You would think buying an air dryer woyuld be easy wouldn’t you. It is if you follow the rules.
Compressed air dryers are designed to a set of standards, these are;
- A set volume as stated within the suppliers sales literature
- 7 bar working pressure
- a dew point of 3 °C
- An ambient temperature of 25 °C
- An inlet temperature of 35 °C
When a dryer is working with a given design volume within these operating conditions you will get compressed air at 3 °C dew point at 7 barg. Compressed air, once drier, stays dry. To get any more water out of the compressed air the atmospheric air temperature would have to “hover” between 3 and 0 °C. If this were to happen then you “might” see a very small amount of water for the period that the compressed air hovers between 3 and 0 centigrade. In reality this temperature scenario rarely happens.
Matched Sets
The main components in a dryer, the Freon compressor, the condenser, and the heat exchangers are thermally matched for a given air volume to achieve a set contact time for the compressed air and the cold Freon gases to ensure compressed air is cooled to 3 °C, and to impose the smallest possible pressure drop. At 3 °C water condensate is removed to waste. Condensers have to have sufficient surface area to dissipate the heat adsorbed generated in tghe Freon circuit. If the heat load cannot be ejected, the Freon pressure / temperature will rise and create thermal overload leading to a trip out. Condensers are cooled by the ambient air temperature.
In order for the Freon circuit to function continuously and reliably, the gas circuits cooling ability must exceed the work / heat load given by the air compressor and the prevailing atmospheric conditions. If this is achieved, you will receive dry compressed air.
If the heat load from the compressor exceeds the cooling ability from the Freon compressor circuit then the air dryer will fail. It will fail either because; the Freon High Pressure gas switch shuts the circuit down due to high heat / work load. Or the Freon compressor burns out due to high returning gas temperatures. If you are on the edge of a trip out, the dew point will be in the 10 °C region or even higher. High dew points are not acceptable as poorly dried compressed air loitering in the piping system will allow water vapour to condense in the pipeline when airline temperatures drop during the night, or in the cooler months of the year.
Selecting for Africa
We live in a warm climate, we only see the design conditions shown above for a few months of the year. For a large percentage of the year our temperatures are higher than ther design criteria. Let’s assume we have a 30 °C ambient in tghe compressor room and a 40 °C air inlet to the dryer. All the other critera remain uncghanged. The two “jumps” of 5 °C actually increase the work load by 22%.
If we are working in the hotter areas such as Cape Town, Pretoria, Nelspruit etc. then we can expect ambient’s of 35 and inlet air temperatures of 45 °C. These conditions increase the heat load even more by some 40%.
The Effects of the increase in temperature means we have to reduce the air flow volume by up to 40%, this is really the heat load, this reduction in work load allows the dryer Freon circuit to gain control of the heat load again and to function continuously without mechanical trip outs or failures.
How to Create a Problem
If you do not reduce the heat load, in other words the air volume stated in the European sales brochure, you are destined for constant air dryer mechanical failures and a life time of poor compressed air quality. This will lead to downtime, spoilt products, higher operating costs, leading to a waste of profits.
If the suppliers sales brochure does not provide a set of de-rate factors for the dryervolumes and de-rate figures for higher temperatures, I suggest that you do not purchasde the product offered, as you have no way of determining the real capacity of the air dryer under your own operating conditions. Any reputable supplier should have the de-rate factor in the sales literature.
Supply Chain Issues
When Artic Driers was formed 23 years ago there were about six air dryer suppliers. By my count now, there are some 26 importers of air dryers in the republic.
Some suppliers have some knowledge of dryers, some may even have a qualified and experienced refrigeration air dryer mechanic on the staff. Please don’t use a normal fridge mechanic, they invariably setup compressed air dryers incorrectly. A few xsuppliers will have a good history of supplying industry.
However, sad to say, that many of the current supply chains have little experience in compressed air drying and frequently get the selection wrong, or worse still sell a low cost imported product into the market with little or no support structure.
Get the Facts
Before you lay money down on the table, do the homework, make enquiries, check the track record of the supply chain. Are they really commited to the compressed air industry?? How long have they been selling this range, what aftercare is offered, can you check the selection by using the factors in their literature.
At the risk of sounding pretentious, there is nothing for nothing in this world, you what you pay for. We rarely buy the cheapest car for the family, so why do we buy the cheapest dryer? Dryers, like cars have differences! Consider the purchase carefully, compressor companies may be great at compressors, are they equally competent on air dryers? Or are you buying a caravan from a car dealer, when you should be talking to the caravan manufacturer…
Consider your purchase carefully before commititing your hard earned Capex budget!