Refrigerated Air Dryers
These are available in two separate types:
a) Gas bypass dryers.
b) Chilled mass dryers.
These are both different forms of direct expansion dryer and work on the principle that the compressed air is chilled as close to 0 deg C as possible, without dropping below zero. The moisture in the compressed air then condenses into liquid, for the same reason that condensation occurs on a window.
One of the most important things to consider is the control circuit, the more expensive the circuit the more accurately the minimum air temperature can be held to just above zero.
An expensive control circuit will keep the compressed air temperature floating between about +1 and +3 deg C. This means that you have an average pressure dewpoint of about 2 deg C. A cheap control circuit allows the dewpoint to float between +2 and +12 deg C, giving you a mean pressure dewpoint of only 7 deg C. The problem is that the dryer manufacturer using the cheap control circuit may tell you that the dryer achieves a minimum pressure dewpoint of + 2 deg C.
Quite true, the problem is that it only achieves this for a short period.
Cheap temperature controllers are usually called Klixons. The reason is simple; they click on to turn the fridge compressor on and click off again to turn the fridge compressor off. They are only useful in dryers that use a quantity of chilled glycol, silica sand or the like to 'store up the cold'. (Please excuse the grammar).
The more expensive controllers are used for refrigerant control. These gas bypass dryers will be fitted with two types of capacity control, a Thermo-expansion valve and/or a Thermo-bypass valve.
Gas Bypass Refrigerated Dryers:
These use a tube in tube type system, the compressed air flows through one tube, usually the inner tube, whilst the refrigerant flows through the outer tube. The tubes are sometimes wound into a coil, or sometimes contained in a bundle. If there is only one heat exchanger, then the compressed air leaves the dryer very cold, resulting in condensation on the outside of the compressed air pipework.
It is fairly simple to overcome the condensation problem and also to increase efficiency by having another heat exchanger. This is also a tube in tube affair, but the warm incoming compressed air passes down the outer tube, over the cold air that is leaving the dryer, having already been chilled. In this way two things are achieved.
i. The air being discharged from the dryer is warmed up, preventing condensation on the compressed air pipework in the factory.
ii. The compressed air going into the dryer is chilled, reducing the load on the refrigeration cycle. In fact the load is reduced by a staggering 55%.
From the energy point of view, it is always better to opt for a dryer with a primary heat exchanger and a secondary heat exchanger. The running costs are halved.
An advantage of the gas bypass type dryer is that the refrigeration compressor will be 100% off-loaded, once the required minimum temperature has been achieved. This is similar to an industrial air compressor going off load once it gets up to pressure, the only difference being that with the refrigeration compressor the gas gets short circuited so that the gas goes round in a loop from the discharge port into the inlet port. Although the refrigeration compressor continues to operate, the only power it absorbs is the power it needs to keep the crankshaft turning over.
Chilled Mass Dryers:
The initial principle is the same; the compressed air is reduced in temperature thereby allowing the moisture to drop out. Whereas the previous dryer is designed so that the refrigerant acts directly upon the compressed air, the chilled mass dryer cools down a tank containing glycol, sand or even aluminium fillings which in turn contains the heat exchanger. This means that the refrigerant is indirectly chilling the compressed air via something else. In principle the refrigeration compressor chills the mass down to about 2 deg C, then it turns off. The mass slowly creeps up in temperature to a pre-set point, then the compressor cuts back in again to pull the temperature back down again.
An argument put forward by manufacturers of this type of dryer is that the refrigeration compressor only runs intermittently - this must be cheaper in terms of running costs. Is this true? Let's study some of the basic points.
1. This type of dryer has by design introduced another medium, the refrigerant is not directly acting upon the compressed air. Nothing in life is 100% efficient, and the chilled mass dryer has introduced another inefficiency.
2. Refrigeration compressors use electric motors. Like all electric motors these are limited to a specific number of starts per hour. A hermetic fridge compressor may be limited to, say 12 starts per hour - once every five minutes - and the timer ensures that the compressor cannot start for a minimum of five minutes after it has stopped. This means that regardless of the temperature of the chilled mass, regardless of the condition of the compressed air, the dryer will not start again until the five minutes has passed. You could theoretically be up to a +20 deg C dewpoint by the time the dryer cuts back in again.
3. If you consider a case where the dryer is running at between 70% and 100% load, the bypass type dryer will be going on load and off load to suit the fluctuating demand. The chilled mass dryer will be attempting to cool the compressed air, AND it will be attempting to pull down the temperature of the chilled mass at the same time. This means that whilst the gas bypass dryer is running off-load, the chilled mass dryer is still rattling away at full load.
These three points indicate that the chilled mass dryer may not be as cheap to run as the manufacturers would like you to believe.
On the plus side, if the refrigerated chilled mass dryer is oversized, then it will genuinely shut down for long periods. The compressed air dewpoint will gently float up to the pre-set point, and the compressor will cut in again, pulling the temperature of the chilled mass down again. To achieve this you would need a refrigeration system much bigger than the one installed in the hot gas dryer, which means higher running costs, albeit for a shorter running time.
A second plus point is that the refrigeration circuit is simple. A thermostat turns the compressor on and off, which avoids using expensive capacity control systems. A box of sand or a tank of glycol is comparatively cheap so this should mean a cheaper dryer, just remember how much sand costs the next time you walk past a builders merchant.
An interesting page that discusses refrigerated air dryers in a little more detail can be found at
www.filterworld.co.uk/VT-dryers.html