Compressor and receiver sizing for ancillary systems

       Introduction

    Compressors are commonly used in industry to transfer various media and are essentially mechanical devices to compress working medium in gas form. There are a wide variety of compressors, thus, proper selection and calculation of compressors is required to best fit with industry-associated application .

 Compressors with rotating blades are used for flows with large volume rates and low discharge pressure, while piston compressors are destined for high pressure. Operational considerations , namely purpose , designed values of intended users,gas data , required flow rates, temperatures -operational or final, cooling medium and operational area  play a vital role in deciding the compressor and or  receiver combination.

Types of compressor units

The basics steps for selecting a compressor unit.

Compressors are separated usually in two large subgroups: 

dynamic 

positive-displacement

Inlet  and outlet pressure shall vary from deep vacuum to surplus pressure depending on operational needs. This is one of conditions to match type and configuration of compressor.

Properties of working medium. Gas compression

Compressor may compress different gases. Gas thermodynamic or compressible gas mixture properties must be furnished to a vendor to properly configure the compressor unit. Full content, common name and chemical formula of gas are required for calculating the compressor unit. Data sheets of compressor units shall clearly indicate the gas testing data with each component name, molecular weight, boiling point and etc. listed. This data are very important for identification of correct compressor values. Ratio between general gas values (pressure, temperature and volume) is called gas equation.

The simplest gas equation is the ideal gas equation.

P · V = R · T

where:
P — pressure,
V — molecular weight,
R — gas constant,
T —temperature.

This equation applies to gas only, temperature of which is higher than critical temperature and pressure is way lower than critical pressure. The air shall abide by this law under atmospheric conditions.

Real gas differs from ideal gas by the factor called compressibility (“N”). The term “compressibility” is used in thermodynamics to explain deviations of thermodynamic properties of real gasses from properties of ideal gasses.

P · V = N.R.T , T is in kelvin (T=T deg C + 273)

N value –  gas constant , its pressure and temperature.

Compression process

Compression ratio (R) – is the pressure ratio at discharge to suction pressure:

R = Pd/Ps (where Pd and Ps are absolute).
One stage compressor has only one R value.
Two stage compressors have 3 R values.

R = overall compression ratio
R1 = first stage compression ratio
R2 = second stage compression ratio.

R = Pd/Ps

R1 = Pi/P

R2 = Pd/Pi

Ps – suction pressure
Pd – discharge pressure
Pi – pressure between stages

While compressing the air in compressor unit, molecular weight becomes lower to result in less spacing between molecules. Since quantity of gas molecules is increased in fixed volume, its weight and density of fixed volume also increase. Growth of density results in pressure .

Compressor operations are focused on gas pressure and temperature to be increased and heat to be removed from compressor. In most cases it is required to increase the gas pressure with least capacity values. 

If the compression process is adiabatic, no heat is transmitted between compressor and environment to result in less operation during isentropic compression. This assumes no losses in compressor, which indeed is unachievable.This can be used for indicative compression performance index. 

Isentropic compression performance index is identified as operational compression during isentropic process divided by actual operations used for gas compression. Compression performance index is often indicated as isentropic performance index.

However, it is possible to make a compressor with over 100% isentropic performance indexes. Operations in reversible isothermal process are less than in isentropic process. Gas temperature in reversible isothermal process is maintained with reversible heat transmission during compression at suction temperature. This process  has negligible losses. However, consumed capacity is almost always more than isentropic capacity ,thus, isentropic performance index is used for compressor classification.

Two types of compressors - displacement and dynamics- currently present differ in principles of working medium compression. Displacement compressors compress gas to detain significant gas volumes in closed environment with subsequent decrease of volume. Compression starts when certain amount of gas enters the process chamber of the unit with subsequent decrease of inner volume of the process chamber.

Dynamic compressor type is used to compress gas by means of mechanically-operated blades or impeller to transfer speed and pressure of gas. Larger impeller diameter, larger molecular weight of gas or higher rotations will produce more pressure. Usually displacement compressors are selected for fewer amounts of gas and larger pressure values. Dynamic compressors are selected for larger amounts of gas and lower pressure values.

Basics steps to compressor selection to include

1. Calculate compression ratio.
2. Select whether one stage or multiple stage compressor needed.
3. Discharge temperature calculation.
4. Identify volumes required.
5. Identify operational volumes required.
6. Select compressor model.
7. Identify minimal rotation torque of selected compressor.
8. Select actual rotation torque.
9. Calculate actual operational volume.
10. Calculate capacity required.
11. Select suitable configurations.
12. Select proper compressor.

Data sheet of compressor equipment

Most important data sheets of compressor equipment are emphasized below:

• Outlet pressure in atm/ bars. Most  compressors produce from 6 to 8  bar and more . Operational pressure of compressor is an average between maximum producible pressure to stop the discharge process and minimum pressure in the system to start-up the compressor. Normally, the pressure differential is 2 bars between stop and start-up the compressor for up to 7 bars.Compressors are divided into compression machines of low, medium and high pressure. Not all compressors have sufficient capacity to compress the air while producing high pressure; as a matter of fact only heavy-duty piston units may reach 30 atm or higher pressure index. 
• Screw-type compressor versions may not operate at such high indexes;
  Inlet / outlet pressure. Least inlet gas flow shall be indicated in data sheet of compressor units. This is required to ensure the compressor capacity.
• Inlet temperature. Volume flow rate, pressure requirements and required power may affect the inlet temperature. Thus, maximum inlet temperature shall be also specified.
• Discharge temperature. Discharge temperature (Td) depends on inlet temperature, compression index, gas specific heat and compression performance indicator. This temperature is important for mechanical design of compressor, selection of compression stage, calculations of cooler and pipelines.
• Suction capacity or air discharge (amount of induced or forced air at outlet). This is the first value is stated by manufacturers of compressor equipment . Running compressors lose capacity at discharge due to air loss and suction capacity is always a bit higher. Compressor capacity is volume of compressed air flow upon a particular given time and is expressed in m3/hour or l/min.Capacity is expressed at 293 kelvin (20deg cent) in industrial standards(+/- 5% dev) as NM3/hr, which gives a reference standard . When selecting the proper compressor about 30% oversized capacity shall be taking into account.Theoretical capacity of compressor is defined by geometric measurement of air in the working area during one suction period. Then this volume is multiplied by the number of periods (cycles) per time unit. This theoretical capacity is higher than actual capacity of compressor unit. Theoretical and actual capacity difference is compensated by the capacity factor (Cf) to depend on suction conditions and compressor unit design specifics (valve losses: suction and discharge, amount of volume not completely displaced) to enable decreasing the volumetric efficiency (piston compressor). Capacity factor of industrial design compressors amounts from 0.6 to 0.8.

Difference in theoretical and actual compressor calculations at inlet and outlet may reach a significant value. When indicated in the data sheet theoretical capacity of compressor unit shall be recalculated for capacity at outlet, thus, to decrease the value by 30-40%.

• Motor power is measured in kW. Motor may be diesel, turbine or electric. Motor power is one of the core parameters to ensure compressor to manage air discharge. The higher the capacity the more power is consumed. Failure to calculate the power correctly may result in power consumption without effect. Usually, heavy-duty motors are installed in high power units requiring this particular drive;
• Compressor weight and overall dimensions. These values may vary from typical small size units easy to transport and operate in a garage to heavy-duty ones to require more space . compressors with rotating blades may be used to constantly discharge air without the need for receivers.
• Air receiver is also an important parameter to enable idle operations. Air receivers are tanks designed for collecting compressed air. Receiver volume enables continuous off-line operations of pneumatics. It serves to energy saving, when properly sized.
• Corrosiveness of transferred gas. Composition of corrosive gas shall be identified for all operating conditions. It is important because of cracks coming from corrosion under pressure in high-strength material.
• Liquid in gas flow. Liquid in gas shall be avoided. Otherwise, it may cause malfunction of compressor. Separator shall be installed to dewater working medium; electrical tracing and insulation of inlet port shall be made when outside temperature is below gas dew point or hydrocarbon components heavier than ethane are compressed.

In some industries, like nutrition sector, no contaminants are allowed in compressed air. In this case, when selecting compression unit, power parameters shall be less preferential than design features. Data sheets of compressors shall comply with compressed air purity requirements with unit compression to be processed without any lubricating oils applied to working surface.

Design particulars of compressor equipment

Design particulars of compressor are as follows:

• Type of drive. ICE or electric motor;
• Stage numbers for air compression. This parameter is justified and important for selection of piston compressors to enable gas compression in several cylinders in stages;
• Cooling system (air and water).
• Mobility. Compressors to be fixed on special foundation or on trailer for ease of transportation;
• Part element arrangement. All constituents of compressor unit may be frame or  Data from technical passports of their compressor units are based on theoretical capacity (suction capacity).
• Receiver sizing and installation.

Designing a compressor

Compressor data sheet shall by all means indicate maximum allowable operating pressure. In line with maximum allowable temperature values these data shall be used by manufacturers to design the body and main parts of compressor to withstand maximum allowable operating pressure and temperature. 

Centrifugal and reciprocating compressors maximum allowable operating pressure is computed with adding maximum inlet pressure to maximum differential pressure to take place in compressor at more complex set of conditions. 

Piston cylinders and compressors with rotating blades body maximum allowable operating pressure shall be higher than nominal discharge pressure by 10% or 1.65 bar depending on which value is higher.

Maximum allowable temperature shall be maximum discharge temperature for centrifugal and reciprocating compressors during operations to include some deviation values. Maximum allowable temperature for cylinders of piston compressors and body of rotating blades compressors shall be higher than nominal discharge temperature.

Pipeline flanges and nominal value

End-to-end dimensions, nominal flange value and type shall be clearly stated in data sheets for all compressor inlets and outlets. Shaft seal and piston rod also shall be clearly stated in data sheets.

Materials

Gases under compression may help selecting the compressor materials; in particular, it pertains to contact elements. For example, while compressing H2S, sulfide cracking of high-strength materials may occur. Materials suitable for operations are considered to be thermally treated and yield point lower than 6123.5 bar

Process compression stages

Compression ratio (R) is the ratio of discharge pressure (Р2) to suction pressure (Р1) in compressor, Р2/Р1. When compression to higher pressures is required, compressor calculation assumes several compression stages; in some cases coolers are required to remove heat between compression stages. Additional compression stages are required 

• To reduce temperature at the end of each compression stage applying interim cooling up to allowable level to ensure proper compressor operations.
• To reduce temperature at inlet compression stage to drop the flow required for reaching the set compression ratio.
• To ensure differential pressure limits of various compressor types, e.g. axial load limits in centrifugal compressors, stress limit of rod in piston compressors, axial load in compressors with rotating blades.
• To reduce power consumption compressor drive due to interim coolers operation between stages and to maintain safe temperature limits.

Choosing a one-stage or multiple-stage compressor

The choice of proper number of compression stages is largely based on the compression ratio.

Discharge temperature and operational mode are also considered when identifying the proper number of compression stages.

R value	# stages
1-3	Single-stage
3-5	Normally single-stage, occasionally two-stage
5-7	Normally two-stage, occasionally single-stage
7-10	Two-stage
10-15	Normally two-stage, occasionally three-stage
15	Three-stage

Comparison of a single-stage and two-stage compressor both installed to do the same application (same capacity, gas and pressures):

	Single-Stage	Two-Stage
Discharge Temperature	higher	lower
Initial cost	lower	higher
Overall System Complexity	lower	Higher

As with many engineering decisions, a suitable compromise between initial cost and operating / maintenance costs must be found.

1.  Estimation of air consumption of all consumers, (Q) l/min / M3/h
 This ratio is close to maximum parameter, should a large number of consumers is involved. It may be reduced by load coefficient, since not all consumers are involved at the same time in operations. The goal is to introduce corrections for reduction to be at sole discretion of compressor unit owner to ensure sufficient air volume in pneumatics.

2. Next parameter for calculation is compressor capacity (Q1), All compressor manufacturers indicate maximum inlet air consumption in technical passports or catalogues. This parameter may not be applied as outlet compressor capacity since this parameter does not include compressor performance and design specifics. In this regard calculation of compressor capacity shall be as follows:

Q1 = Q · (b/h)

where
Q – total air volume to be consumed by all consumers of pneumatic system to be measured in l/min;
b- coefficient to count for design specifics of compressor unit by manufacturer;
h– performance index of compressor unit.
b and h values (for reference) are for compressor operations within working pressures from 6 to 8 bars are given below.

Compressor design	b	h
Semiprofessional compressors	1,7	0,55
Professional compressors	1,5	0,65
Heavy-duty compressors	1,3	0,75
Rotary compressors	1	1

3. Duty cycle of the compressor 

Duty cycle is defined as the amount of time the compressor runs to fulfill the operational requirements of the system in the total time.

If a compressor runs 7 minutes every 10 minutes, the duty cycle is 70%.(3 min rest).

Typical values considering cooling, wear and tear , deterioration and use the duty cycles have been defined for compressors as follows

Reciprocating compressors : 30- 70%

Screw compressors             : 100% (pressures up to 10 bar) , lower for higher pressures.

This is the main factor to be used for receiver sizing of units.

Receiver size is the buffer volume within the pressure range for duty rest cycle.

Receiver Volume (Cft) = (Q1 -Q)* T filling*Patm / ( P max-P min)

Q1 : Air requirement, CFM

Q : Capacity of compressor, CFM

P max -P min : operating range PSIG ( differential start /stop)

P atm :  14.7 psi or absolute air pressure.

Standard rule : 1.25 to 1.5 times CFM requirement, rounded for small units.

Manufacturers recommend following range  Q1, when selecting the volume of receiver:

V = (1/2 to 1/8)·Q1

Rest time of the compressor, should be within the expended time of air bottle within the restart at max consumption basis duty cycle fixed.

5. Compressor characteristics, especially, capacity values are affected by elevation above sea level, ambient temperature and atmospheric pressure. The higher the elevation is the lower the temperature and ambient pressures are. This should be noted when operating an air compressor under such conditions because these conditions affect capacity values of compressor and nominal consumption of compressed air. Thus, should the compressor be operated at high altitudes, output characteristics would differ from those specified in technical passport in certain manner.

In fact, the air is discharged at heights to result in deterioration of cooling of electric motor of air compressor and its heat-affected parts. Motor to operate under nominal characteristics at maximum elevation above sea level of 1000 m and maximum temperature of 40°С 

Algorithm for choosing an air compressor based on capacity and pressure values. Scheme of compressor selection

Proper type of compressor may be selected based on general initial data following the scheme below.

Below compressor characteristics are based on compressor type:

Type of compressor:	Maximum values:
Piston	Q = from 2 to 5 m3/min РН = from 0.3 to 200 Mn/m2 (laboratory study indicated up to 7000 Mn/m2) n = from 60 to 1000 rpm N = max. 5500 kW
Rotary	Q = from 0.5 to 300 m3/min РН = from 0.3 to 1.5 Mn/m2 n = from 300 to 3000 rpm N = max. 1100 kW
Centrifugal	Q = from 10 to 2000 m3/min РН = from 0.2 to 1.2 Mn/m2 n = from 1500 to 10000 (max. 30000) rmp N = max. 4400 kW (for sky-borne - 10 000 kW and above)
Reciprocating	Q = from 100 to 20000 m3/min РН = from 0.2 to 0.6 Mn/m2 n = from 2500 to 20000 rpm N = max. 4400 kW (for sky-borne max. 70000 kW)