat transfer coefficient between ice and water | 400 W m^{−2} K^{−1} [36] |

h_{siw} | Convection heat transfer coefficient of water and thermal storage tank | 100 W m^{−2} K^{−1} [36] |

h_{spa} | Convection heat transfer coefficient of spray precooling box and air | 20 W m^{−2} K^{−1} (Measured) |

h_{sta} | Convection heat transfer coefficient of thermal storage tank and air | 6 W m^{−2} K^{−1} (Measured) |

m_{lit} | The mass of litchi precooling | 1000 kg (Assumed) |

m_{wat} | The mass of water in thermal storage tank | 1200 kg (Assumed) |

P_{lod} | Cooling loss rate caused by opening the door | 300 W (Assumed) |

P_{thb} | Power of heat bridge heat transfer | 200 W (Assumed) |

P_{wpp} | Water pump power in precooling | 280 W (Calculated) |

q_{con} | Heat load per unit area of the condenser | 260 W m^{−2} [34] |

q_{eva} | Heat load per unit area of the evaporator | 3500 W m^{−2} [34] |

r_{coi} | Radius of evaporation coil | 0.008 m (Assumed) |

r_{iic} | Radius of circulating water pipe | 0.032 m (Assumed) |

r_{isc} | Inner radius of evaporation coil | 0.007 m (Follow determination) |

r_{oic} | Radius of circulating water pipe insulation layer | 0.072 m (Follow determination) |

t_{chr} | Charging time of thermal storage | 12 h (Assumed) |

t_{ctp} | Time from the end of charging to the end of precooling | 12 h (Assumed) |

t_{otp} | Operation time of water pump | 8 h (Assumed) |

T_{amb} | The ambient temperature | 30 °C (Measured) |

T_{ilt} | The initial temperature of litchi precooling | 29 °C (Measured) |

T_{itw} | The initial temperature of water | 31 °C (Measured) |

T_{osi} | Outer surface temperature of ice | 0 °C (Assumed) |

T_{ttl} | The termination temperature of litchi precooling | 8 °C (Assumed) |

T_{ttw} | The termination precooling temperature of water | 7 °C (Assumed) |

T_{wat} | The water temperature of thermal storage tank | 15 °C charging (Assumed) 3 °C after charging (Assumed) |

α | The power coefficient of water pump | 0.57 (Measured) |

δ_{ils} | Insulation layer thickness of spray hydrocooling box | 0.01 m (Assumed) |

δ_{ilt} | Insulation layer thickness of thermal storage tank | 0.05 m (Assumed) |

λ_{coi} | Coil’s thermal conductivity | 377 W m^{−1} K^{−1} [36] |

λ_{ice} | Ice layer’s thermal conductivity | 2.22 W m^{−1} K^{−1} [36] |

λ_{ilc} | Insulation layer’s thermal conductivity | 0.038 W m^{−1} K^{−1} [36] |

λ_{ils} | Insulation layer’s thermal conductivity | 0.038 W m^{−1} K^{−1} [36] |

λ_{ilt} | Insulation layer’s thermal conductivity | 0.038 W m^{−1} K^{−1} [36] |

ρ_{ice} | Density of ice | 920 kg m^{−}³ [36] |

Parameters | Property | Values | The Reference Equations |
---|---|---|---|

Q_{lit} | Thermal energy absorbed by litchi precooling | 8.1 × 10^{7} J | Equation (1) |

Q_{chr} | TES capacity of charging | 2.4 × 10^{8} J | Equations (2)–(7) |

m_{ice} | Ice storage capacity of charging | 256 kg | Equation (8) |

δ_{ice} | Thickness of icicle | 0.04 m | Figure 3 and Equations (10)–(14) |

l_{coi} | Length of evaporation coil | 39.6 m | Equation (9) |

P_{0} | Refrigerating capacity of compressor | 5900 W | Equations (15)–(17) |

A_{con} | Heat transfer area of condenser | 22.6 m^{2} | Equation (18) |

A_{eva} | Heat transfer area of evaporator | 2 m^{2} | Equations (19) and (20) |

Component | Size or Model | Material or Company | Values |
---|---|---|---|

Compressor | QXL-30E | Zhejiang Boyang, China | 3180 W (−10 °C) |

Condenser | Small 3-HP | SIMCO | 18 m^{2} × 2 |

Evaporation coil | 0.016 m diameter | Copper | 43.4 m |

Circulating water pipe | 0.032 m diameter | PVC | 5 m |

Thermal energy storage tank | 1.7 × 0.85 × 1 m^{3} | Stainless steel | 1.4 m^{3} |

Spray precooling box | 0.75 × 0.52 × 0.45 m^{3} | Acrylic | 0.18 m^{3} |

Nozzle | 0.0015 m diameter | Silica gel | Variable size |

Sprinkle | 0.25 × 0.25 × 0.006 m^{3} | Stainless steel | 3–10 L min^{−1} × 6 |

Precooling basket | 0.35 × 0.48 × 0.16 m^{3} | Plastic | 200 g |

Water pump | SD-750 | Weller | Frequency conversion |

Test | Charging Time (h) | Litchi Load (kg) | Spray Flow Rate (L min ^{−1}) | Water Storage Capacity of TES Tank (kg) |
---|---|---|---|---|

1 | 12 | -- | -- | 1200 |

2 | 4 | 23 | 30 | 400 |

3 | 3 | 23 | 30 | 400 |

4 | 5 | 23 | 30 | 400 |

5 | 6 | 23 | 30 | 400 |

6 | 5 | 8 | 30 | 400 |

7 | 5 | 13 | 30 | 400 |

8 | 5 | 18 | 30 | 400 |

9 | 5 | 28 | 30 | 400 |

10 | 5 | 23 | 20 | 400 |

11 | 5 | 23 | 40 | 400 |

12 | 5 | 23 | 50 | 400 |

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## Share and Cite

**MDPI and ACS Style**

Huang, H.; Lv, E.; Lu, H.; Guo, J.
Experimental Analysis of a Spray Hydrocooler with Cold Energy Storage for Litchi. *Appl. Sci.* **2023**, *13*, 8195.
https://doi.org/10.3390/app13148195

**AMA Style**

Huang H, Lv E, Lu H, Guo J.
Experimental Analysis of a Spray Hydrocooler with Cold Energy Storage for Litchi. *Applied Sciences*. 2023; 13(14):8195.
https://doi.org/10.3390/app13148195

**Chicago/Turabian Style**

Huang, Hao, Enli Lv, Huazhong Lu, and Jiaming Guo.
2023. "Experimental Analysis of a Spray Hydrocooler with Cold Energy Storage for Litchi" *Applied Sciences* 13, no. 14: 8195.
https://doi.org/10.3390/app13148195