Calculating Rate of Heat Transfer Between Two Working Fluids of a Heat Exchanger

Summary

This video tutorial explores the principles of heat exchangers, specifically focusing on the transfer of heat between hot and cold fluids. The script uses an example of oil as the hot fluid transferring energy to water as the cold fluid. Key concepts such as mass flow rate, specific heat capacity, and temperature changes are explained in detail. The video also demonstrates how to calculate the mass flow rate of water and determine changes in outlet temperatures. The tutorial emphasizes the importance of efficiency and sign conventions in heat transfer calculations.

Takeaways

• 😀 Heat exchangers transfer heat energy from a hot fluid to a cold fluid, with the hot fluid losing heat and the cold fluid gaining it.
• 😀 In the example, oil is the hot fluid, transferring heat to water in the heat exchanger.
• 😀 The heat exchanger is assumed to be 100% efficient, meaning all the energy from the hot fluid is transferred to the cold fluid.
• 😀 The rate of heat transfer can be calculated using the formula: mass flow rate × specific heat capacity × change in temperature (ΔT).
• 😀 The mass flow rate of the cold fluid (water) can be calculated by equating the heat transfer rate from the hot fluid to that of the cold fluid.
• 😀 In the example, the mass flow rate of oil is 0.25 kg/s, with a specific heat capacity of 2150 J/kg°C and a temperature change of 25°C.
• 😀 For water, the mass flow rate can be found by rearranging the formula for heat transfer, resulting in a value of 0.16 kg/s.
• 😀 Changing the outlet temperature of the oil affects the outlet temperature of the water. A lower oil temperature results in higher water outlet temperatures.
• 😀 The formula for heat transfer is adjusted to account for the sign convention: negative for heat loss from the hot fluid and positive for heat gain in the cold fluid.
• 😀 In the second scenario, where the oil's outlet temperature is reduced to 42°C, the water's outlet temperature increases, showing a higher heat transfer rate.

Q & A

What is the fundamental purpose of a heat exchanger as described in the script?

A heat exchanger transfers heat energy from a hot fluid to a cold fluid, where the hot fluid gives up heat and the cold fluid gains that heat.

Why might oil be cooled using a heat exchanger in the example?

Oil may be cooled to prevent overheating of equipment or to recover heat energy for use in heating water or other processes.

What assumption is made about the efficiency of the heat exchanger in the first example?

The heat exchanger is assumed to be 100% efficient, meaning all heat lost by the hot fluid is gained by the cold fluid.

What formula is used to calculate the rate of heat transfer in the script?

The rate of heat transfer is calculated using: mass flow rate × specific heat capacity × temperature change (T2 - T1).

How is the mass flow rate of water determined in the first calculation?

By equating the heat lost by the oil to the heat gained by the water and solving for the water’s mass flow rate.

What is the calculated mass flow rate of water in the first scenario?

The mass flow rate of water is calculated to be 0.16 kg/s.

Why is a negative sign introduced in the second example when calculating heat transfer?

The negative sign indicates that the hot fluid (oil) is losing heat, ensuring consistent sign convention between heat loss and heat gain.

What change is made to the hot fluid conditions in the second example?

The outlet temperature of the oil is reduced from 50°C to 42°C, increasing the heat transfer rate.

What effect does lowering the oil’s outlet temperature have on the water’s outlet temperature?

Lowering the oil’s outlet temperature increases the water’s outlet temperature because more heat is transferred to the water.

What is the calculated outlet temperature of the water in the second scenario?

The water exits the heat exchanger at approximately 41.40°C.

Why does the rate of heat transfer increase in the second example?

Because the temperature drop of the oil is greater, resulting in a larger temperature difference and therefore more heat transferred.

What will be discussed in the next tutorial after this script’s content?

The next tutorial will cover heat exchangers that use refrigerants, including phase changes within the heat exchanger.