How to Perform Mechanical-Integrity Inspections of Air Coolers
Proper inspection methods ensure continued reliability and minimize the risk of an accidental ammonia release.
Owners of air coolers — and any ammonia refrigeration system — should perform periodic mechanical-integrity inspections to ensure the continued reliability of the unit and to minimize the risk of an accidental ammonia release. When performing mechanical-integrity inspections of a typical air cooler, there are 10 key inspection points to help identify potential problems with the air cooler. Look for signs of out-of-design-specification conditions such as:
- Ice or material buildup.
- Corrosion on the tubes, fins or connections.
- Bulging header pipe ends due to excessive pressure.
- Fan vibration and unusual fan noise.
- Signs of tube impingement or rubbing.
- Housing cracks or missing connections.
- Physical damage caused by fork lifts.
- Loose connectors other signs of poor electrical integrity.
- Missing fan protection.
- Systems being operated outside of the designed for operating conditions.
- Know what to look for in each case.
Excessive Ice or Material Buildup
Inspect all air coolers for material buildup on the fins and drain pans. Dust or packaging-material fibers are common sources for this buildup. Inspect the air coolers operating in areas below freezing for excessive ice buildup on the fins and drain pans. Excessive ice and material buildup could interfere with airflow and reduce the coil capacity. Excessive ice buildup between the bottom of the coil tubes and the drain pan could cause mechanical damage to the coil tubes and drain pan.
Remember that hot-gas-defrosted air coolers commonly have an array of heating tubes attached to the drain pan. Any damage to these tubes — or to the tubes in the coil core — could result in an ammonia leak. Follow the manufacturer’s guidelines for defrosting air coolers, which might include adjusting the number of defrosts and their duration. The pressure setting on the defrost regulator also may need to be adjusted to ensure complete defrosting.
If adjustments to the defrost scheme do not prevent excessive ice buildup, contact the air cooler manufacturer, a contractor or a consultant for further analysis.
Corrosion of Tubes, Fins or Connections
Inspect the surface of the tubes, fins and piping connections for signs of pitting or uneven discoloration. Also inspect any insulated pipes where the vapor barrier might be compromised. Corrosion of the fin surfaces results in reduced refrigeration capacity, so it is important to also check these surfaces.
If a pit continues through the wall thickness of the tube or pipe, excessive pitting could result in an ammonia leak. Visually inspect all areas of the coil with a flashlight. For hard-to-reach areas, use a flexible, lighted borescope. If possible, measure the depth of large pits with a micrometer depth gauge. Generally, any pit deep enough to measure is cause for concern, and remedial action to stop the corrosion and repair the materials should be taken. Consider taking digital pictures of any new signs of corrosion for future comparison.
Prevent corrosion of tubes and fins by using only cleaning chemicals that are suitable for the materials of construction. Many cleaning chemical suppliers and your air cooler manufacturer will have recommendations regarding cleaning. In some corrosive environments, a water rinse system can be utilized to keep contaminants from building up on tube and fin surfaces.
Exceeding Maximum Allowable Working Pressure
Trapped liquid in an air cooler could expand and cause the internal pressure to exceed the maximum allowable working pressure (MAWP). This could result in ruptured components and ammonia leaks. Hydraulic shocking, or vapor-propelled liquid, is another internal force that could exceed the air cooler’s MAWP and cause ammonia leaks.
Inspect the ends of the evaporator header pipes for signs of bulging. The largest-diameter end cap typically is the first location where any signs of overpressurization will be evident. Tubes or pipes that look misshapen are also signs of overpressurization.
Prevention measures include using good piping practices per the piping handbook published by the International Institute of Ammonia Refrigeration (IIAR); keeping hot-gas lines clear of liquid; and always allowing the defrost pressure to equalize before opening the suction stop valve.
Fan Vibration and Integrity
The fan assemblies on air coolers should be inspected annually or per the manufacturer’s recommendation for your specific air coolers.
With the fans running, look and listen for any vibrating components. Typically, vibration in larger sheet-metal panels will be the most noticeable. Keeping a safe distance from the fan, and with all guards in place, watch the fan in operation, looking for any vibration as it rotates. Listen for any sudden changes in noise levels or tones. For variable-frequency-controlled fans, check for vibration over the entire range of speeds. For multi-fan units, shutdown individual fans to localize the source of vibration.
With the fan motors off and safely locked out, check the torque on the bolts securing the fan assembly to the motor shaft. Also, check the torque on the bolts holding the fan motor to its mount. Check with your air cooler manufacturer for proper bolt torque values.
If left unchecked, loose fasteners in either the fan hub or the motor mount could result in a catastrophic failure such as mechanical damage to the air cooler tubes, which could result in an ammonia leak.
Refrigerant Distribution Tube Condition
Some air coolers are equipped with refrigerant distributor devices that meter equal amounts of liquid and vapor refrigerant to each coil circuit. The distributors are commonly connected to each coil circuit by means of long, small diameter feed tubes that are usually about 0.25” diameter. It is important to periodically check these feed tubes for signs of rubbing.
Any rubbing or contact typically will show up as a flattened area on one or more distributor tubes. If not repaired, the rubbing could eventually wear through the tube wall, causing an ammonia leak. Visually inspect each tube, paying close attention to those that are touching or close to touching. If wear is noticed, contact the air cooler manufacturer or a refrigeration contractor to reorient the tubes, install wear sleeves or replace the tubing.
Integrity of Housing, Unit Supports and Piping Supports
The condition of the air cooler’s housing and supports should be inspected annually. Visually inspect the unit housing and the hangers for cracks, missing or loose fasteners, and any signs of corrosion. Deteriorated supports could result in the unit falling from the mounting location, which could result in an ammonia leak.
Another area to inspect is the piping supports near the air coolers. Typically, air coolers are not designed to support piping, control valves or hand valves.
Improperly supported piping could overstress the coil connections, resulting in a leak. Any unit and piping supports that show signs of damage should be repaired or replaced immediately.
Protect Against Traffic Hazards
Air coolers that are operating in areas of high forktruck traffic should have their protective barriers inspected annually. Visually check each barrier for evidence of damage due to forktrucks. Damaged barriers should be repaired or replaced.
Traffic impacting an air cooler can cause mechanical damage and ammonia leaks. Prevention measures include adding additional protection, training lift operators to not hit the protective barriers or units and adding warning or caution signs.
Electrical components mounted to air coolers should be inspected annually. Look for loose connectors or signs of overheating such as black marks, melting of wire insulation or cracks in wire insulation. Visually inspect the electrical connectors and use an infrared camera to look for overheating components.
Any electrical issues, if left uncorrected, could result in a fire. Contact a company electrician or an electrical contractor to either repair or replace defective equipment.
Missing Fan Guards
The presence of and condition of fan guards should be inspected annually. In the absence of proper guarding, people could be injured or product could come into contact with moving fan blades. Visually inspect each air cooler to make sure that each fan is guarded and that the guard is in good condition. Contact the equipment manufacturer to obtain replacement fan guards.
Suitable for Operating Conditions
Air coolers should be inspected annually to determine that the unit is still being operated within the intended conditions. One way to verify this is to check the unit serial plate for the minimum and maximum design operating temperatures and pressures.
Operating air coolers below the minimum design temperature could result in material failures and rupture due to embrittlement at low temperatures. This is especially true for air coolers constructed from some carbon steels and hot dip galvanized. Air coolers with aluminum or stainless steel tubes typically can operate safely at low temperatures.
Air coolers operating above design temperature could lead to overpressurization and rupture, which could be possible during higher temperature cleaning processes. Contact the equipment manufacturer to verify design values and only operate equipment within recommended design parameters, or replace equipment operating outside of design. PC