Performance Assessment of a Two-Stage Reciprocating Air Compressor

This paper presents the performance assessment of a two-stage reciprocating air compressor operating at an Oil and Gas Terminal in Rivers State, Nigeria. The main focus was to investigate the effects of control parameters and clogging on the performance of the compressor. Data were obtained from the manufacturer’s manual, field reports and the field operator’s log sheets. Relevant thermodynamic equations were used to determine and analyse appropriate control parameters of the compressor. Data were also analysed using various appropriate compressor equations and a thermodynamic analysis of the compressor was done to evaluate its performance. The outcome of all the analyses showed that the compressor experienced 26% loss or reduction in the volumetric efficiency, 8% loss in the isothermal efficiency, 11.1% loss in volume flow rate and 21% decrease or reduction in the mass flow rate due to clogging when compared with the design specifications. The analysis also showed that the performance of the compressor was affected by several other factors including the climatic and environmental conditions such as the high operating ambient temperature of the inlet air to the compressor. It revealed that the effects of clogging caused a continuous rise in temperature which reduced the discharge pressure, mass and volume flow rates, isothermal and volumetric efficiencies; thereby reducing its performance in comparison with the design specifications. The results further revealed that clogging was a major factor that affected the performance effectiveness of the reciprocating compressor.


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Abstract-This paper presents the performance assessment of a two-stage reciprocating air compressor operating at an Oil and Gas Terminal in Rivers State, Nigeria.The main focus was to investigate the effects of control parameters and clogging on the performance of the compressor.Data were obtained from the manufacturer's manual, field reports and the field operator's log sheets.Relevant thermodynamic equations were used to determine and analyse appropriate control parameters of the compressor.Data were also analysed using various appropriate compressor equations and a thermodynamic analysis of the compressor was done to evaluate its performance.The outcome of all the analyses showed that the compressor experienced 26% loss or reduction in the volumetric efficiency, 8% loss in the isothermal efficiency, 11.1% loss in volume flow rate and 21% decrease or reduction in the mass flow rate due to clogging when compared with the design specifications.The analysis also showed that the performance of the compressor was affected by several other factors including the climatic and environmental conditions such as the high operating ambient temperature of the inlet air to the compressor.It revealed that the effects of clogging caused a continuous rise in temperature which reduced the discharge pressure, mass and volume flow rates, isothermal and volumetric efficiencies; thereby reducing its performance in comparison with the design specifications.The results further revealed that clogging was a major factor that affected the performance effectiveness of the reciprocating compressor.

I. INTRODUCTION
Compressors are of vital importance in modern-day industries.They play tremendous roles in ensuring that industrial processes and works are done [1].Compressors are mechanical devices that can compress a substance in gaseous state [2].When air is used as the working fluid in a compressor, it is called an air compressor.This means that an air compressor is a machine that increases the pressure and temperature of air [3].During the process of air compression, the inlet/intake pressure of the air is usually raised to a higher exhaust pressure [4].
Air compressors have earned a fair amount of popularity in several industries as a result of the various uses of compressed air.Some of such uses and applications of compressors and compressed air are in the industries and in equipment such as refrigerator, air conditioner, trucks and for the filling of apparatus used for breathing, bicycle pump and the painter's spray gun.Usually, a compressor is driven by a prime mover such as a diesel engine or electric motor [5].
In general, air compressors are sometimes classified according to their working principles or principle of operations, according to their actions and according to their number of stages.Furthermore, based on the principle of operation, compressors are classified into positive displacement compressors and non-positive displacement or dynamic compressors [6].Positive displacement compressors are also grouped into reciprocating and rotary compressors.They are also called piston compressors [7].Reciprocating compressors operate on the same principle as the old, familiar bicycle pump which is by means of a piston in a cylinder [8].
A reciprocating air compressor increases the pressure of air via the use of piston by ensuring that the volume is reduced in such a way that it takes in successive volumes of air, confines the air within a space called the cylinder and elevates the air to a higher pressure [9].The piston compressor is considered one of the most widely used equipment for air and gas services and it embraces the use of pistoncylinder arrangement such that the piston is driven by a crankshaft and that causes a displacement of the piston in the cylinder thereby resulting in a rise in pressure [10].
Some of the key components or parts of a reciprocating air compressor are cylinder, crankshaft, piston, suction/inlet and outlet/delivery valves and connecting rod [11].Reciprocating air compressors may be single or double acting, single stage or multi-stage [11], [12].In a single stage reciprocating air compressor, compression takes place in one stage while the multi-stage type involves more than one compression stage for achieving the required high pressure and low temperature by using intercooler [13].An example of the multi-stage compressor is the double or twostage compressor under consideration in this work.
Apart from the segmentation of the compression stages into different stages to help increase the pressure and reduce the temperature during the compression process for the improvement of the performance of the compressor, one major concern is the problem of compressor clogging which it experiences -especially in a humid environment.The air compressor under study is located at an Oil and Gas Terminal in Rivers State, Nigeria.The said compressor had suffered breakdown in the past due to clogging thereby reducing production because of the period used in maintenance.The aim of this study, therefore, is to assess the performance of the compressor with regard to the problem of clogging.The objectives of this research are to investigate the effects of the control parameters (pressure, temperature, mass and volume flow rates) on the Performance Assessment of a Two-Stage Reciprocating Air Compressor Ikpobari Amuele Nwakpang, Barinaadaa Thaddeus Lebele-Alawa, and Barinyima Nkoi performance of the compressor, to investigate the effects of clogging on the performance of the compressor and to do a thermodynamic analysis of the said compressor to assess or evaluate its performance.
In a bid to improving the efficiency of two-stage reciprocating air compressor due to high temperature, an experimental investigation was performed involving the use of different intercoolants such as air, water, cooling source, radiator coolant and ethylene glycol.Consequently, it was concluded that the isothermal work required to compress the air and the power required to drive the reciprocating compressor reduced with respect to the intercooling [13].Reference [14] carried out a thermodynamic analysis of a reciprocating air compressor.During the thermodynamic analysis, they found out that the clearance volume is one of the parameters on which the performance and efficiency of the compressor depend.
Reference [15] presented a paper on improving the performance and development of two stage reciprocating air compressor with particular focus on the case study of a reciprocating air compressor of a locomotive with the associate problems, diagnosis and solutions highlighted.It involved a simulation model which they did.The simulation model can predict volumetric efficiency, free air delivered, indicated power, shaft power, cylinder air pressure, cylinder air temperature, resultant torque and the mass of air drawn in or discharged out per cycle, by varying any operating parameter like speed, discharge pressure, etc. and physical parameters like clearance volume, crank radius, connecting rod length and cylinder diameter.Also, compressed air was produced without the use of electricity.That was done with the use of speed breaker setup in roadways.Instead of using electric drive, mechanical drive was used to generate the compressed air.This method of using speed breakers for the generating of electricity is of great value especially because of the over reliance on electricity for the production of compressed air which has been the norm for many years now [16].

A. Materials
The manufacturer's manual, operational data and field reports for the air compressor under investigation were used for this study.From these data, values were computed and used for the assessment of the performance of the compressor.Also, relevant operating parameters such as the inlet and outlet temperatures and pressures of the compressor were obtained for its thermodynamic analysis.

B. Methods
The analysis and assessment of the compressor were done to achieve the aim and objectives of this research with the use of relevant fundamental equations that govern a twostage reciprocating air compressor, relevant models and thermodynamic analysis.

1) Analytical Model
A thermodynamic analysis was chosen as a suitable method for analyzing the compressor under consideration.There are three types of compression processes which are possible in the reciprocating air compressor.They are polytropic, isothermal and adiabatic compression processes.The area under Fig. 1 shows the workdone in compressing the air to P2 from P1 and the line with the least slope (isothermal compression) produces minimum or lowest work.As seen from the graph, isothermal compression produces minimum work and it is the condition under which minimum work can be achieved in a compressor.

2) Polytropic Compression
This process lies between the isothermal and adiabatic processes.In Fig. 1, 1-2p is a polytropic process and the relationship between pressure (P) and volume (V) is given as; PV n = C (1) Also, workdone during a polytropic process is given as;

4) Adiabatic Compression
This type of air compression takes place in the absence of heat transfer [8].The workdone during adiabatic compression process is calculated using: where  = Index of compression for air

5) Volumetric Efficiency (ηv)
This is the ratio of the actual volume of air taken into the cylinder during suction stroke to the pistol displacement volume (PDV) or swept volume.
where C = Clearance ratio Isothermal efficiency is given as the ratio of the isothermal work to the actual work.

7) Multistage Compressor
Multistage compression involves more than one stage of compression and it is done when high pressures are required [17].It has the two-stage (low and high pressure stages) compression as its typical example where compression of the air is done in two stages with an intercooler incorporated between the two stages [18].The essence is basically to reduce the discharge temperature and increase the discharge pressure to the required pressure {19].Achieving lower work, better capacity and efficiency while maintaining isothermal condition is very difficult since increasing pressure ratio, rp, causes a decrease in volumetric efficiency and an increase in delivery temperature, T2 [20].This therefore requires a multistage compressor (a two-stage type in this case) to solve the problem as shown in Fig. 2.

9) Overall Pressure Ratio (rp)
The overall pressure ratio of the compressor is given as the ratio of the discharge pressure of the air to its inlet pressure.
Where P1 = Inlet Pressure (N/m 2 ) and P2 = Discharge Pressure (N/m 2 ) 10) Intercooler Pressure For minimum work condition with perfect intercooling to be obtained, the intercooler pressure is evaluated as: Px =

A. Thermodynamic Performance Assessment of the twostage Reciprocating Air Compressor
A thermodynamic analysis of the compressor under study was done to evaluate its performance behaviour with regard to the problem of clogging with the use of the equations already deduced, collected data and the operating parameters of the compressor.

B. Reciprocating Compressor Analysis
With the aid of equations deduced and substitution of the appropriate values of the operating parameters, some parameters were evaluated and tabulated as shown in Table I.Tables II and III represent clogging and normal periods respectively, show the various daily average operating values over a 30-day period when the compressor under investigation was operating under the influence of clogging and when it was operating almost normally.In Table II, the values are for a period when the compressor experienced the problem of clogging which negatively affected the performance of the compressor and thereby caused an increase in discharge temperature and decrease in discharge pressure, mass and volume flow rates respectively as the days went by.As observed in Table III, the mean values of the control parameters (discharge temperature, discharge pressure, mass and volume flow rates) over another 30-day period referred to as "normal" did not change or fluctuate significantly; they only varied around their daily mean values thereby making the average values per day over the afore-mentioned period almost constant.

C. Off-design Behaviour of the Compressor
The Tables I and II clearly illustrate the trend in terms of the changes in the operating conditions of the thermodynamic parameters of the compressor.The clarity of the effects of these parameters on the performance assessment of the compressor is therefore simplified via graphical representations as shown in Figures 3-12  From Fig. 3, it is observed that the discharge pressure is a function of the operating ambient temperature of the intake air such that as the operating ambient temperature of the intake air increases the discharge pressure also increased correspondingly.Also, a decrease in the operating ambient temperature also led to a drop or decrease in the discharge pressure.The trend in Fig. 6 shows that the change in isothermal work of the compressor affected the isothermal efficiency such that an increase in the isothermal work brought about an increase in isothermal efficiency.The reverse is the case when the isothermal work decreased as the isothermal efficiency also decreased.Fig. 7 shows what happened to the discharge temperature during clogging and normal periods.During the clogging period, the effect of clogging caused a gradual increase in the discharge temperature of the compressor over time as the days went by and this is a disturbing trend considering the danger of continuous rise in temperature on mechanical systems which can cause wear and tear, damage and consequently lead to a complete breakdown of the compressor if clogging is not minimized.During the normal period, the discharge temperature did not experience any significant change as the days went by but only the daily average temperature experienced small changes in their values around their mean values per day.Fig. 8 shows that the discharge pressure continued to drop every day during the clogging period but this was not the case when the compressor discharge pressure operated under normal condition as the discharge pressure did not experience any significant change as the days went by; rather it only experienced small changes in its values around the mean values per day.Fig. 9 shows that the volumetric flow rate of the air kept decreasing as the number of days increased thereby negatively affecting the flow capacity of the air compressor and lowering its performance.The case was different and better when the compressor operated under normal condition.Fig. 11 shows that during the clogging period, every increase in temperature caused decrease in the mass and volume flow rates.The negative effect of clogging on the performance of the compressor was such that, as the temperature increased daily, the volume flow rate and the mass flow rate of the discharged air decreased.Fig. 12 shows that the effect of clogging (during the clogging period) caused a gradual temperature increase that resulted in a gradual decrease in the discharge pressure over time as the days went by.

IV. CONCLUSION
The performance of the two-stage reciprocating air compressor located and operating at an Oil and Gas Terminal in Rivers State, Nigeria was assessed and analysed via the application of some relevant thermodynamic equations.From the analysis of the performance of the compressor, it was evident that the performance of the reciprocating air compressor depended significantly on the effects of clogging on the compressor.
From the thermodynamic analysis done, it was evident that the compressor experienced 26% loss in the volumetric efficiency of the compressor, 8% reduction or decrease in the isothermal efficiency, 11.1% loss in the volume of air discharged every second and 21% loss or reduction in the mass flow rate when compared with the design specifications.These low efficiencies could be as a result of clogging which may have taken place to cause the reduction in the performance of the compressor to such a level.
Table II and Figures 7 to 12 show that clogging had negative effects on the performance of the compressor which caused increase in discharge temperature, decrease in discharge pressure, mass and volume flow rates.Therefore, it may be justifiable to say that minimising clogging in the compressor would likely cause improved performance that may meet the required design performance.This work therefore shows an effective thermodynamic performance evaluation of the compressor at an Oil and Gas Terminal in Rivers State, Nigeria; hence, the objectives of this research work have been achieved.This study also contributes to the understanding that thermodynamic analysis is a veritable tool that can be used in analysing and accessing the effects of clogging in two-stage reciprocating air compressors.
In order to solve or minimize the menacing problem of clogging for an improved performance of the air compressor, the double-filter installation method should be applied to limit the degree and amount of unwanted elements gaining entrance into the compressor.Periodic and timely change of the desiccants inside the compressor's air dryer is also recommended to minimise clogging and to prevent moist air from being discharged; thereby ensuring that discharged air are always dry.Periodic cleaning of the cylinders of the reciprocating air compressor helps to minimize clogging caused by accumulation of deposits and unwanted build-ups due to corrosive effects on the internal surface of the cylinders.Further research could be carried out to detect the presence of clogging and the exact quantity of clog in reciprocating compressors without opening the compressor during maintenance.

12 )
Temperature For complete or perfect intercooling in a two-stage reciprocating air compressor, the intercooler temperature (also known as the intermediate temperature) is given as; Heat rejected in the intercooler The rate of heat rejected in the intercooler in kg/s is given as flow rate, kg/s Tx = Intercooler Temperature, K T1 = Inlet Temperature, K Cp = Specific heat at constant pressure, KJ/kgk III.RESULTS AND DISCUSSION .

Fig. 4
Fig.4illustrates a direct proportionality which shows that the isothermal work increased with an increase in the ambient temperature.This means that a reduction in the ambient temperature also caused a corresponding reduction in the isothermal work of the compressor.

Fig. 5
Fig.5clearly illustrates the fact that, as the ambient temperature increased, the volumetric efficiency of the compressor also increased and verse versa.

Fig. 7 .
Fig. 7. Effect of Clogging on discharge temperature over a period of time.

Fig. 8 .
Fig. 8. Effect of Clogging on discharge Pressure over a period of time.

Fig. 9 .
Fig. 9. Effect of Clogging on Volume flow rate over a period of time.

Fig. 10 .
Fig. 10.Effect of Clogging on Mass flow rate over a period of time Fig. 10 also illustrates the fact that as the days went by, the mass flow rate reduced continuously.This gradual decrease in the mass flow rate continued until after the 26th day when a sharp decrease began.

Fig. 11 .
Fig. 11.Effect of Temperature during clogging on Volume flow and Mass flow rates

Fig. 12 .
Fig. 12.Effect of Temperature during clogging on discharge pressure over a period of time