Case Study: Novel Refrigerant Applications
The drive for increased efficiency and the use of alternative energy sources is taking refrigerants to places they’ve never before been used. There’s a whole new spectrum of devices being created that go well beyond traditional refrigeration or Heating, Ventilation & Air Conditioning (HVAC) systems. These systems have new challenges and virtually no developed testing methods or standards. Frequently, these new devices require developing new-to-the-world technology.
A client of Mackenzie Design required a stand to test novel refrigerant devices. This test stand had to be portable and easy to use. Technicians operating the stand would only have a working knowledge of refrigeration and no background in thermodynamics. It would also be shipped off site where other professionals – with no experience – would perform specialized Highly Accelerated Life Testing (HALT).
The science behind refrigerant behavior – thermodynamics – is complicated and involves intangible properties. Properties like temperature and pressure are easily recognizable to technicians. But some properties aren’t so easy to ascertain. For example, there’s no such thing as an entropy meter. In fact, entropy is an important variable but its really something only engineers and scientists talk about.
Envelope dimensions were a critical consideration because of the desired portability. The test stand had to fit through an average sized door and the weight needed to be minimized. There were also power source and heat sinking issues to deal with. Finally, to have off site technicians perform testing the stand had to be very straightforward to use.
Complex System, Easy Controls
The inlet, or “head” pressure at the Device Under Test (DUT) was the critical variable needing precise control and measurement. What is head pressure? Imagine the pressure created by your hometown’s water tower. Placed at the highest geographical point and filled regularly, it ensures that everyone’s faucets have steady, pressurized water below. Head pressure is measured in distance (i.e. feet) below the fluid surface. For example, if your house were 100 feet below the water tower you would have 100 feet of head pressure in your faucets, or about 45 psi of pressure.
Building a tower is the simplest way to create head pressure, but its not what the customer needed. Utilizing the Engineering Design Process, Mackenzie Design was able to design a stand that controlled head pressure thermodynamically. By adding and removing heat to the refrigerant at critical points in the testing circuit, the 6 foot tall stand could actively control head pressure from 0 to 100 feet!
The design was portable and minimized capital cost, but it was complicated. It required precise control of temperature and pressure at changing flow rates. Were any one variable to change, all set points needed to be updated simultaneously. Multiple Proportional-Integral-Derivative (PID) control loops were designed to work together. Combined with computer controlled set points, all of the complexity was managed with a simple LabView interface. The computer utilized equations and state tables to simultaneously adjust variables and perform engineering calculations. Operators could easily adjust the one variable they were testing, forget about the rest and be confident their test would be set up properly.
To achieve all this and maintain portability the stand was designed to function with a wide range of power sources. Anything from a household dryer circuit to an industrial 3-phase power source could be used. Cold water was utilized for a heat sink, supplied by either a cooling tower or a city water source. The stand was outfitted with a special set of controls to limit wasted water during testing.
Upon completion, the new test stand:
- Safely segregated the refrigerant space, the electronics space and the operator space to eliminate risks to untrained operators.
- Had two independent testing loops whose complicated thermodynamic processes were easily controlled
- Controlled refrigerant head pressure from 0-100 feet in a 6 foot tall unit.
- Allowed testing with sub cooled liquid, super heated vapor and saturated liquid with quality control.
- Was portable and flexible, fitting through normal doorways and connecting to common power sources.
- Had numerous data logging options that were failsafe during power outages.
- Had a system to minimize wasted water during testing.