Most supermarket refrigeration equipment breakdowns are repeat problems. When responding to refrigeration service calls at supermarkets, refrigeration technicians immediately play “problem percentages” upon entering the store—this allows them to get to the root source of the problem as quickly as possible, to prevent perishable product loss.
A common problem with refrigeration systems is when the thermostatic expansion valve (TEV) doesn’t feed enough refrigerant and the refrigeration technician sees high superheat.
7 steps to restoring refrigerant flow
The following checks will help the HVAC technician identify the TEV problem quickly and restore refrigerant flow.
- Check the sensing bulb location. The sensing bulb should be attached to the horizontal section of the suction line at the evaporator outlet. On suction lines 7/8 inches and larger, it should be installed at 4 or 8 o’clock on the side of the horizontal line. On smaller lines, the bulb may be mounted at any point around the circumference, except the bottom of the line where oil may influence the sensitivity.
- Check for vapor-free liquid to the TEV. When the system is pumped down, install a liquid line sightglass directly ahead of the TEV. If flash-gas is present, check for a liquid line restriction or pressure drop caused by incorrect pipe size or poor piping practices. Correct restriction as required.
- Check design pressure drop for required TEV capacity. The capacity of the TEV is a variable dependent on the pressure differential across the valve. Greater pressure drop across the TEV = greater TEV capacity. Remove the source of the pressure loss on the inlet, or pressure increase at the outlet. If the reduced inlet pressure to the valve is due to low condensing pressure, install the appropriate head pressure controls.
- Check for element charge migration. Gas type elements ZP, CP, and VGA charges have a limited volume of constituents. The bulb temperature must be lower than the element or the bulb constituents can migrate into the element, causing the valve to throttle and close, causing high superheat. Warming the element with hot water can return the charge constituents to the bulb and return the superheat to normal. Long term, a change must be made to keep the element from getting colder than the bulb, or the thermostatic element can be changed to another type so charge migration does not occur.
- Check for moisture. Water or a mixture of water and oil frozen in the valve port (or working parts of the valve) prevent proper operation. Since the valve is the first cold spot in the system, moisture will freeze here and restrict flow. A See-All Moisture Liquid indicator can identify a system with moisture issues. When moisture is present, the Catch-All filter-drier should be replaced.
- Check the TEV adjustment. Calculate the evaporator superheat. If an adjustment needs to be made, turn the adjusting stem in increments of ¼ to ½ turn. Then let the system stabilize before making additional adjustments. Clockwise adjustment will increase superheat.
- Check for contaminants in the valve body. Restrictive dirt or foreign material such as copper oxide scale, metal chips, oil breakdown, sludge, etc. can be a problem. Conventional strainers allow some types of these materials through, ultimately obstructing the port of the TEV.
Checking these seven points first will save the technician considerable time in troubleshooting and repairing TEV issues with refrigerant flow. Regular maintenance is, of course, essential for maintaining proper TEV flow and function and keeping downtime to a minimum. For more information, contact Sporlan Division technical support at 636-239-1111.
You can also download a copy of 12 Solutions for Fixing Common TEV Problems - Form 10-143 for more troubleshooting tips.
Article contributed by Glen Steinkoenig, product manager, Thermostatic Expansion Valves, Sporlan Division of Parker Hannifin.
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