Department of Technology

Lab Station: Air Heater

Description of the system

Figure 1 shows an air tube with heater and temperature sensor(s).

Figure 1

Here are features of the lab station:

  • Fan: A fan makes air flow through the tube. The fan is operated manually with a knob. The fan position is indicated or measured by a voltage signal which is in the range 2 - 5 V (min, max fan speed). This voltage signal can be measured between two terminals. The normal fan speed is defined to be the maximum speed. (We do not know the actual volumetric flow, but this information is not necessary here.)

  • Heater: The air is heated by an electrical heater. The supplied power is controlled by an external voltage signal in the range 0 - 5 V (min power, max power). This heater control signal must be applied between two terminals. This voltage is used to control a Pulse Width Modulator (PWM) which connects/disconnects the mains voltage to the heater.) The PWM signal is indicated by a lamp. The PWM device requires 24 VDC power supply, which is produced by an AC/DC converter.

  • Temperature sensors: Two Pt100 temperature elements are available. You can use any of these, but Temperature sensor 1 may be regarded the default sensor. These two sensors have been calibrated equally. The sensor signals are available as voltage signals at their respective terminals. The range is 1 - 5 V, and this voltage range corresponds to the temperature range 20 - 50 oC (with a linear relation). The normal sensor position is defined to be the outermost position in the tube.

  • Analog I/O device: Figure 1 shows the NI USB-6008 device, but any I/O device supporting the above voltage ranges can be used.

  • Controllers: A temperature control system can be implemented using one of the PID control functions in LabVIEW. Alternatively, an industrial process controller can be used, for example the Fuji PYX5 PID-controller.

Mathematical model

A simple mathematical model that gives an ok description of the air temperature at the tube outlet (where one of the temperature sensors are mounted) is as follows:

Tout = Tenv + Theat

Here:

  • Tenv  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).

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

thetat * d(Theat)/dt = - Theat + Kh * u(t-thetad)

Here,

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

  • thetat [s] is time-constant.

  • Kh [deg C / V] is heater gain.

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

In a simulator based on this model a proper initial value of Theat should added. (This initial value is applied to the integrator in a block diagram representation of the differential equation given above.) 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. Reasonable values are

  • Kh = 4 K/V

  • thetat = 20 sec

  • thetad = 2 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

Updated 20. February 2010 by Finn Haugen. E-mail Finn.Haugen@hit.no