The Analysis of Heat Transfer on Hot Air Coffee Roasters at the Appropriate Technology Research Centre

Roasting occurs due to heat transfer from the heating surface into the material. Coffee roasting plays an important role in determining the chemical composition, aroma, and taste of coffee. There are two main types of coffee roasters, namely drum and hot air roasters. This research aims to determine the heat transfer in hot air coffee roasters. The data collected consists of roasting time, coffee bean temperature, roasting room temperature, fuel consumption and air flow speed in the cyclone system chimney and sample was carried out at medium roasting level, with three repetitions. The method used is theoretical analysis and experimental testing. The results showed that the heat transfer mechanism in hot air roasting is a conduction and convection mechanism. The amount of conduction and convection heat transfer in the coffee roasting equipment was carried out in three repetitions. It was also found that factors that influence the rate of heat transfer in hot air roasting equipment are due to conduction and convection, as well as influenced by heat transfer surface area, thermal conductivity, air flow speed from blower pressure and pressure on the gas fuel. The results of the research show that the heat transfer mechanism in hot air roasting is a conduction and convection mechanism. The amount of heat transfer in one repetition of the conduction quantity (Q) is 9222.8 W and the convection rate (Q) is 3209.4 W with an average efficiency of 34.7%, in two repetitions the conduction quantity (Q) is 9883.3 W and the convection rate (Q) is 3444.6 W with the average efficiency is 34.8%, at three repetitions the conduction (Q) is 9690.3 W and the convection rate (Q) is 1123.3 W, the average efficiency is 11.6%. Factors that influence the rate of heat transfer in hot air roasting tools are conduction and convection, heat transfer surface area, thermal conductivity, air flow speed due to blower pressure and pressure on LPG gas fuel.


INTRODUCTION
Coffee has become a popular drink all over the world.Many people enjoy the taste and aroma of coffee.Coffee processing to produce coffee rice can be classified into three types of processing, namely: dry processing, semi-wet processing and wet processing.one of them is dry processing by roasting coffee.
One of the important aspects in coffee production is the roasting process that affects the taste, aroma and final quality of coffee beans [1].Drying coffee that has been picked and sorted should be dried immediately so as not to undergo chemical processes that can degrade the quality.Drying can be natural and drying artificially [2].Roasting is a crucial stage in the coffee processing, that is aimed to change markedly the chemical, physical, structural and sensorial properties of the green beans by heat induced reactions.In this way roasting process makes coffee beans suitable for brewing.The green beans are, in fact, characterized by only a weak and greenly aroma and a hard texture hinders their use as food [3].The beans change color to a dark brown.Initially the process is exothermic [4].
The process of forming the aroma and flavor of coffee can not appear just by drying, but done using high temperatures.Aroma and flavor can be raised using the method of roasting or roasting.Roasting is the core of coffee bean production until it becomes powder, roasting is done using a tool called a roaster [5].
Roasting is a complex process involving the transfer of energy (from the roaster to the beans) and mass (moisture and volatile compounds from the beans to the environment) implicit in the main changes in coffee beans in terms of weight, density, moisture, color and flavor [6].The best coffee quality can be obtained based on the roasting temperature, roasting time, water content, caffeine, and chlorogenic acid.Existing research only indicates coffee quality based on roasting time [7].
Coffee roasting is a drying process to reduce the water content of coffee beans, usually indicated by weight loss at the end of the process.The roasting process is carried out using a specific temperature until the bean develops, giving the product a distinctive aroma [8].Classification of roasting by degree of color it is divided into three: light, medium and dark [9].The roasting process involves applying heat to the coffee beans to produce the chemical and physical transformations necessary to develop the desired flavor characteristics.In addition, roasting induces major physical changes within the coffee beans and determines their behavior in storage, grinding, and brewing.In the present understanding of the phenomena that appears during roasting [12].
Roasting is one of the important processes of coffee production, which significantly affects the quality of roasted coffee.There are many approaches to grilling, applying a rotating cylinder (the standard method of the grilling process), a fixed bowl, and a fluidized bed method [8].
In the coffee industry, roasting is a very important fight against coffee brewing.Some factors that need to be considered during roasting, including roasting machine, roasting time and temperature [13].Coffee roasting equipment is used to heat coffee beans to the right temperature with optimal timing.In a coffee roaster, heat transfer affects roasting efficiency and results.Therefore, the analysis of heat transfer in coffee roasting equipment is very important to understand in order to improve the roasting process.The results of research on heat transfer in coffee dryers show that modification of the design of the dryer can improve the efficiency of heat transfer and produce better quality coffee beans [14].Heat is defined as energy transferred by virtue of a temperature difference or gradient.It is a vector quantity, flowing in the direction of decreasing temperature, with a negative temperature gradient.In the science of thermodynamics, the important parameter is the quantity of heat transferred during a process [15].
The heat transfer that occurs during the roasting process takes place.In the study, researchers determining the effect of temperature on mechanical Coffee Roasters, because it is suspected that the effect of temperature on yield, processing capacity and water content in coffee [16].There are three main heat transfer means in a conventional coffee roaster, conduction, convection and radiation [17].
Heat transfer analysis in coffee roasters by measuring the temperature at various points on the roaster and analyzing the heat distribution during the roasting process.The study found that the right temperature profile and good temperature regulation are essential in producing coffee beans that are roasted evenly and with the desired quality [18].
Heat transfer in the process of heating coffee beans before roasting process by using mathematical models to predict the temperature profile and heat distribution during the heating process.

RESEARCH METHOD
In

RESULT AND DISCUSSION
Roasting is a thermodynamic process involving the application of a certain amount of heat energy to food ingredients which causes the absorption of heat from the roasting environment into the coffee beans.
The purpose of roasting is to bring out the flavor of the chemical components dissolved in the coffee which form the flavor of the coffee when it is brewed [19].This process is carried out as an effort to form the sensory character of coffee.The greater the temperature used when roasting, the less time required for roasting [20].
Based on the research conducted, it is stated that roasting at a temperature of 200℃ for 10 minutes can produce well-roasted coffee beans [21].
According to research [19], the roasting stage can cause several changes in coffee beans, including the following.
1.The color changes from green to yellow, (light brown), brown then black.
2. Increase the size of the coffee beans to twice the original size.
3. Density reduction in coffee beans to half the initial density.

A loud explosion (pop) occurs in the coffee
beans resulting from the release of pressurized gas and water vapor.
The essential things in the formation of sensory character during roasting are the transfer of energy (heat), the presence of water, and a reduction in the density of coffee, so that it can be explained about the stages that occur during the roasting process [22] Three important stages in the roasting process, namely drying, flavor and color development, and cooling [22].The hot air coffee roaster is a coffee roasting tool that uses a convection heat transfer mechanism using hot air flow in a pipe for the roasting process of coffee beans.
In its process, air is produced from pressure on the blower, the aim of which is to provide air pressure in the pipe so that air can flow into the coffee bean roasting room.To produce hot air in the pipe, there is a heating medium, namely a burner at the bottom along the heating pipe in the heating furnace.The presence of a combustion burner in the heating stove pipe burns along the heating pipe so that the air in the pipe also heats up and this hot air will later be used for the roasting process of the coffee beans.Heat transfer occurs when heat energy moves from one object to another object that has a lower temperature [24].
Heat transfer is the science of energy transfer in the form of heat which occurs due to temperature differences between objects or materials [25].In the energy transfer process, there is a speed of heat transfer that occurs, or it is known as the heat transfer rate.Heat transfer can be defined as a process of moving energy from one area to another due to differences in temperature in that area.There       From the results of the calculation of the heat transfer rate that occurred in the hot air roaster, which is viewed in terms of conduction and convection, the results of the heat transfer coefficient and average efficiency of the three treatments are known.In the 50th treatment at a burner temperature of 700℃, it was found that this type of research, the theoretical calculation analysis research is used, the results of theoretical calculations are used as parameters during testing.The test was carried out three times and the test results of the heat transfer performance on the hot air.Theoretical calculations require several data parameters, including the actual number of heating pipes in the furnace, pipe length, pipe diameter, blower air speed, burner gas temperature and roasting temperature to be achieved.The research on heat transfer analysis on hot air coffee roasters was carried out from December 2022 to June 2023 at the research center of Subang National Research and Innovation Agency.In carrying out this research requires the following tools and materials.
went through several stages of completion starting from observation, tools testing, and analyzing the heat transfer.

Figure 2 .
Figure 2. Flow diagram of research procedure

Table 2 .
Time required to reach the cracking stage on the roasting drum (Source:[23])

Figure 4 .
Figure 4. Coffee bean roasting levels (Source: [20]) are three main mechanisms in heat transfer: conduction, convection, and radiation.Conduction heat transfer is a heat transfer mechanism that occurs through direct contact between particles in an object or between two objects that are in contact with each other.In a coffee roaster, conduction can occur when the raw coffee beans in the tool come into contact with the hot surface of the tool.The heat will then travel through the coffee beans gradually, changing the temperature of the beans and causing the roasting process.The heat transfer that occurs in a radial cylinder system, such as in the construction of a heat exchanger in a hot air roaster, can be assumed by solving the following figure and equation.

3 .
Surface area (A), the greater the surface area, the faster the heat transfer.Convection is heating transfer that occurs through the movement of fluid or air that carries heat energy.In the context of a coffee roaster, convection occurs when hot air moves through the roaster and flows around the coffee beans.Hot air carries heat energy which then transfers heat to the coffee beans.Good air movement in the roaster is very important to ensure efficient and even heat transfer to all parts of the coffee bean.Judging from its working principle, the hot air roaster uses a blower to provide pressurized air to flow air into the roasting room, so the convection heat transfer in the hot air roaster is a type of forced convection.

Figure 7 . 2 . 3 .of pipes in row 1 𝑁2=
Figure 7. Convection Heat Transfer Process in Cylinders (Source:[28]) The process of heating and cooling a fluid flowing in a closed channel is an example of a heat transfer process.The heat transfer rate at that the results of theoretical calculations to determine the rate of conduction and convection heat transfer in a coffee roaster showed that the time required to reach 700℃ burner temperature was 7.8 minutes.The effect of the burner temperature on increasing the temperature in the heating furnace was 650℃ with a conduction heat transfer rate of (Q) 24131.1 W. It was also found that the effect of the temperature on the gas burner on increasing the temperature in the roaster room was 208.9℃ with a convection heat transfer rate to the roaster room of (Q) 3677.2W and it was also found that the convection heat transfer coefficient for coffee beans was (Q) 3926.6 W with an average heat transfer efficiency of 61%.From the results of the calculation of the overall heat transfer coefficient of the hot air type coffee roaster, it was found that the overall heat transfer rate was high in terms of the heating furnace (conduction) of the hot air roaster to transfer heat to the roaster chamber (convection) from three treatments.After knowing the overall heat transfer coefficient of conduction and convection, the average efficiency will then be calculated to determine the average heat transfer efficiency of the hot air coffee roaster in the three treatments.

Table 3 .
Test parameter results data

Table 4 .
Theoretical calculation of heat transfer in hot air roaster

Table 5 .
The results of heat transfer coefficient and average efficiency of the hot air roasters