COILS, COOLERS AND CONDENSERS
Looks in cool O
Joe Fazzari of Colmac Coils looks in detail at mechanical integrity inspections of air coolers.
wners should perform periodic mechanical integrity inspections on air coolers to ensure the continued reliability of the
unit and to minimize the risk of an accidental ammonia release. This article lists some of the more common issues to look for when performing mechanical integrity inspections of a typical air cooler.
Excessive ice or material build-up Inspect all air coolers for material build-up on the fins and drain pans. Dust and/or packaging material fibres are common sources for this build up. Inspect the air coolers operating in areas below freezing for excessive ice build-up on the fins and drain pans. Excessive ice and material build-up could interfere with air flow and reduce the coil capacity.
Excessive ice build-up 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.
Adjustment of the defrost regulator pressure
setting may also be needed to ensure complete defrosting. If adjustments to the defrost scheme do not prevent excessive ice build-up, 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 vapour barrier might be compromised. Excessive pitting could result in an ammonia leak if the pit continues through the wall thickness of the tube or pipe. Corrosion of the fin surfaces results in reduced refrigeration capacity, so it is
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important to also check these surfaces.
Visually inspect all areas of the coil with a flashlight. For hard to reach areas, use a flexible, lighted bore scope. 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 beginning corrosion for future comparison. Corrosion of tubes and fins can be prevented by only using cleaning chemicals that are suitable for the materials of construction. Many cleaning chemical suppliers and/ or your air cooler manufacturer will have recommendations regarding this. In some corrosive environments, a water rinse system can be used to keep contaminants from building up on tube and fin surfaces.
Exceeding maximum allowable working pressure (MAWP) Inspect the ends of the evaporator header pipes for signs of bulging. The largest diameter end cap is typically the first location that any signs of over-pressurization will be evident. Tubes or pipes that look misshapen are also signs of over- pressurization. Trapped liquid in an air cooler could expand and cause the internal pressure to exceed the MAWP. This could result in ruptured components and ammonia leaks.
Hydraulic shocking, or vapour propelled liquid, is another internal force that could exceed the air cooler’s MAWP and cause ammonia leaks. Prevention measures include using good piping practices per the IIAR Piping Handbook, 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
Ice accumulation in drain pan of hot gas defrosted air cooler.
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, usually about a quarter inch in diameter. It is important to periodically check these feed tubes for signs of rubbing. This will typically 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. Contact the air cooler manufacturer or a refrigeration contractor to reorient the tubes, install wear sleeves, or replace the tubing.
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