The photo is showing air cooled chiller condenser maintenance being carried out by Maximus Chillers. 6 water cooled chillers are located in the plant room below. They are 750 kw single screw compressors with a control panel located to the side of each. On this visit, the emphasis was to carry out a thorough maintenance of the condensers.
Fan Speed Controllers
The refrigerant for the systems is R134a, so to allow for the saturation of the refrigerant, 8 bar is the head pressure set point. This pressure corresponds to the desired condenser temperature of 36°C. Subcooling of 6°C to 8°C is achieved during the nominal operation of the plant. Each fan speed controller runs the 12 condenser fans together. Other condenser designs where fans bang on forwards and backwards cause vibrations resulting in reoccurring leaks on the condenser. Chiller No. 2 had tripped during a 'discharge override' system message. On inspection of No. 2 condenser- the fans speed inverter had tripped on 'over temperature' alarm. The panel fan, which is the kind to cool computers, was found to be still trying to run but seized. This failure had caused the alarm on the inverter. The panel fan was replaced from the stock of parts in the onsite stores. The chiller was reset and a detailed Tick Sheet was completed noting the occurrence of this fault on arrival.
Air Cooled Chiller Condenser Maintenance Cleaning
Because of the large size of the plant, the customer had installed a fireman’s hose for the cleaning of the condensers. This is located in the free space beneath the condensers. It is fitted to a portable buggy so it can be moved under each condenser. Setting the nozzle to the correct attitude, our engineer moved the buggy sideways, so as to rinse the condenser in the direction of the fins. The condenser was relatively clean as the onsite maintenance engineers carry out this task as part of their monthly schedule.
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Design Considerations for Air Cooled Chiller Condenser Maintenance
The kind of condenser in this article is:
Multiple Row Tubing
It has tubing of ½” diameter which is 3 rows deep. Each pass starts at the discharge header which is at the inlet to the condenser, goes down to the far end, through a condenser end turn, back to the discharge end, through another condenser end turn and back again to the liquid pipe end. The heat removed from the condenser, per kg of refrigerant, is the heat content of the vapour as it leaves the compressor, minus the heat remaining in the liquid at the end of the condenser.
Some condenser designs are up to 6 rows deep. This allows a small footprint of space where there is not much room for the location of the chiller. The downside to this, is that the chiller engineer can often struggle to locate the exact location of a leak when it is deep into the coil. If a condenser is too small, it will cause a higher head pressure and reduce the life of the compressor. We can arrange the lift out and repair of deep row condensers in our workshop.
This condenser has aluminium fins which are pressed around the copper tubing. This increases the surface area of the condenser heat exchange medium and so increases the dissipation of heat. The same amount of heat delivered to a condenser from the compressor must also be rapidly removed. For this condition to be reached, enough head pressure will need to be built up so that the condenser temperature is at least 15°C above the ambient. This is why the same chiller can be picked up and shipped to the Middle East and still work. It will just run at a higher discharge pressure/ temperature. There will, however, be a loss in the coefficient of performance as the higher pressures will result in more electricity in, versus the same amount of refrigeration effect out.
Forced Convection Type
The fans mounted on the ducting provide this forced convection. The air is sucked through the bottom of the condenser, across the 3 rows of tubes, along the fins and up through the fans. The air flow is stable as it enters the fins so good heat transfer is achieved. As it leaves the fins at the top, there is lower heat transfer as the air is turbulent.
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Fan Replacement during Air Cooled Chiller Condenser Maintenance
Spare fans are available onsite for replacement when individual fans fail. As mentioned above, these condensers are controlled with FSC's which reduce the occurrence of leaks. As well as this, fan speed controllers increase the lifespan of the fans. Because all of the fans run together, they speed up and slow down steadily. When high pressure control switches are used, they are set at different pressures. This means that some of the fans never come on until the head pressure is too high. This is usually due to a blocked condenser, failed fans or a high ambient. Because the fans have not come on for a long time, they are often seized or have suffered water ingress from the rain. An onsite maintenance engineer is available to help with the lift out and lift in of the replacement fans.
Pressure Relief Valves
Each of these condensers is fitted with a pressure relief valve (PRV) It is fitted into the discharge pipe on the inlet to the condenser. This is so that if the fans and the HP switch were to fail, the dangerous levels of pressure in the system would be vented. It is unlikely, however, that the HP switch would fail as this is a very reliable part. A PRV being fitted is often the requirement of insurance companies. The testing or replacement being arranged at scheduled intervals.
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