This bench top computer controlled service unit enables the operation of the Armfield range of small-scale heat exchanger systems. It provides controlled cold water flow, bi-directional hot water flow and the instrumentation required to do a series of in-depth investigations into heat exchanger performance. The individual heat exchangers can be quickly changed over, to enable comparisons between different types of heat exchangers to be made. The HT30XC requires a personal computer with USB for the operator interface.
The service unit provides two fluid streams to the heat exchanger, a hot water stream and a cold water stream. The hot water stream is heated in a vessel fitted with an electric heater. The heater is switched on and off by a solid state relay (SSR) which is under software control. A thermostat limits the maximum water temperature to 85°C for operator safety. A gear pump circulates water from the vessel, through the heat exchanger and back into the heater vessel. Both the pump speed and direction are under software control, allowing for co-current and counter-current investigations over a wide range of flow rates.
The cold water stream is generated from a mains water supply. The flow through the heat exchanger is adjusted by a variable flow valve, again under software control. A manually adjustable pressure regulator is used to minimise the effect of mains pressure fluctuations.
Conditioning circuits for up to 10 K-type thermocouples are included, (the thermocouples themselves are supplied with the heat exchangers). The instrumentation also includes flow meters to measure the flow rates of the two fluid streams.
Switching on the unit puts it into ‘Standby’ mode. From this mode it is necessary for a regular series of pulses to be received from the software (via the built in USB interface) to fully power up the unit. This ensures that unless the control software is running, the heaters, the pump and the cold water control valve cannot be switched on. The unit also includes an emergency stop switch. All electrical circuits are located in a bench mounted ABS supporting base, and protected by a Residual Current Device for operator safety. The ABS base includes a drip tray and drain tap in case of water spillage or leakage.
Educational software is provided for all Armfield heat exchangers. Included are drivers to enable other software applications to communicate with the HT30XC via the USB interfrace. Users can also write their own software typically in LabVIEW™, MatLab™, 'C', 'C++', Visual Basic™, and Delphi™, among others, to remotely coontrol the equipment, for example, over a Local Area Network (LAN) or even over the internet. The HT30XC is ideal for this, as it has been designed to shut down safely in the event of a commuincations failure. Once the heat exchanger has been installed and configured, the student does not need to be present with the equipment.
The tubular heat exchanger is the simplest form of heat exchanger and consists of two concentric (coaxial) tubes carrying the hot and cold fluids. In these miniature versions the tubes are separated into sections to reduce the overall length and to allow the temperature at points along both fluid streams to be measured. Two versions are available, the HT31 is a basic version with two sections and a single interim temperature measurement point. The HT36 is a more sophisticated unit with four longer tube sections, giving four times the overall heat transfer area and three interim temperature measurement points in each fluid stream.
The HT36 has sufficient heat transfer area to demonstrate the classic counter current flow conditions where the outlet of the heated stream is hotter than the outlet of the cooled stream.
The plate heat exchanger is extremely versatile and commonly used in the food and chemical processing industries. Different combinations of plates and gaskets can be arranged to suit a particular application. The miniature exchanger supplied consists of a pack of plates with sealing gaskets held together in a frame between end plates. Hot and cold fluids flow between channels on alternate sides of the plates to promote heat transfer.
The shell and tube heat exchanger is commonly used in the food and chemical process industries. This type of exchanger consists of a number of tubes in parallel enclosed in a cylindrical shell. Heat is transferred between one fluid flowing through the tubes and another fluid flowing through the cylindrical shell around the tubes.
The miniature exchanger supplied is designed to demonstrate liquid-to-liquid heat transfer in a 1-7 shell and tube heat exchanger (one shell and seven tubes with two transverse baffles in the shell).
Vessel heating or cooling of a process liquid in a tank, either batchwise or with continuous product feed, is common practice throughout industry.
The characteristics of the heat transfer using an external jacket or internal coil can be demonstrated together with the effect of stirring the vessel contents.
The Cross Flow Heat Exchanger is commonly used in applications such as heating, ventilating and air conditioning. It is also encountered as vehicle engine radiator.
This type of heat exchange occurs when the flow direction of the two fluids cross each other. In the HT35, hot water flows in and out of a radiator, perpendicular to air stream, which is being pulled into the radiator by an axial fan.
The convection between the two fluids through fins surface on the radiator implements the heat exchange.