Lab Station: Air Heater

Description of the system

Figure 1 shows an air tube with heater and temperature sensor(s). University College of Southeast Norway, Porsgrunn, has 26 copies of this lab station, being used in several control courses in both bachelor and master programmes in technology.

Figure 1

Video presenting the air heater

Mathematical model

A mathematical model that has proven to describe quite well the dynamic behaviour of the outlet air temperature (where one of the temperature sensors are mounted) in simulations is as follows:

T_out = T_env + T_heat

where

• T_env  is the environmental (room) temperature. It is the temperature in the outlet air of the air tube when the control signal to the heater has been set to zero for relatively long time (some minutes).

• T_heat is the additive contribution to the total temperature T_out due to the heater. T_heat is given by the following "time-constant with time-delay" differential equation model:

theta_t * d(T_heat)/dt = - T_heat + K_h * u(t-theta_d)

where

• u [V] is the control signal to the heater.

• theta_t [s] is time-constant.

• K_h [deg C / V] is heater gain.

• theta_d [s] is time-delay representing air transportation and sluggishness of the heater.

In a simulator based on this model a proper initial value of the state variable T_heat must defined. If you assume that the heater has been turned off for a while, you can set the initial value to zero.

You can adjust the parameters of the model by some simple experiments where you run the simulator in parallel with the real process. Experiments have shown that reasonable parameter values are

• K_h = 3.5 K/V

• theta_t = 23 sec

• theta_d = 3 sec

Technical information

Each air heater consists of the following items:

1. One plywood plate on which the devices are mounted

2. Plastic box containing all electrical devices

3. One plastic tube

4. One air fan (originally a PC fan)

5. One potensiomter (variable resistance) for manual adjustment of the voltage controlling the fan speed.

6. One electric power cable (for connection to mains outlet, e.g. 220 V)

7. Two temperature sensors, type Pt100, with measurement signal converter from resistance to current: INOR miniPack-L

8. One heating element (coil) for electric heating of air. The coil is originally used in a shoe dryer. Power (assuming 220 VAC) is 250 W.

9. One electrical AC-DC converter from 220 VAC to 24 VDC. Datasheet_power_supply.pdf

10. One Pulse-width modulator (PWM): Carlo Gavazzi RN F23V30. Datasheet_ssr_pwm.pdf

Publication

• F. Haugen, Fjelddalen E, Edgar T., Dunia R., Demonstrating PID Control Principles using an Air Heater and LabVIEW,

Updated August 24, 2016 by Finn Aakre Haugen. E-mail Finn.Haugen@hit.no.