solenoid valve design - Page 1
Forum section: electronics / projects

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2013, 05:31:06 am
content: So I'm designing a high efficiency air conditioner (sort of a research project for learning about stuff) and decided that instead of a mechanical ("thermostatic") expansion valve, I'm going to use a solenoid expansion valve. (For those who don't know, the expansion valve is the part that controls the flow of refrigerant.)The valve bore is very small (on the order of 0.5mm) and the stroke only needs to be a millimeter or two. It does need to be very well sealed, so the mechanical part would be welded inside a copper tube and the actual coil wound on it afterwards. (The valve runs on DC and changes position slowly, so eddy currents in the copper tube can be ignored.) I'm planning on having the weight of the plunger push against the seat (about 250 PSI pressure difference worst case, planning on designing for 300) and the coil applies a force pulling the plunger upwards and reducing the pressure difference that starts opening the valve. It does have to be continuously variable, so the pressure actually tries to open the valve (otherwise it becomes unstable) and the magnetics must be designed so that it avoids the plunger "snapping" into position.How do I calculate the dimensions in order to reduce the power needed to hold the valve open?

author: SeanB
author_rank: Super Contributor
timestamp: June 29, 2013, 10:11:21 am
content: These electronic expansion valves are a standard part, look at the Danfoss or Thermoking website for more info on them, and sizing info as well. They normally run on 24V, and use the current through the coil to open the needle against a spring, so that you get a somewhat monotonic relationship between current and position. Not linear, but monotonic, and normally the needle and tapered seat is dimensioned to make it more linear, but it still only has a limited range of control that is matched to the refrigerant, the flow rate and the desired evaporator temperature. Most have the ability to change out the actual valve parts ( after pumping the system down and reclaiming the refrigerant away from the valve area) to a range of different sizes.Sorry I cannot give more, I very rarely work with these, mostly only TXV units or simple cap tube ones.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2013, 01:15:46 pm
content: I looked those up and to my surprise, they just seem to be solenoid valves with controlled open/close rates. As in they're fully on or off at any given moment. I don't think that's the right way to go since it pressure cycles the plumbing. I presume the actual pressure cycling is quite small in a large system, but I'm only dealing with a 1/2 ton system that is mainly designed for dehumidification and water heating.I can pretty easily build a valve for less than $10 of common refrigeration plumbing parts. I'm just not sure exactly what sort of dimensions to design the coil in order to minimize the power needed. (Piezoelectric valves are the best since they use basically no power except when changing position, but it's very hard to find one and building your own is even harder.)

author: Uup
author_rank: Regular Contributor
timestamp: June 29, 2013, 03:54:46 pm
content: Those type of EEVs (electronic expansion valves) are an early design. Most air conditioners that have an EEV (usually an inverter or multihead unit) use the stepper motor type of EEV. The stepper motor type of EEV is the dominant type now.There's a lot of R&D in the engineering of an EEV and that information is not openly available. It would take a lot of experimenting, trial and error and data collecting to perfect your home brew EEV, and in the end it likely won't meet your 'high efficiency' target. Although making an EEV would be an interesting project in itself.However, for your project, why reinvent the wheel? You're no doubt not going to make your own high efficiency compressor, refrigerant, heat exchanger, etc...

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2013, 10:24:12 pm
content: It's actually possible to make a good heat exchanger from copper tubing, it just wouldn't be very much cheaper than buying one unless you only need a very small unit. I'm going to do just just for the subcooler (only a foot or two of tubing), but the condenser is going to be a plate HX. (That would need about 30 feet of tubing to get the same performance as the plate HX I'm looking at, at which point the plate HX is about the same price or even slightly cheaper.)I'm designing it with a two stage evaporator with an ejector.http://www.sae.org/mags/aei/6741The idea is that the energy in the pressure difference is used instead of being wasted. (I'm starting from a window A/C, so what was the original condenser is going to become the second evaporator.) I'm going to have some sort of valve (might be as simple as using a pinch off tool to intentionally restrict the flow) to adjust the flow from the phase separator to the second evaporator, and another one (planning on a float valve) to let the gas bypass the evaporator to reduce the pressure drop. (The outlet from the ejector, a mix of liquid and gas, flows into the separator, at which point the liquid is directed into the evaporators and gas to the compressor inlet through an accumulator.)I'm not planning on inverter driving the compressor (it's only 1/2 ton, so it will just run for a few hours every day depending on humidity and hot water demand), but the water pump is definitely going to need inverter drive to get the correct outlet temperature and the fans are also inverter drive so I can choose between high speed for better cooling or lower speed for better dehumidification.