In industrial environment, compressed air is considered an essential utility for operation of various equipment, tools and instruments, but less is understood in terms of real cost of owning, operating and maintaining such system. What is the life cycle cost of such utility? How much does it cost to operate a compressor? How to do industrial audit, analysis, procure funding, implement projects, monitor and verify savings, and maintain such system in top conditions (O&M measures).
A systems approach is necessary to get an holistic overview. This article will outline an approach that is used fundamentally in a new system design and debottlenecking of an existing system to improve operation and reduce the life cycle cost of compressed air system in an industrial setting which uses reciprocating and rotary screw compressors. This approach is presented from a process and systems engineers' perspective.
The compressed air system can be broken down into mechanical system and process system.
The mechanical systems may consist of various end-users of compressed air, delivery or distribution system (piping, valves, fittings, controls, etc.), compressor (centrifugal or positive displacement), its associated equipment (aftercooler, receiver, dryer, filter, etc.), and a driver (electric motor).
The process system will consist of air pressure and temperature at end-users, quantity and quality of air requirement, physical properties (molecular weight, polytropic exponent), ambient air temperature and air pressure, moisture in the suction air, etc. The energy consumption is impacted by the design of mechanical and process systems as well as end-user load profile and hours of operation. It is very important to understand the technical equations used in the calculations of flow rate at standard temperature and pressure (STP), STPs, inlet air density, outlet air density, compressor capacity, adiabatic or polytropic head, discharge temperature, break horsepower, compression ratio.
A systems analysis integrates mechanical as well as process systems. This will allow us to understand the impact of change in one variable or multiple variables on energy consumption. Let us start with determination of load.
Most of the industrial processes are continuous in nature, operating
7 days a week, 24 hours a day, with a planned yearly shut-down
of week or two. In many cases, not all of the processes are down
at the same time and rotating shut down requires that compressed
air utility be available around-the-clock. Whether designing a
new system or re-rating an existing operation, it is prudent to
make a reasonable estimate of end-user load profile, resulting
in total computation of compressed air flow rate in cubic foot
per minute (CFM) or pounds per hour (Lbs/hr) that a compressor
system must deliver. The load profile will also give a projection
of peak load, part load and no load time periods. A graphical
presentation of total CFM versus 24 hour time period will allow
development of lead/leg compressor selection and deployment strategy.
The following parameters are usually logged to determine load
estimate and load profile.
The computed total load must be adjusted upward by leak losses (10 to 20%) and future expansion factor (10%+) to arrive at the total compressor capacity.
A load profile curve will determine base load to be delivered by one or more full load "lead" compressors and fluctuating load by a part-load "leg" compressor. Maximum compressor efficiency is obtained by operating compressors at full load and have minimum numbers of compressors running on 'swing' mode at part-load. Compressors, electric motors, adjustable frequency drives enjoy maximum operating efficiency at full load if selected right.
An establishment of base line operation is utmost important in energy savings and conservation analysis. The base line energy consumption should be correlated to one or more dependent variables such as plant production and other process variables or major events when compressed air is predominantly required (for example, sheet breaks on paper machines). A regression analysis that correlates energy use to multiple plant or process variables is useful in determination of impact production variability has on the compressed air system energy use.
The simplest way to measure the electricity usage is to connect
power metering equipment to each compressor motor in the motor
control central and log amperage, voltage, power factor, and kW
over a period of time. The data can be averaged over 15 minutes
or one hour interval and stored in the logger for further analysis.
The Graph 1 plots compressor load in kW over one month time period.
During the same time period, plant production data and other dependent variable data must also be collected and correlated to energy consumption. Other process variables such as discharge air pressure, temperature, and flow rate (if available) may also be noted at each compressor for creating existing compressed air operating system computer model. It should be noted that part-load compressor air capacity can be estimated from the electrical load data using vendor furnished part-load compressor curves.
As a part of base line estimation and establishment of future
requirement, more than one interviews with plant operators, process
engineers, maintenance personnel, and management are required.
The questions such as following must be asked and additional questions
can be added based on the circumstances.
This process of data gathering can take one to several days depending upon the complexity of the plant and compressed air system. A thorough understanding of their current operation, current design, and future requirement will help energy engineer evaluate all facets of their operation and propose energy conservation measures that not only save energy but provide a more reliable system. A system design and engineering experience is extremely valuable in this phase.
An ideal goal will be to model and simulate their original design
operation and current operation under various case study scenario.
Once the current operation of various compressors is modeled off-line
on a computer program accurately, the energy engineer can then
create various "what-if" case studies with proposed
energy conservation measures.
An example of a base line case model using C-MAX software program
is given below.
A good process and systems engineer understands the engineering
formulae and technical relationship between compressor motor power
draw and process variables.
Reciprocating/ Rotary Screw Compressor Formulas
Flow at Standard temperature and
pressure (STP):
Qs : Volumetric flow rate at Standard Condition.
w : Mass flow rate, lb/hr.
MW : Molecular weight.
Ts : Absolute Temperature at Standard Condition, oR.
Ps : Pressure at
Standard Condition, psia.
There are three standards available to enter Flow@STP.
API Standard: 14.7 psia, 60 oF, 0% relative humidity.
ASME Standard: 14.7 psia, 68 oF, 36% relative humidity.
CAGI Standard: 14.7 psia, 60 oF,
36% relative humidity.
Compressor Capacity (ICFM)
Q1: Compressor capacity at inlet T and P, cubic ft/min (ICFM).
Z1 : Compressibility factor of gas at inlet.
T1 : Inlet temperature, oR.
P1 : Suction pressure, psia.
Inlet Gas Density:
p1 : Inlet gas density, lb/cubic ft.
Outlet Gas Density:
p2 : Outlet gas density, lb/cubic ft.
P2 : Discharge Pressure, psia.
T2 : Discharge Temperature, oR
Adiabatic Head:
For BHP, FREE AIR and FLOW calculation:
Had : Adiabatic head, ft-lb/lb.
Zav : Average compressibility factor.
R : Gas constant, 1545/MW.
T1 : Inlet air temperature, oR.
r : Compression ratio (P2/P1), unit less.
k : Adiabatic exponent.
Discharge Temperature:
Gas Horsepower:
If mass flow rate is available:
GHP : Gas horsepower, hp.
Hp : Adiabatic head, ft-lb/lb.
Ep : Adiabatic efficiency.
If capacity is available:
Q1 : Capacity (ICFM), cubic ft/min.
Zav : Average compressibility factor.
Z1 : Compressibility factor of gas at
inlet.
Brake Horsepower:
BHP = GHP (1 + %Mechanical Losses)
The industrial efficiency improvements (that includes compressed air systems) must be evaluated in conjunction with other main processes. Any proposed changes that may compromise plant reliability will be rejected regardless of high pay back. The proposed Energy Conservation Measures (ECM) must be discussed and presented with their benefits to plant decision makers before conducting a "paper" evaluation. Ask a question to yourself: Does this change benefit plant operation, reduce cost, increase reliability, and automate operation to "eliminate" human intervention to realize energy savings? In reality, saving electrical energy may be on lower priority list for plant personnel. Other non-energy related benefits must be evaluated and integrated with the complete "pay back" or "ROI" analysis.
Some of the energy conservation measures are "soft" and can be categorized as O&M measures. A good example is plant air leaks. Truly, it is not an ECM. Another good example is an operating strategy to use full load lead compressors and low efficiency compressors for part load leg operation or shutting down compressor(s) when not needed. These soft measures can be implemented with a better training and becomes no cost/low cost O&M measures.
The "hard" savings are realized by measures that fundamentally
changes the design (mechanical and/or process) of the compressed
air systems and compressed air need at end-users. Some of the
such measures can be implemented with low investment and others
require substantial investment to realize the savings. Each case
or measure must be evaluated carefully, documenting its impact
on operating cost, life cycle cost, and estimating interactive
impact of more than one measures on the system energy savings.
A partial list of ECMs are listed below.
Let us evaluate the second energy conservation measure listed
above. How does cooler air inlet temperature reduce energy consumption?
The cooler air temperature increases the density, thereby, mass
flow rate increases (remember for positive displacement compressor,
the inlet compressor capacity in ICFM would be the constant. The
ICFM is volumetric flow rate at compressor inlet temperature and
inlet pressure). This will, however, reduce the adiabatic head,
keeping energy draw at motor essentially the same at constant
adiabatic efficiency. To deliver the same compressed air load
(pounds of air delivered over a year), the operating time will
reduce saving the electrical energy. This is illustrated in the
following case study ("Cool Inlet Air") where average
inlet temperature is changed from 100 to 50 oF. The compressor
capacity has remained the same, but the cooler air increases the
mass flow rate, reducing the annual time of operation from 8,520
to 7,759 to deliver the same air quantity in lbs/year. The energy
savings at same adiabatic efficiency and full load operation would
be approximately 9.6%. A good computer modeling tool will allow
you to do this complex "what if" study with ease.
The analysis should also account for interactive impact on energy
usage due to multiple process or mechanical system changes. Once
pre-retrofit and post-retrofit case studies are completed, they
must be compared and operating cost savings must be quantified.
The following report compares five existing compressor
operation to two new compressors.
There will be industry specific process changes which can reduce the use of expensive compressed air and save money. An example would be to use electric motor driven small mixer in a tank versus agitating the tank content with compressed air. The economics of such improvements can be looked at using different yard-stick other than a simple pay back or rate of return. A dollar saved in operating cost is equivalent to ten dollars of revenue for an industry which has a 10% net income to revenue ratio! The company has become 10 times more productive!! The challenge comes in assuring that these predicted savings are realistic and persistence over the life of the measure or project. This is discussed in the next section.
It is relatively simple to verify energy savings from an upgrade of a lighting system. You count the lighting fixture, their wattage and multiply with operating hours. The difference in energy consumption between post retrofit and pre retrofit plus any interactive impact on HVAC system would be the energy savings. The compressed air system energy use in a process plant can vary dramatically with processes and production throughput. The energy usage must be normalized or correlated against dependent variables such as production rate or feed composition or number of operating events that consume higher amount of the compressed air during the event time period. It is the energy engineers' experience, education, skill-set that determine and establish which variables most impact the use of compressed air and why? A sensitivity analysis and/or regression analysis may be required to correlate these process and operating variables to compressed air system energy use.
For example, at a paper mill it was determined that in addition
to the paper production in tons/day, number of sheet breaks at
paper machine winder requires significant amount of compressed
air. The daily data was correlated for one month and presented
as shown below. The correlation with net daily production was
significantly better than with number of sheet breaks at the paper
winder.
The regression analysis of each dependent variable (production
and number of sheet breaks) indicated poor compressed air energy
consumption correlation to # of breaks. This was indicated by
R2=0.0353. However, daily paper production has much stronger correlation
(R2=0.9612). Both graphs are shown below.
Similarly, the post retrofit metered data indicated a much better
correlation of compressed air energy use with the net paper production.
A regression analysis gave R2 value of 0.7922, indicating higher
degree of data fit over a range of paper production.
Once a reasonable energy consumption correlation has been established at different production level, the verified energy savings can be accurately predicted at a given constant or at annual average or at variable daily production rates.
An understanding of what a proposed system change would cost over its productive life is an important item for an investment justification. Most justifications are done on the basis of simple pay back using operating cost savings and first time or initial project investment.
A better and accurate approach would be to determine cost to own, operate, and maintain a new or improved system over its useful life. This should factor in present value analysis of energy savings and non-energy savings benefits over the life of the project. It should also take a credit for energy recovery (removal of heat of compression for productive uses). The method should allow user to account for initial project investment, salvage value, cost of capital, depreciation, tax implication, project operating cost, maintenance cost, other utility cost (such as cooling water to seal or lube oil), compressed air energy recovery, energy savings and non-energy savings benefits (such as production increase, yield improvement, reduction in pollution, etc.). All utilities and labor items must be assigned appropriate cost and individual inflation factors (for example, electric cost may increase at a lower rate than cooling water cost).
Integrating these variables into financial analysis would give a better and accurate picture of economics of such a project. The total cost can be normalized using total compressor capacity over its predicted life. In some cases, it may reveal that a compressor equipment that cost less initially to purchase may actually be significantly more expensive using normalized cost indices. Undoubtedly, experience and professional judgment will play a key role in evaluating the life cycle cost of a system improvement. Again a powerful software tool can integrate, calculate and present various life cycle case study scenarios with ease, and help you make an optimal decision.
A professional audit and analysis require excellent understanding of end-user processes, compressed air system design and process variables that impact the energy usage. A methodical approach starts with field visits, interviews, data collection, computer modeling of current system, pre and post retrofit metering, ECM analysis with previously set-up computer model, selecting cost-effective measures and system using comprehensive life cycle cost and economic analysis, installation of measures, verification of energy savings with statistical analysis, and finally training operating people to maintain the new compressed air system in optimum condition to secure persistence of energy savings over a long period of time. In industrial environment, non-energy benefits must be quantified and factored into the justification process. A well planned energy analysis and implementation will meet success criteria of the project - that is to reduce operating cost while increasing system reliability and meeting the rate of return goal of a plant capital investment.
ABOUT THE AUTHOR
Paresh S. Parekh, P.E., PMP, is president of Unicade Inc., a consulting engineering firm in Bellevue, Washington. Unicade has developed C-MAX software (a suite of fluid flow systems software applications which includes pump, fan, blower, centrifugal compressor, reciprocating compressor, piping, life cycle cost analysis, etc.), which is being used extensively for energy audits and analysis in various industries.
Mr. Parekh has 26 years of industrial, commercial, and international business management experience. He is an expert on chemical process engineering, systems engineering & classical project management techniques. His projects in industrial energy conservation and process optimization for pulp & paper, petroleum refineries, petrochemicals, heavy chemicals, chlor-alkali, plastics, food, electric utilities, and forest products industries have resulted in millions of dollars in operating cost reductions. He has presented articles at number of symposiums and technical meetings. Mr. Parekh is past treasurer, secretary, and chairman of Puget Sound section of American Institute of Chemical Engineers. He is a member of ASHREA, AIChE, PMI and AEE. Mr. Parekh has a Master of Science degree. in Chemical Engineering and is a registered professional engineer in the states of California and Washington. He is a certified Project Management Professional.
He can be reached at UNICADE INC., 13219 NE 20th
Street, Suite 211, Bellevue, WA 98005. Tel. No. (425) 747-0353,
Fax No. (425) 747-0316, e-mail: unicade@unicade.com,
Unicade web site: http://www.unicade.com.