TE LAB-TWO STAGE RECIPROCATING AIR COMPRESSOR

 

Ex. No:                                                                                                                                             Date:

 TWO STAGE RECIPROCATING AIR COMPRESSOR

 

Aim: To conduct a test on two stage air compressor and to determine the volumetric efficiency and isothermal efficiency

 

Theory: Air compressors are used for supplying high pressure air. In a multistage, air is compressed in several stages, instead of compressing the air fully in a single cylinder, via the intercooler where air is cooled, to result in saving of work done.

 

Area under the curve in the above PV diagram indicates work done. This PV diagram is for a two stage air compressor with perfect inters cooling I.e., when air sis cooled to intake temperature in the intercooler (T₂ – T ₁). The low pressure cylinder (LP) diagram is shown as P₁ – 1 – 2¹- P₂. High pressure cylinder (HP) diagram is shown as P₂ – 2 – 3 – P ₃. The reduction of work done due to inter cooling is shown by shaded area 2 – 3 – 3 ¹ – 2 ¹. Point 2 lies on the isothermal line 1 – 3”. In case of imperfect inter cooling i.e , when outlet temperature of LP cylinder T₂ is not cooled up to inlet air temperature. T₁ the point 2 does not lie on the isothermal curve 1 – 3”.

 

Although isothermal compression is economical it is not possible to achieve it in practice. To have an isothermal compression, compressor will have to run extremely slow, while in practice, it is driven at high speeds, so that as much air, as possible is compressed in a given time. Since there is saving of work by compressing air isothermally, it is necessary to make an attempt to obtain approximately and isothermal compression.

 

Procedure:

 

·        Check all electrical connections fill the manometer to zero mark by water

 

·        Start the compressor and allow it to run for few seconds

 

·        Take down the manometer reading, speed indicator  and temperature at no load( zero Pr.)

 

·        At a constant speed of the compressor the delivery pressure From 2.0 kgf/cm² up to 10.0 kgf/cm² by varying both the main valve and valve below the storage cylinder i.e, by adjusting in such way that delivery pressure remains at 1.5 kgf/cm² or so),

 

·        Note the readings of like , speed of the compressor in rpm, manometer readings like, intermediate and delivery pressure, suction air temperature, discharge air temperature etc

 

·        Repeat the trials for different pressure varying from 2.0 to 10.0 kgf/cm² of the compressor.

 

Observations:

 

Observation table:

S. No.	Inlet Pressure, P1	Intermediate Pressure, P2	Delivery Pressure, P3	Speed of Compressor, N	Manometer Reading, hm = (h1+h2)	Temperature Reading, oC			Time  for 5 rev of energy meter
						T1	T2	T3
				rpm	mm				sec

 

 

 

Formulae:

·         Area of orifice, a  = πd² / 4

·         Head of air,    h_air = 1000 x h_m / ρ_a

 

Where,     h_m = head of water in manometer = h₁ + h₂

                 ρ_w = density of water                            = 1000 kgf/m³

                               ρ_a = density of air at STP         = 1.169 kgf/m³

 

·         Volume of free air delivery, v₁ = C_d x a_orifice x 2gh_air

            Where,      C_d = Coefficient of discharge of orifice = 0.65

                             a   = area of orifice

 

 ·         Swept volume: V_LP = Area x Length = πd²l / 4

 

Where,       d = diameter of LP cylinder

l = stroke length of LP cylinder

 

·         Volumetric efficiency:

η_vol = volume of free air delivered / (swept volume of LP cylinder x N/60)

·        Indicated power, IP = IP_LPC + IP_HPC

            

·        Isothermal Power, P_iso = p₁.FAD.l_n (p₂/p₁)

·        Isothermal Efficiency, η_vol = P_iso / IP

·        Compressor Efficiency, η_com= IP / Power input from energy meter

                Where, Power input from energy meter (Shaft power) kW-hr

                                    = (3600 x revolutions considered x η_mech x η_tran) / (time x energy meter const.)

Mechanical efficiency, η_mech  = 0.85

Transmission efficiency, η_tran = 0.95

 

 

Pre Lab Questions:

1.   What is the principle of compressor?

2.   Differentiate various types of compressors?

3.   What is the actual thermodynamic process during compression?

4.   Explain concept of multi staging?

5.   What is the necessity of orifice?

6.   What is the pressure control device incorporated in the setup and explain its use.

Post Lab Questions?

1.   Differentiate single stage and multistage compressor?

2.   What is the purpose of an inter cooler in an air compressor?

3.   What is isothermal efficiency?

4.   How do you define volumetric efficiency and isothermal efficiency of a compressor?

5.   Define swept volume.

6.   Plot PV diagram and show how will you calculate the isothermal efficiency?

 

Conclusion:

 

 

PO Attainment:

Composed By:      

 R.Satheesh, M.E., Asso.Prof.,