Refrigeration question - Expansion valve regulation strategy - Page 1
Forum section: electronics / general

author: Miyuki
author_rank: Frequent Contributor
timestamp: June 23, 2023, 07:51:44 am
content: Hi folks, is any refrigeration expert here, or maybe just someone with better searching skills.I'm trying to find out how are electronic expansion valves controlled in systems without pressure sensors when you have only temperature sensors.With a pressure sensor, it is simple, you can calculate superheat and just open the valve more to decrease it.But cannot find how they do it from temperature only, do they estimate pressure based on evaporator inlet temperature?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 23, 2023, 12:36:27 pm
content: Quote from: Miyuki on June 23, 2023, 07:51:44 amThat's actually a very accurate way to measure the low side pressure in steady state operation.

author: Miyuki
author_rank: Frequent Contributor
timestamp: June 24, 2023, 10:12:37 am
content: Quote from: NiHaoMike on June 23, 2023, 12:36:27 pmIt makes sense with proper coupling and insulationI just wasn't sure if there isn't any hidden catchThanks

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 24, 2023, 12:21:34 pm
content: I have done it with a diode connected transistor and a TMP513. It was at least as accurate as my Yellow Jacket HVAC gauges. (That was despite the fact I used R433b and had to interpolate a PT lookup table.) Even getting the condensing temperature was pretty easy by putting the sensor in the middle part of the condenser.

author: noidea
author_rank: Unknown
timestamp: September 29, 2023, 04:01:12 pm
content: Quote from: Miyuki on June 23, 2023, 07:51:44 amSorry for the late reply I don't frequent the forum as much as I used to. There is nothing wrong with calculating Superheat by using sensors instead of transducers, most of the Japanese brands use this control for the expansion valve on their VRF indoor units. Two sensors minimum, midpoint of the coil for an approximation of evaporating temperature and exit of the coil for superheated temp. Better arrangements use an inlet sensor as well and calculate either Exit - inlet or Exit - midpoint to regulate the expansion valve. They will also ensure the inlet and midpoint sensors are on different circuits of the coil to take into account a blocked circuit. It all works great until you have a sensor that plays up and gives erroneous inputs when the unit is running...You can calcualte superheat from sensorsor I would have an

author: MarkS
author_rank: Unknown
timestamp: September 29, 2023, 04:21:48 pm
content: Quote from: Miyuki on June 23, 2023, 07:51:44 amNon-electronic expansion valves use a sealed tube with a bulb at one end that is attached to the line set. It operates on temperature. The fluid in the bulb expands and contracts with the temperature of the line and moves a diaphragm in the valve to meter the refrigerant. Using a pressure sensor would just add complexity and expense to the system, not to mention a point of failure. Or to simplify: If it ain't broke, don't fix it.

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 29, 2023, 05:08:25 pm
content: Quote from: MarkS on September 29, 2023, 04:21:48 pmBut those expansion valves work in pressure mode and the bulb adds superheat accuracy. Ones for bigger evaporators even use an external pressure tube (from the end of the evaporator, where the bulb is placed) to be more precise.It is the same mode as when electronic ones use a pressure transducer.