Installation And Operation Of Liquid Level Probes
Such controls are often focused directly on supermarket racks. The application supports monitoring, performance, and charge management, as well as troubleshooting, via a central microprocessor-based control panel.
These capacitance-based probes interpret liquid level in a receiver, accumulator, or similar pressure vessel as a selectable DC voltage range of either 0-5 V or 1-6 V when powered by an 11- to 47-VDC regulated remote source.
The signal is proportional to the vertical liquid level surrounding the probe. The probe is mounted, sealed, and grounded electrically via a 3/4-inch NPT thread or ProbeLok installed into a coupling on the top of the vessel. Depending on the vessel, an available surrounding, vented steel sleeve reinforces and protects the probe while providing repeatability and precision output.
INSTALLATIONTo select the proper size, the receiver inside height and fitting should be known. Subtract the value of the minimum clearance from the probe bottom plus the typical engagement length from the vessel height to determine the maximum insertion length. Actual probe insertion length may be less than the maximum length allowed. Probe insertion length must be long enough to measure the lowest receiver level desired. Probe lengths are not field adjustable.
Proper location should allow for adequate installation and removal without bending. Avoid locating it near the liquid inlet to the receiver. The probe location should not interfere with dip tubes or other internal parts.
OPTIONSAs all systems are not alike, there are options available for most applications. For extreme cold systems, risers are available to provide a thermal block for the electronic head or provide clearance for insulation. They come with or without digital displays. Remote digital display is available for unusual installations, where access is difficult. The ProbeLok feature allows for drop-in installation and eliminates the corkscrew effect on longer probes. Standard probe lengths of 6, 8, 12, 19, 23 inches are available. Custom probes up to 12 feet in length can be ordered.
ELECTRICALRun wiring through 3/4-inch NPT electrical opening on top of the enclosure. Wiring should be at minimum 20 AWG. To protect electronics from moisture, dirt, etc., seal the electrical opening with a watertight cable connector, or, if a conduit fitting is used, install conduit laterally lower than the electrical house on the probe. It needs to be sealed with a silicone sealant immediately above the probe. To prevent electrical noise, it is important not to run wires with or near power wires.
CALIBRATIONAll probes are precalibrated at the factory for a specific refrigerant. Shielded probes CLPS and CLPSD should not require field adjustment unless a different refrigerant is being used or if only a segment of the probe is represented. To calibrate, disconnect the probe from the controller or computer. Install a voltmeter in parallel with the output of the control loop. Lower the refrigerant level in the receiver to a level equal to 0%; the voltmeter should display 0.0 VDC. If not, adjust using the zero adjustment screw.
Raise the liquid level in the receiver to the 50% level or some other known level. The voltmeter should display 2.5 VDC for a 50% probe immersion level and the probe jumper is set for 0-5-VDC output. (For 1-6 VDC output, the display should be 3 VDC for 50% level.) For other levels, the corresponding equivalent voltage should be displayed. If not, adjust the probe span adjustment screw.
SAFE OPERATIONWhere cold refrigerant liquid lines are used, it is necessary that certain precautions be taken to avoid damage that could result from liquid expansion. Temperature increase in a piping section full of liquid will cause high pressure due to the expanding liquid, which can possibly rupture a gasket, pipe, or valve. All hand valves isolating such sections should be marked, warning against accidental closing, and must not be closed until the liquid is removed.
Check valves must never be installed upstream of solenoid valves or regulators with electric shut-off; nor should hand valves upstream of solenoid valves or downstream of check valves be closed until the liquid has been removed. It is advisable to properly install relief devices in any section where liquid expansion could take place.
Avoid all piping or control arrangements that might produce thermal or pressure shock. For the protection of people and products, all refrigerant must be removed from the section to be worked on before a valve, strainer, other device is opened or removed. Flanges with ODS connections are not suitable for ammonia service.
Finally, it is important to say that people doing any work on a refrigeration system must be qualified and completely familiar with the system and the valves involved, or all other precautions will be meaningless. This includes reading and understanding product bulletins and safety bulletins prior to installation or servicing work.
For more information, contact Parker Hannifin Corp. Refrigerating Specialties Division at 770-681-6300 or 800-506-4261 or via e-mail at RSD_info@parker.com.
Publication date: 11/04/2002