Heat Exchanger/Radiator Design for Drying Systems

JUNE FIRST WEEK

AIM

To design and analyze a finned-tube radiator capable of transferring heat from hot water to air and producing heated air suitable for drying applications.

BACKGROUND

Drying is one of the most widely used methods for preserving agricultural products. The efficiency of a drying system depends on its ability to remove moisture from the substrate. Which in turn depends on the airflow, air humidity, and air temperature. Vigyan Ashram is looking to build a system where hot water, either from Solar water heaters or biomass gasifiers, can be used as a heat source for drying applications. A radiator acts as a heat exchanger that transfers thermal energy from hot water to air, which in trun will act as drying media. Therefore, proper radiator design is essential for achieving energy-efficient and uniform drying.

INTRODUCTION

A radiator is a heat exchanger designed to transfer heat from a hot fluid to a colder fluid without direct contact. In drying, we envisage applications where hot water flows through tubes while ambient air passes over finned surfaces. Heat is transferred from the water to the tube walls, then through the fins, and finally to the air.

The use of fins significantly increases the available heat transfer area and arrangement of tubes breaks laminar resulting in higher thermal efficiency. The design of a radiator involves selecting appropriate tube dimensions, fin geometry, tube arrangement, and flow rates to achieve the desired outlet air temperature.

OBJECTIVES

1. To understand the working principle of a radiator as a heat exchanger.
2. To calculate the heat transfer rate required for air heating.
3. To determine radiator dimensions such as tube length, tube diameter, fin spacing, and fin thickness to suit the objective no. 2
4. Design and fabricate the said system
5. To evaluate the effect of airflow and water flow rate on heat transfer performance.
6. To optimize the radiator design for maximum thermal efficiency.

WORKING

The radiator functions as a heat exchanger between hot water and air. Hot water enters the radiator tubes at a high temperature (around 80–90°C) and flows through the tube network. Simultaneously, ambient air is forced across the finned tubes using a blower or fan.

As the hot water flows inside the tubes, heat is transferred to the tube walls by convection. The heat then conducts through the tube material and spreads into the attached fins. Because fins provide a much larger surface area than plain tubes, they significantly enhance heat transfer to the surrounding air.

The moving air absorbs this heat and its temperature increases before entering the drying chamber. Meanwhile, the water temperature decreases as it releases heat, typically leaving the radiator at 50–60°C.

For improved performance, tubes are often arranged in a staggered pattern. This arrangement increases air turbulence, improves contact between air and the heat transfer surfaces, and enhances overall heat transfer efficiency.

The heat transfer process occurs are:

1. Convection from hot water to the inner tube surface.
2. Conduction through the tube wall and fins.
3. Convection from the outer finned surface to the airflow.

JUNE SECOND WEEK

Design Requirements and Parameters

Hot Water Side Parameters

• Inlet water temperature: 80–90°C
• Outlet water temperature: 50–60°C (???)
• Water flow rate: To be determined based on heat demand
• Working fluid: Water
• Operating pressure: As per system requirements

Air Side Parameters

• Inlet air temperature: Ambient temperature
• Desired outlet air temperature: Approximately 65°C
• Air flow rate: Depends on the rateof loss of moisture/drying rate

Geometrical Parameters

• Tube diameter
• Tube wall thickness
• Tube length
• Number of tubes
• Tube pitch
• Number of tube rows
• Fin thickness
• Fin spacing
• Fin height

Radiator Design Calculation

Step 1: Moisture Removal Requirement

Assume:

• Moisture to be removed = 40 kg
• Drying time = 6 hours

Moisture removal rate:$$\dot m_w=\frac{40}{6}$$$$\dot m_w=6.67\;kg/h$$$$=0.00185\;kg/s$$

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Step 2: Energy Required for Water Evaporation

Latent heat of vaporization of water:$$h_{fg}=2257\;kJ/kg$$

Heat required:

$$Q=\dot m_w\times h_{fg}$$ $$Q=0.00185\times2257\times1000$$$$Q=4175\;W$$

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Step 3: Include System Losses

Dryers experience losses due to:

• Wall losses
• Air leakage
• Tray heating
• Product heating

Assume:$$20\%$$

Additional heat.$$Q_{total}=1.2\times4.2$$$$Q_{total}=5.0\;kW$$

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Step 4: Water Temperature Selection

Assume solar hot water or boiler water:

Parameter	Value
Water inlet	90°C
Water outlet	70°C

Therefore:

ΔTw​=20∘C

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Step 5: Water Flow Rate

Using:$$Q=\dot m C_p \Delta T$$

where$$C_p=4180\;J/kgK$$$$5000=\dot m\times4180\times20$$ $$\dot m=0.0598\;kg/s$$ $$\dot m=3.59\;L/min$$

Water Flow Required :

$$\boxed{3.6\;L/min}$$

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Step 6: Air Flow Rate

Air temperatures:

Parameter	Value
Air inlet	30°C
Air outlet	65°C

$$\Delta T_a=35^\circ C$$

Using:$$Q=\dot m_a C_p \Delta T$$ $$5000=\dot m_a\times1005\times35$$$$\dot m_a=0.142\;kg/s$$

Air density:$$\rho=1.15\;kg/m^3$$$$\dot V=\frac{0.142}{1.15}$$$$\dot V=0.124\;m^3/s$$

Air Flow Required

$$\boxed{450\;m^3/h}$$

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Step 7: LMTD Calculation

Counter-flow arrangement:$$\Delta T_1=90-65=25^\circ C$$ $$\Delta T_2=70-30=40^\circ C$$$$LMTD= \frac{40-25} {\ln(40/25)}$$$$LMTD=31.9^\circ C$$

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Step 8: Required Heat Transfer Area

Assume overall heat transfer coefficient:$$U=50\;W/m^2K$$ $$A=\frac{Q}{U\times LMTD}$$$$A=\frac{5000}{50\times32}$$$$A=3.125\;m^2$$

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Step 14: Fan Selection

Required airflow:$$450\;m^3/h$$

Convert to CFM:$$450\times0.5886$$ $$265\;CFM$$

Conclusion

The radiator was designed based on the heat required to evaporate 40 kg of water in 6 hours from a 100 kg dryer batch. The calculated heat load of approximately 5 kW can be achieved using a finned copper tube radiator with aluminum fins, hot water at 90°C, and an airflow rate of approximately 450 m³/h, producing drying air near 65°C. This design serves as the basis for the prototype and future scale-up of the dryer heating system.

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JUNE THIRD AND FOURTH WEEK

Design Discussion and Prototype Planning

After studying the fundamentals of radiator design and identifying the key parameters affecting heat transfer performance, I discussed the design approach with Dixit Sir. During the discussion, we reviewed important parameters such as tube diameter, tube length, tube arrangement, fin spacing, fin thickness, airflow rate, and water flow rate.

Based on the available resources and the need to validate the design experimentally, it was decided to first develop a 1 kW radiator prototype before proceeding towards a larger system for drying applications. The prototype will help in understanding the actual heat transfer performance, pressure losses, fabrication challenges, and overall system efficiency.

Following this discussion, the design parameters required for the 1 kW prototype were finalized. The next step involved identifying and sourcing the necessary materials, including copper tubes, aluminium fins, manifolds, fittings, blower components, and supporting structural materials. Material specifications and availability were then studied to prepare for the fabrication stage of the prototype.

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Proposed Parameters for the 1 kW Radiator Prototype

Parameter	Value
Heat Transfer Capacity	1 kW
Hot Water Inlet Temperature	80–90°C
Hot Water Outlet Temperature	65–70°C
Air Inlet Temperature	Ambient
Target Air Outlet Temperature	55–65°C
Tube Material	Copper
Tube Inner Diameter	8mm
Tube Outer Diameter	10 mm
Tube Length	200 mm
Tube Arrangement	Staggered
Fin Material	Aluminium
Fin Thickness	0.16 mm
Fin Spacing	4 mm
Number of Fins	50
Horizontal Tube Pitch	25 mm
Vertical Tube Pitch	30 mm
Water Flow Rate	To be validated experimentally
Air Flow Rate	To be validated experimentally

These parameters were selected as an initial design basis and may be refined after fabrication and experimental testing of the prototype.

JULY FIRST AND SECOND WEEK

Material Procurement and Fabrication Progress

After finalizing the design parameters for the 1 kW radiator prototype, the required materials were identified and procured. The purchased materials included copper tubes, aluminium fin sheets (0.30 mm thickness), and other components required for fabrication.

Following material procurement, the fabrication of the radiator prototype was initiated. The first stage of fabrication involved preparing the aluminium fins according to the finalized dimensions. The fin cutting process was completed, followed by drilling the required holes for the copper tubes. These operations ensured proper alignment of the tubes and laid the foundation for assembling the finned-tube radiator.

With the fin cutting and drilling completed, the project is now ready for the next fabrication stages, including tube assembly, manifold fabrication, brazing, and final radiator assembly.