Comparison of systems

The following sequence shows the structure of a system comparison of adsorption dryers. The system assessment is based on the following performance range :

Compressed air output Q = 1000 - 5000m³/h
Operating pressure p = 5 - 10 bar
Inlet temperature ti = 25 - 45° C

This output range is of interest because it is relevant to all dryer systems and not based on idealistic values. The characteristic compressed air cost parameter to be established for adsorption dryers is based on £/1000 m³. The design data is based on the average values of the overall range with

volume flow at inlet 3000m³/h
operating pressure at inlet 8 bar absolute inlet temperature 35°C
relative humidity 100%
and subsequently indicated in the form of diagrams.

a) Design data

The design data within the assumed performance range is available without the need for calculations. The individual basic parameters are :

Volume flow at inlet V m³/h
volume flow at outlet Vo m³/h
inlet temperature T_¡ °C
relative humidity rH %
operating pressure at inlet Pa bar abs.
pressure dewpoint P_dp -°C

The pressure dewpoint for this comparison is laid down at P_dp -40°C. Heatless and heat regenerated adsorption dryers achieve this pressure dewpoint reliably throughout their service life. However, closed loop systems will only achieve this pressure dewpoint for limited periods under favourable conditions.

b) Cycle times

Differing periods for the adsorption and desorption processes are established depending on the system and laid down as minimum values for optimum drying. All time periods of importance for a comparison can be determined by means of the formulae (see part 7) as individually stated. For a general estimate, the following cycle periods apply under normal conditions^2.

With an optimum layout of heat regenerated adsorption dryers, the heating period t_h will not exceed 0.65 times the adsorption period t_B, so that the required temperature decrease can be reached within the remaining cooling time t_c.

c) Installed electrical capacities

Physical laws as well as the heat allowance determine the values of the installed electrical capacities and are to be obtained using the formulae for heating energy P_h (see Formula 7.2.7.2) and blower power P_b (see Formula 7.2.7.3).

These values can also be obtained from the adsorption dryer sales literature. Table 9.7.2 shows the installed electrical capacity requirement P_iel of the adsorption dryer systems 3 without the component used by the control system.

The difference between the values for internal heat regeneration systems as opposed to external blower regeneration is based on the fact that heat is applied to the desiccant bed in different ways. Adsorption dryers with internal heat regeneration permit direct contact between the heating element and the drying material with optimum heat transfer accompanied by minimum losses. With blower regeneration, the ambient air drawn in and heated by the blower is the means of conveying externally generated heat. Heat losses arise from the heating element to the air and, additionally, from the air to the drying material.

d) Air quantities

The air requirement for regeneration as well as for purge air has been explained (see Part 7) with the calculations for adsorption dryers. The quantity for regeneration and/or purge air, which is withdrawn from the volume of dried compressed air, i.e. purge air is no longer available to the process. For adsorption dryers with blower regeneration, not only cooling air but also additional purge air from the system is required and has to be taken into account when costs are compared.

The quantity of purge air V_fl with the unit m³/h depends on the heat allowance and the blower running time. Table 9.7.3 shows the regeneration and purge air requirement at reference conditions⁴.

e) Differential pressures

In order to establish the differential pressures, i.e. the pressure drops, in an accurate manner, a theoretical balance sheet has to be drawn up in detail, looking separately at regeneration, adsorption and cooling.

Differential pressures of the filters, regeneration coolers and separators are taken from technical data sheets. The pressure losses within the adsorber are obtained by means of diagrams (see Diagram 7.2.1.13). Individual pressure losses for fittings and pipe connections are also considered by the manufacturers and generally applicable values taken from specialist literature offer sufficient information. The sum of individual pressure losses, if added up, provides the total pressure loss of the adsorption dryer installation including all pre and after filters (when new),

so that the effective operating pressure at the outlet of the drying installation can be determined through:

This threoretical differential pressure loss can be equated with the loss of compressor performance. The power requirement through pressure loss is allocated to the total compressor performance. This also makes it possible to determine the pressure loss as a component of the comparable electrical performance as an objective value.

The pressure loss through adsorption dryers including pre and after filters proportionally reduces the compressed air output which is passed to the compressed air system. This value should not be underestimated as a critical comparison criterion.

f) Compressor performance

Using the tables and equations of this Part, the listed performance components can be specified. The performance values have to be determined for differing regeneration and purging volumes as well as differential pressures in relation to the required compressor performance.

Thus two items have been allocated as a standardised form of energy, items which had, previously been ignored and not applied in practice. Manufacturers of compressors quantify the specific performances (see Part 3) of compressors, which are then used as basic parameters in the course of system comparison between adsorption dryers. For approximations, the following value from the below Table was used, referred to the operating pressure p = 8 bar abs.

4 p = 7 bar; tin = 35°C; Pdp = -25°C

Design Compressor	Energy Requirement Efl kW/m3/min
-
Piston Compressor
Oil Free	5,38 - 5,89
Oil Lubricated	4,82 - 4,87
-
Screw Compressor	-
Oil Free	5,89
Oil Injected	6,02
-
Turbo Compressor	5,89 - 6,92

The appropriate basic value from Table 9.7.4 is multiplied by the volume flow of regeneration, purging and cooling air quantity. The product is the comparable performance P_c allocated to the compressor in accordance with

g) Average performance requirement

Derived from the compressor shaft power, needed to achieve the volume flow at the inlet to the adsorption dryer, differentiated performances are established.

Taking into account adsorption and regeneration time, the average performance requirement for regeneration air has to be established by:

For the purging air by:

The average performance requirement for the performance loss through the pressure differential is calculated by:

The method for system comparison shown here, an old and questionable analysis has been eliminated and replaced with a well founded, consistently logical approach.

The specified topics are supplemented by the formulaes. for average energy requirement for heating and blower, and allocated to the appropriate adsorption dryer system wherever this applies. For each adsorption dryer, the sum of average performance is established in accordance with:

The energy and performance data can be established by this scheme and is qualitatively comparable. Through the mathematical determination of these well founded values, we can in principle do without the random figures for the costs of compressed air. The following overall view reproduces the most important basic data for the different adsorption dryer systems.

The following table shows the characteristic specific energy requirement of adsorption dryer systems.

Design Adsorption Dryers	Energy Requirement E kW/m3/min
	Elect. Energy	Reg. Air	Flush Air
Heatless Regeneration	-	0,697	-
Heat Regeneration	0,176	0,203	-
Blower Regeneration	0,342	-	0,053
Heat of Compression System	-	-	-

Other energy requirements, such as steam, hot water, heated oil, for externally heat regenerated adsorption dryers, or even cooling water for adsorption dryers with closed loop systems, are taken into account when calculating operating costs.

A system comparison is not the same as an operating cost calculation. The system comparison is used in order to select the best adsorption dryer. The operating cost calculation is used to find the ideal supply of energy for the particular application.

h) Cost of energy

The energy costs C_O for electrical power, steam, hot water, heated oil or cooling water, must be entered by the operator. With the input data of energy costs and the sum of the average performance, the energy costs per hour of operating time are calculated through :

To form a uniform parameter, the energy costs per unit of volume of compressed air are determined as a basic value in £/1000 m³. This basis of comparison has proved its worth in practice.

Regarding energy costs, the result will generally correspond to the expectation that adsorption dryers with heatless regeneration are less economical, while adsorption dryers with heat regeneration are more economical. This conclusion refers to the result in accordance with the performance ranges

The apparently high cost of compressed air purification must be set against the benefits derived from the compressed air treatment. The operating cost calculation is only complete when the difference between cost of investment including operating costs and effective cost savings have been established.

The diagrams in the following section show how the differentiated energy costs have to be allocated to the differing adsorption dryer systems.

i) Energy costs diagram

At the beginning of this section, the performance ranges of adsorption dryers valid for this comparison were specified. Using the formulae presented and the solution procedures indicated, an evaluation on the basis of industrially based values involves relatively complex work. The data are evaluated by computer analysis and the result presented in a clear manner in the form of diagrams.

It shows the energy costs of compressed air systems in £/1000 m³ at constant volume and unchanging inlet temperature but variable pressure.

This diagram establishes clearly that heat regenerated adsorption dryers used for higher operating pressures are assessed more favourably than adsorption dryers with heatless regeneration, when used at the same high pressures. In the case of adsorption dryers with heat regeneration, the higher operating pressure not only leads to an optimum relationship between volume flows from the inlet to the operating state but, at the same time also to a more favourable load factor through the pressure dependent secondary relative humidity . This factor does not apply in the case of adsorption dryers with heatless regeneration.

This diagram shows the energy costs of compressed air systems in £/1000 m³ at constant volume and pressure but various inlet temperatures.

It is clear from all the curves that, at higher inlet temperature, adsorption dryers with heatless regeneration should operate more favourably than adsorption dryers with heat regeneration. In the case of adsorption dryers with heat regeneration, the higher inlet temperature influences the secondary relative humidity

In Diagram 9.7.3, the curves for adsorption dryers with heatless regeneration and closed loop adsorption dryer systems remain about the same, for these systems the change in volume results in a change in energy cost. Heat regenerated adsorption dryers with internal and external regeneration show a reducing cost per 1000m³ as flowrate increases. This is due to the differences in the equipment and the combined types of regeneration such as electric heater and regeneration air requirement. The quantity of these regeneration energies, electrical energy and compressed air requirement, is caused by different factors and results in a different shape of the curves.

There are three widely differing diagrams which, make clear statements about the energy costs of the individual dryer systems, based on a differentiated but factual assessment.

j) Capital service

Before an adsorption dryer is installed, capital resources for the investment have to be made available. For a comparison which includes investment, capital costs have to be incorporated in the evaluation. The annual capital amortisation is calculated while taking into account equipment costs A, writing off period n and the interest rate p (z = p / 100) as follows :

Operational running time and differentiated flows at the outlet of the adsorption dryer systems have to be determined by converting the capital service K distributed over the service life t_Oa per year and expressed in capital amortisation per hour.

System comparisons claiming to arrive at an objective evaluation can be meaningful only if arrived at on the basis of a uniform framework. Due to their design, adsorption dryers with heatless regeneration are cheaper to manufacture than systems using hot regeneration. With these systems, capital and energy costs move in opposite directions.

k) Maintenance/spare parts costs

Maintenance and spare parts costs should also be included in any comparison. For reasons of simplification, these values are expressed as percentages, depending on the cost of the installation and on the basis of data gathered from experience.

Independent of the numerical values arrived at through estimated flat rate costs, differentiated tabulations concerning the service life of adsorption dryers can contain accurate costs per unit of time. The following values are realistic as guideline values:

Maintenance and spare parts costs

Maintenance Costs Spare Parts Costs	Heatless Regeneration	Heat Regeneration
		Internal	External	Closed Loop System
Maintenance Costs	2,0%	4,0%	4,0%	3,0%
Spare Parts Costs	1,0%	2,0%	2,0%	1,5%
Prefilter Element	0,1%	0,1%	0,1%	0,1%
Outer Filter Element	0,1%	0,1%	0,1%	-

percent values stated in the table for maintenance and spare parts costs are based on the manufacturers’ standard. Adsorption dryers based on a different specification have to be separately assessed. The maintenance costs C_mh per operating hour t_Oh taking into account the annual operating hours t_Oa have to be established by :

The spare parts costs C_sph per operating hour t_Oh are to be calculated by:

l) Total costs

The sum total of energy costs C_eh , capital servicing C as well as maintenance and spare parts costs C_sph are to be collated for the annual operating period as follows :

The total costs £/1000 m³ of treated compressed air referred to the usable volume flow at the outlet of the dryer find their definitive determination through the concluding formula.

Only the energy obtained by multiplying volume V_ox pressure po at the outlet of the adsorption dryer is at the disposal of the user. The data indicated at the inlet to the dryer is relevant for determining the size of the adsorption dryer and, for this reason, only of secondary importance for comparison purposes.

Figures on their own are informative but can, nevertheless, lead to flawed decisions if the comparison of systems does not take into account the subjective, and thus variable, decision criteria for the adsorption dryer in question.

The system comparison contains a numerical result from the values supplied from the basic data. The below Diagram shows the energy and capital costs of compressed air systems in £/1000 m³ at constant inlet temperature and pressure but variable volume.

Capacities above 5000 m³/h are intentionally not shown as, above this performance range, we enter the field of heat regenerated adsorption dryers. This range was established on the strength of realistic assessment aids, so that unbiased evaluations could be built up with other basic data in a reproducible manner.

The decision for or against a particular adsorption dryer system should not be taken on the basis of energy and capital costs calculation alone. What does make sense, is an application catalogue, by means of which all aspects are weighted and assessed, always with the actual requirement in mind. There is no uniform and universally valid answer to the question of “Which adsorption dryer system is the best one?”. For each case and application there is only one correct dryer which, must be individually evaluated following the various criteria.

m) Cases of application

The question of which adsorption dryer system should be utilised can, generally be assessed by the following scheme. This scheme does not replace a system comparison, but makes a general and preliminary selection possible.

Typical application case