Inside every chemical plant we can find compressors very easily. These rotating devises we use to compress and pump or transfer the compressible fluids like air, ammonia, natural gas, refrigerant, carbon dioxide, etc. from one point to another. These equipment are power guzzlers and consumes major part of total electricity bill inside the plant. There are various types of compressor which we use in our chemical process industries, such as reciprocating, screw, centrifugal, rotary, diaphragm, etc.
So, the selection of an efficient compressor is very critical from the aspect of longer life-cycle with minimum maintenance. Which is very important for the overall cost of plant and manufacturing. This compressor selection depends of various physical and chemical properties of the fluid. Therefore, it is very important for a process engineer to prepare a correct and complete specification sheet for the compressor. So that, this compressor specification sheet we can send to a compressor supplier. Based on which supplier can select and supply a suitable and efficient compressor for our requirement.
So, in this article we will try to understand various sections of a process specification sheet of a compressor. Moreover, we will go through various type of compression and will see how to estimate the power of a compressor.
Table of Contents
Process Specification Sheet for Compressor
So, let us consider an example for reciprocating compressor process data sheet as given below. This compressor we need to pump air inside a reactor for oxidation reaction. In this specification sheet you can see various sections as described below:
Top Section
This includes identification details of the process data sheet or specification sheet. This information includes, the logo and name of your organization. Details like compressor description, tag number and project name are very important during erection work. As, this helps the people at site for easy and errorfree identification of that equipment and correct installation.
Other details are document number, revision number which are important from documentation point of view. Apart from this, details of maker, checker and approver are important to trace the engineers involved in this project. Also, information for which this PDS we are issuing is important like, it can be “Issued for Inquiry” or “Issued for Comments” etc.
Process Data Section
In this section we need to provide the details of service fluid for which this compressor will be running. We need to provide capacity of the pump, for this we use FAD or free air delivery. FAD is the actual quantity of air, which air compressor deliver at outlet and converted back to the inlet conditions of the compressor. We can also represent FAD in terms of NTP (normal temperature & pressure), which is at 20 0C and 1.0 atm. pressure.
Apart from FAD we need to provide ambient pressure, temperature conditions and required discharge working pressure. The discharge temperature will depend upon type of compression (i.e., adiabatic, isothermal or polytropic compression) and will be furnished by vendor. In this section other includes heat capacity ratio, compressibility factor, compressor efficiency & power requirement (will be provided by vendor).
Mechanical Data Section
For reciprocating compressor these data include design conditions for the compressor. Other details are supply & discharge sizes, which depends upon the compressor model selected by the vendor. You need not be worried about these data because vendor will provide all these details.
Material of Construction Data
For a compressor working life longevity this information is very important and critical. So, for suitable material of construction either we can provide the data or can provide fluid details to the vendors. This detail should include corrosion data of fluid, any solid or abrasive particle presence, solidification or polymerizing nature of the fluid, possibility of any explosion under pressurized conditions, etc.
Utility Data Section
As we know compression of gases increases the temperature and to maintain the compressor temperature, we need to remove heat of compression. So, for this purpose there are interstage coolers in a compressor. These coolers can be air cooled in case of small size compressor. While, for large size compressors we require cooling water or refrigerant in some cases.
Therefore, we need to provide the details for utility (i.e., inlet temperature & pressure of cooling water available, fouling factor of the cooling water). For the air we can provide its min/max/nor temperatures and average relative humidity data.
To drive a compressor, we need electricity so information which are also important like, voltage and frequency. Also, the details for compressor driving system is important, it can be pully driven or direct coupled. This information depends upon the compressor model and provided by the vendor.
Remarks Section
In this section we can provide miscellaneous information, which are common. Based on your specific requirements this information can be customized. Whole purpose here is to provide all the general clarification to the compressor vendor, which are important for the performance and long service life of the equipment.
Machines for Transfer & Compression of Gases
So far, we discussed about the specifications for a reciprocating compressor. Now let us look into various type of machines which we use to pump and compress the gases. We can divide these devices in three categories based on capacity and compression requirements as below:
Centrifugal Fans
In our industries we use centrifugal fans to pump large quantity of air or gases to low discharge head. For example, such applications are combustion air supply in boilers, boiler furnace FD and ID fans, air supply for AHUs, spray dryer air supply fans, etc. We can see in all these cases, required air flow is very high and discharge head ranges between 100 to 1500 mm WC or water column. Since this differential pressure is very low, we can assume fluid as incompressible. Therefore, we can use the equations applicable in centrifugal pump power calculation for the centrifugal fan also. The efficiency for a centrifugal fan can be in the range of 60 – 75%. So, to estimate the power requirement of a centrifugal fan for given capacity, discharge head and discharge velocity, we can use below equation.
Here, m= mass flow rate of gas (kg/s), p2 & p1 = suction and discharge pressure for fan (kg/cm2g), V2 = discharge velocity (m/s), η = fan efficiency and ρave = average fluid density (kg/m3)
Let us consider an example: m = 30000 m3/h = 500 m3/s, p2 = 500 mm WC g = 106391 kg/m-s2, p1 = 101325 kg/m-s2, V2 = 40 m/s, η = 70% and ρave = 1.02 kg/m3
Putting all above numbers in Eq. 01 we get,
Centrifugal Blowers
For higher pressure services say greater than 0.1 kg/cm2g and up to 0.5 kg/cm2g we generally use single stage centrifugal blowers. However, with multistage centrifugal blowers we can achieve pressure up to 2.0 kg/cm2g. We can find many types of centrifugal blowers in our industries such as rotary blower, axial flow blower, multi-stage centrifugal blower or compressor. These centrifugal and axial flow blowers are high RPM machines and can handle larger volumes of gas say around 600,000 ft3/min.
Compressors
For high compression requirements we use compressors, from reciprocating compressors we can achieve discharge pressure around 2300 to 3300 kg/cm2g. As at increased pressure temperature of gas increases adiabatically. So, compression ratio which is p2/p1 is a parameter for the blowers and compressors, which help us to decide the number of required compression stages. For a compression ratio around 3 and 4 or lower, adiabatic temperature rise is not too much and no special cooling provisions are required. However, for higher compression ratios we need multiple compression stages and with provision of interstage cooling system. For smaller size compressors we use air cooling by providing fins on hot surface. While, in case of large size compressors we need to provide cooling water or sometimes other refrigerant also.
Generally, compressors follow polytropic compression which is the path between isothermal and isentropic compression. This is because limitation in cooling, as it is difficult to achieve complete isothermal compression. For isothermal compression discharge temperature is equal to compressor inlet temperature.
So, positive displacement type compressor which we most commonly see in our industry is reciprocating compressor. Other compressors are screw type, multistage centrifugal and rotary type. Ideally, we can estimate power required for a compressor for isothermal or adiabatic compression using below equations:
Power Required for Adiabatic Compression
We can calculate power requirement for adiabatic compression using below equation.
To estimate number of stages required for a compressor, we can calculate using below equation:
Power Required for Isothermal Compression
In case of isothermal compression, we remove heat of compression and maintain outlet temperature equal to inlet temperature. So, power requirement for the isothermal compression we can calculate as follows:
In both the cases, first we need to estimate the number of stages required for compression using Eq. 02 and then estimate the power requirement for each stage. And, total power requirement we estimate by multiplying single stage power requirement with total number of stages.
Power Required for Polytropic Compression
As we discussed above for large size compressors the type of compression neither follow isentropic or isothermal compression. It is somewhere between both of these and termed as polytropic compression. To estimate the power requirement, we can use Eq. 02, which we use for adiabatic or isentropic compression power. Only we need to replace the ratio of specific for gas ϒ by n. The value of n we can calculate by below relation.
Where, ρ1 & ρ2 are the gas densities (kg/m3) at compressor inlet and discharge conditions.
Also note in all above equation η is compressor efficiency which around 80-85% for reciprocating compressors. While in case of centrifugal compressors it can be up to 90%.
Compressor Power Calculation
So, we take an example given in above datasheet and will estimate the adiabatic power requirememt. Various data required are as follows:
q0 = gas volumetric flow rate, std m3/s = 7500/3600 = 2.083 std m3/s
T1 = compressor inlet gas temperature, K = 35 + 273 = 308 K
‘p1 = inlet pressure = 1.0 kg/cm2
‘p2 = discharge pressure = 3.2 kg/cm2
ϒ = 1.4 (in case of air)
‘η = 78%
So, number of compression stage from Eq. 3
N = (1/1.3868)*ln(3.2/1.0) = 0.84 (since it is <1, hence single stage compressor is suitable)
Power requirement for adiabatic condition use Eq. 02
PB = ((0.371*308*1.4*2.083)/(1.4-1)*(0.78))*((3.2/1.0)^(1-(1/1.4))-1) = 421 kW
Conclusion
So, this article should help a process engineer to prepare a compressor specification sheet. Which is very important to purchase a suitable efficient compressor for the requirement. You can upload format for compressor specification sheet by clicking here.
Thanks for reading.