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This paper was presented by domnick hunter at the I Mech E European Conference on Developments in Industrial Compressors and their systems in April 1994

SYNOPSIS

This paper describes the application of an energy saving technique for heat regenerative desiccant dryers. This technique is well established and the equipment used to implement the technique is well proven and operating successfully in many compressed air plants around the world. The energy savings arise as the cycle of the dryer is matched to the demands placed on it by the compressed air system. Several field studies were undertaken in the U.K., the results of which are tabulated. Payback periods averaging 12 months were often observed. Before the energy saving system is described, a brief outline of the operation of a standard PNEUDRI® desiccant compressed air dryer will be presented.

Standard cycle of a desiccant dryer

A desiccant dryer consists of two chambers, each filled with desiccant material which adsorbs water vapour from the compressed air. At any time, one of these chambers is on-line drying the process air, whilst the other is being regenerated (see Figure 1). The regeneration process uses a portion of the dried process air as a counter current purge. This purge air may be un-heated as in the so-called heatless dryers. However, in heat-regenerative dryers, the amount of purge air is reduced and its effectiveness increased by using heat energy.

Figure 1: Schematic of a desiccant dryer.

The desiccant chambers are repeatedly regenerated and brought on line using some form of cyclical controller. The drying/regeneration cycle is optimised for the rated airflow of the dryer at certain fixed inlet air conditions, usually 35°C (95°F), 100% relative humidity and 7 bar g (100 psi g) line pressure (see ISO 7183, 1986 E). The controller continually cycles the dryer whilst it is switched on, regardless of the loading placed on it. This is where the benefits of a dewpoint switching system become evident.

Dewpoint dependent switching

The adsorption capacity of the desiccant within the dryer is essentially constant whereas the moisture loading and the air flow through the dryer are continuously varying. In order to maintain the specified air quality downstream of the dryer, it has to be sized for the worst case conditions, namely minimum pressure, maximum flowrate and inlet temperature. These conditions may only occur for a small part of the service life of the dryer, for example, the highest inlet temperature may only be present during the summer months. This means that the moisture loading on the desiccant beds is below the dryer's capacity for much of its service life e.g. quiet periods in-between shifts usually have lowest air supply requirements. To gain access to this dynamic adsorption capacity, a moisture sensor is fitted which monitors the downstream dewpoint. This interrupts the normal sequence of the controller which then instructs the dryer to change over only when the desiccant has adsorbed moisture to its capacity. This effectively elongates the drying cycle. However, as regeneration has been optimised for a fully laden desiccant bed, this remains of constant duration resulting in a period of zero energy consumption; both purging and heating are discontinued (see Figure 2). In this way energy savings are obtained when compared to the operation of a fixed cycle, without sacrificing the resultant air quality. Perhaps an analogy will help our understanding. The desiccant beds are buckets which are being filled with water. Due to the size of the buckets, they have a finite capacity. If the rate at which the bucket is filled is halved, it takes twice as long to fill, but the same time to empty. A standard controller empties the bucket after a fixed period, whether it needs to be emptied or not. The moisture sensor, however, is like a level detector; it tells you to empty the bucket when it is full but before water is spilt onto the floor.

a) Desiccant begins absorbing moisture.

b) End of normal cycle. Some residual desiccant capacity.

c) Moisture sensor detects change over point. No purge or power consumption during this period.

d) Regenerated bed comes 'on-line'.

Advantages of hygrometer sensing

In order to gain maximum energy savings from the dryer, all of the dynamic adsorption capacity of the online desiccant bed must be utilised without causing loss of dewpoint. The moisture sensing system must, therefore, be able to react to all of the changes in inlet conditions which may effect the adsorption process. It must also have a sufficiently rapid response time to detect the leading edge of the adsorption front before breakthrough has occurred. Sensing the moisture content of the effluent air using a hygrometer represents the only measurement which directly confirms the quality of the dry air produced by the dryer. It can also respond to changes in the inlet or process conditions, many of which have complex interaction mechanisms. As the shape of the mass transfer front changes e.g. for low dewpoint applications or throughout the service life of the desiccant, the dryer's cycle is automatically adjusted to suit. The sensing method described above has many advantages over alternative systems. In-bed capacitance probes infer the adsorbed moisture content of the desiccant by monitoring the electrical capacitance of a sample of the desiccant bed. The assumption made here is that the sample of the desiccant bed adjacent to the probe is representative of the whole, which may not be the case. Other systems attempt to infer when the dynamic capacity of the desiccant beds is fully utilised indirectly by monitoring the inlet air conditions using temperature and pressure measurements. Another energy saving system monitors the temperature of the exiting purge air and terminates regeneration when a pre-set temperature has been reached. This system focuses on the regeneration process rather than the delivered air quality and is prone to fluctuations in environmental conditions e.g. ambient temperature and conduction and radiation to surrounding equipment and pipework. These alternative systems may offer the advantage, in some cases, of lower initial sensor costs, but in comparison to the overall costs and energy savings obtained, their choice is, indeed, a false economy.

Moisture loading As energy savings are inversely proportional to the moisture loading on the dryer, a short discussion of the parameters on which this is dependent may be of interest. The moisture loading on the dryer is proportional to 1. Inlet air flow rate. 2. Inlet air moisture content (fixed by temperature, relative humidity and pressure). The moisture content of the air can be calculated as follows:-

where:
MC = moisture content (Kg water/Kg air)
Psat = saturated vapour pressure of water at given temperature (bar) (source ISO 7183 Ref 1)
B = absolute barometric or line pressure (bar)
RH = Relative Humidity (%) The factor 18/28.9 is the ratio of the molecular weights of water and air. Using this formula we may calculate the moisture content of the air under a range of conditions. For example, figure 3 shows how the moisture content changes with air temperature at 7 bar g and 100% Relative Humidity.

Figure 3: Calculated from saturated vapour pressures given in ISO 7183.

As this graph shows, a reduction in moisture content of 44% is obtained if the inlet temperature is reduced from 35°C to 25°C (95°F to 77°F) (all other things being equal). The dewpoint dependent system can translate this directly into an energy saving of 44%. Changes in inlet air flow rate and moisture loading may occur due to the particular requirements of an operators site or due to seasonal variations in ambient conditions. Figure 4 shows the variation in average daily temperatures in London throughout the year (Ref 2). As the dryer will have to be sized to suit the highest moisture load (highest ambient temperature in July would result in highest inlet air temperature) other periods during the year would allow the dewpoint dependent system to provide energy savings.

Figure 4

Case studies and energy savings

In order to estimate the energy savings available using this system, an experiment was started using the factory air supply at domnick hunter. This is a fairly typical system utilising two 220 scfm rotary screw compressors with filters and desiccant dryers. The dewpoint dependent system was fitted to the desiccant dryer. Also fitted was an hours run meter which was activated whilst regeneration of either of the beds took place. The system is left on 24 hours a day, 7 days a week with the compressor left in the automatic, pressure switching mode. The hours run meter was set to zero at a fixed point in time. After a week, the elapsed time and the reading from the hours run meter were recorded. The difference between the readings represents the energy savings obtained during the test period, calculated as follows:-

Where:-
T1 = the elapsed time (mins)
T2 = the time spent regenerating (mins)

Encouraging results were obtained and it was decided that the test should be continually monitored at weekly intervals. Measured energy savings averaging around 85% were repeatedly obtained. This equates to annual savings of approximately 35,500 KWh or £1,775 (@ 5p/KWh). As it was felt that field evidence across a range of installations would be desirable, the measurements were repeated at customers sites (see Table 1). This table shows the energy saved over an 8000 hour working year. The energy saving has been broken down into that derived from reduced consumption of heat and purge energies. The payback period has been calculated using the current list price for the dewpoint dependent system. As it can be seen, all of the 8 sites investigated had payback periods of approximately 18 months or less.

Notes:

1) Heat Energy = 0.55 KW per column

2) Purge Energy = 0.75 KW per column

3) Annual Running Hours = 8000 hours

4) Power Costs Assumed To Be 5p/KWh