Remember the good old days when an instrument air system upgrade meant adding another reciprocating compressor? Well, the controls and equipment that use compressed air are now sophisticated. We worry about air quality, parts per million oil carryover and condensate disposal.
This article
provides an overview of basic information to understand the development and set
up of a compressed air system.
The instruments, controls and equipment determine air quality requirements. An oil-free air system avoids plugging orifices in pneumatic control devices with oil and moisture. On the other hand, pneumatic actuators work better with some oil in the air and need a lubricated compressor.
Investigate
individual components to set the system design. The main determinant is the oil concentration in parts per
million. Equipment vendors give oil
concentration limits either in their operation manuals or by calling the
customer service department.
Provide filtration to protect against build-up or erosion
caused by particulate matter in the large volumes of air the compressors
handle. Moisture in compressed air can
lead to scaling, rust, frozen lines, wear and malfunctioning controls and air
logic devices. The pressure dew point
defines the amount of moisture removal necessary.
When air
is compressed, it is heated; when cooled, condensate forms. Lubricated compressors leave oil in the
condensate. In many areas this
condensate is considered hazardous waste so evaluate maintenance and future
laws before selecting a water/oil separator system.
Select an
oil-free system for applications that cannot tolerate lubricant. An air receiver downstream of the compressor
stabilizes system pressure, acts as a demand reservoir, and collects some
moisture. Put an air dryer, selected to
provide the proper pressure dew point, downstream of the receiver to trap
remaining moisture.
A coalescing
filter after the dryer provides protection if upstream components fail. For instance, the coalescing filter captures
a large portion of moisture traveling downstream of malfunctioning condensation
traps.
Installing a
“dry” receiver after the coalescing filter further stabilizes pressure and acts
as a reservoir for heavy demands.
Lubricated compressors and downstream purification
A modern,
lubricated compressor and high-efficiency purification system produce
instrument quality air with the minimum of stages of efficient
compression. The built-in separator in
the compressor removes the bulk of the oil.
This system is
similar to the oil-free system with a “wet” receiver, air dryer and coalescing
filter. An activated charcoal filter
between the coalescing filter and “dry” receiver removes residual oil
vapors. The following guideline will
assist you on your selection journey.
Air Compressors
The key
issues in purchasing a compressor are reliability, cost-effectiveness, ease of
operation and maintainability.
Compressor reliability is based on the following factors.
Type of control
system – State-of-the-art electronic controls eliminated problems, mechanical
switches, and relays. Older pneumatic
compressor controls using compressed air taken before the air dryer can prove
troublesome because moisture in the air leads to sluggish performance and
damage to the compressor. The rubber
diaphragms used with these pneumatic control systems are a common weak link in
control systems.
Ambient
temperature – The compressor must be capable of operating in ambient
temperatures approaching 110-115 F because compressor rooms are 5-10 F higher
than the outdoor temperature. Higher
temperature ratings mean longer, more reliable periods between maintenance.
Motor design –
As a minimum, motor insulation must be class F. Temperatures inside the sound attenuating enclosures for motor
and compressors are warmer than the ambient air. Summertime operation gives internal temperatures from 110-115
F. Standard Class B insulation motors
are designed for a maximum installed temperature of only 104 F.
Cooling system
– Compressing air produces heat of compression that must be removed. The compressor oil removes some of the
heat. Lubricated compressors remove
even a higher portion of the heat since the oil is in the compression
chamber. The oil is then cooled in a
forced draft air-cooled heat exchanger.
That portion of the heat remaining is removed in inter-cooler and after
coolers of sufficient capacity to permit continuous, fully-loaded compressor
operation in the ambient temperatures.
The after cooler approach temperature, that is, the temperature
difference between the compressed air outlet and ambient air temperature,
should be in the 15-20 F range.
Power,
maintenance, and downtime costs outweigh first cost over the life of the
compressor. Since you pay for a kW, not
horsepower, assemble the data to calculate input kW. Identify all power into the package including compressor brake
horsepower at shaft and motor efficiency at this BHP level, fan horsepower and
motor efficiency, oil pump horsepower and efficiency. Calculate input kW and operations costs by the following
formulae:
Input kW=0.07457BHP+0.07457 Fan HP +
Motor efficiency
Fan Motor Efficiency
Operations cost ($/yr)=Input kWxPower Cost ($/kWh)x operating hours per year.
Insist that
vendors supply performance numbers based on the same criteria. Air-end performance testing does not account
for losses in the compressor package.
Requiring testing in accordance with an industry standard such as
acceptance test PN2CPTC2 that is endorsed by the compressed air and gas
institute and the European committee of manufacturers of compressors, vacuum
pumps, and
pneumatic tools insures valid performance comparison.
The control
system should be easy to use and provide required data. Microcontrollers provide real-time
adjustments, but beware of those systems needing arcane codes or hand-held
programmers.
The sound level
can mean the difference between hearing and not hearing. Sound levels from 75-80 kBA are acceptable
with 85 kBA being the maximum allowable.
Avoid unenclosed rotary compressors and others that exceed these noise
levels.
The compressor
should be easily accessible for maintenance.
If enclosed, the panels should be easily removed. Leave at least three feet of clearance
around the compressor.
The primary
maintenance items on an air compressor include: inlet filter, oil drain, oil
fill, motor greasing, condensate traps, and control calibration. Each should be easily accessible. Service indicators help guarantee timely
maintenance.
In lubricated
compressors, the oil travels downstream and must be replenished regularly. Synthetic lubricants provide superior
lubricating characteristics, longer service life, and lower vaporization
rates. Polyglycols extend change out
intervals to 8,000 hours, have the lowest vaporization rate, and are
biodegradable. Oil-free compressors
require limited amounts of lubricant for bearings and gears.
Use SAE O-rings on fittings along with 37 flared connection to avoid oil leaks. Standard pipe fittings will leak in time, given the temperatures and the viscosity of the lubricant.
Focus on their
purification after selecting the compressor.
The air system designer must consider the following:
-
Delivering the required air quality
-
Maintaining air quality during upsets
-
Minimizing operating costs.
Generally, air purification falls
into one of three categories: filters, dryers and receivers.
Filters remove
condensed liquids, particulates, and oil vapors. Coalescing filters to remove water and oils have efficiencies
from 99.98% at 0.1 micron particle size to 99.9999% at 0.01 micron. The filters should have a maximum wetted
pressure drop of 3 to 3.5 psi. The
maximum pressure drop, normally 10 psi, determines the service life of these
filters. Expect to replace the filter
elements every six to twelve months.
High-performance
coalescing filters require change out every five years. Although these filters have a higher first
cost, the lower pressure drop and reduced energy and maintenance costs provide
a simple payback of less than one year.
Particulate
filters installed downstream of a desiccant dryer should have a different
pressure gauge to indicate the condition of filter elements rated for a nominal
efficiency of 99.95% at 1 micron particle size and initial pressure drop of 1
psi. Coalescing filters must have high-quality
automatic condensate drains.
Vapor removal
and filters absorb oil vapors with activated charcoal. Location and the oil concentration determine
filter element life. Normal pressure
drop for a vapor removal filter is 1 psi.
An aftercooler discharging compressed air at 100’ F passes 67 gallons
of water per 1,000 scfm per 24 hours.
Instrumentation fails when water and lubricant condense as the air is
further cooled in the piping system or as the air expands through the orifices.
The air exiting
the aftercooler is saturated and any further temperature drop results in more
condensation. A useful rule of thumb
states that a 20’ reduction in temperature condenses one half the water vapor
in saturated air.
Air dryers reduce
the moisture content as measured in terms of a pressure dew point (pDp) that is
based on a specific set of inlet conditions to the dryer.
Dew point is
the temperature at which water vapor condenses-saturated, 100% relative
humidity. Pressure dew point is the dew
point of the air at operating pressure.
Atmospheric dew point refers to air expanded to atmospheric
conditions. To avoid confusion, specify
dryer performance in terms of pressure dew point.
The instruments
and the lowest expected ambient temperature determine the drying method. The most common dryer is a refrigerated unit
that cools the compressed air, condenses water and oil vapors, separates them,
and drains them from the system.
Dryer performance is specified as a pressure dew point
class that is based on a specific inlet and ambient conditions. The lowest pressure dew point class with
refrigerated dryers is Class H. This
class delivers a pressure dew point that of 33’ to 39’F. Refrigerated dryers should not operate below
the Class H range because the water vapor will freeze in the dryer.
The highest
practical pressure dew point for a refrigerated dryer is 60’F because higher
pressure dew points give condensation in downstream piping.
In the United
States, most dryer manufacturers base the pressure dew point performance on
standard conditions: inlet air flow, 100’F inlet air temperature, 100 psig
operating pressure. 100’F maximum ambient temperature (aircooled units), 85’F
cooling water temperature (water-cooled units), and 5 psi maximum pressure
drop.
Adjust air
dryer sizing to account for deviation from standard conditions. For example, elevating the inlet air
temperature 10’ increases the load on the dryer by more than 25% and raises the
outlet pressure dew point above 50’F.
Maintaining the original 33-39’F dew point now requires a dryer 35%
larger.
Desiccant
dryers give pressure dew points below 33’F if piping is exposed to freezing
temperatures. Desiccants dry air
through absorption in which a hydroscopic material – chemical, alumina, silica,
molecular sieve- removes the water and oil to reduce the dew point to the
standard pressure dew point of 40’F.
Special designs produce dew points of 100’F or lower.
Standard
conditions for rating a desiccant dryer’s pressure dew point inlet air flow in
scfm, 100’F inlet air temperature, 100 psig operating pressure, and outlet air
flow in scfm to account for the inlet air flow used during regeneration.
Non-cycling and
cycling are the two types of refrigerated dryers. On a non-cycling dryer, the refrigeration compressor runs
continuously regardless of dryer load.
A thermostatic expansion valve and hot gas bypass valve regulate the
flow of refrigerant into the heat exchanger to maintain dew point and minimize
“freeze-up.” Since the unit uses full
input power at all times, a non-cycling dryer should be selected for systems
with a constant air flow.
In cycling
dryers, the refrigerant cools an intermediate fluid that cools and “dries” the
air. During low-load operation, the
refrigeration circuit stops its compressor and restarts it when the fluid
temperature rises. The cycling type
dryer conserves energy and minimizes dryer freeze-up making cycling dryers the
choice with fluctuating air flow and inlet temperatures. Over-sized cycling dryers provide additional
drying capacity for future air system upgrades.
Review the
systems when selecting the refrigerated dryer.
Refrigerant
system – look for:
-
Low input power (Kw) refrigeration compressor (ignore
compressor horsepower. You pay for Kw);
-
Hermetic compressors above 2,500 scfm; below, use
semi-hermetic with valve unloaders;
-
Refrigerant HFC-134A on dryers to 100 scfm, HCFC-22 above
100 scfm.
-
Refrigerant pressure below 100 psig for 100 scfm and
smaller dryers to increase compressor reliability and
-
Air-cooled refrigerant condenser designed for 130’ maximum
ambient temperature to assure trouble free operation during hot summers.
Air system – look for:
-
Pre-cooler/ reheater to remove up to 65% of the heat from
the compressed air to allow using a smaller refrigeration compressor;
-
Smooth copper tubes on heat exchangers to reduce
maintenance and eliminate prefiltering air entering the dryer; and
-
Water/polypropylene glycol solution as the intermediate
fluid in cycling dryers.
Instrumentation and controls –
look for:
-
Easy operation monitoring with parameters displayed
digitally
-
Simple, manual adjustment of pressure dew point in cycling
dryers
-
Controls that sense ambient temperature to maintain dew
point suppression.
There are two
designs – heatless and heated. Heatless
dryers provide a consistent pressure dew point with minimal maintenance and
maximum desiccant life. However, the
air compress must deliver excess flow consumed for desiccant regeneration. If the desiccant absorbs oil vapor then it
must be replaced, so desiccant life is lower on lubricated systems.
Use a heated
dryer when the compressor cannot deliver the required excess flow. The four types of heated dryers are
internally heated externally heated, blower purge, and heat of
compression. Both the internally and
externally heated designs use a heater and a low rate air purge to regenerate
the desiccants.
The bower purge
design uses a heater and a 3 psig blower instead of compressed air for
regeneration. The heat of compression
dryer, specifically designed for use with an oil free compressor, uses the hot
compressed air to regenerate the desiccant and yields the lowest utility costs.
Be sure to
check temperature limits on instrumentation.
Heated dryers produce a spike in dew point and a 180-200’F temperature
spike immediately after regeneration.
Other things to look for are:
-
Vessels that avoid fluidizing the desiccant while drying;
-
ASME coded vessels for quality and safety;
-
Easily accessible low maintenance valves with externally
mounted valve actuators to permit cool operation;
-
Energy savings control systems to match purge consumption
and heater usage to actual dryer load; and
-
Purge mufflers to reduce depressurization noise.
Review the application with a
reputable manufacturer because the desiccant dryer selection can be a rime
consuming and tricky process.
The final
components needed, the air receivers:
-
Provide storage capacity to prevent rapid compressor
cycling
-
Reduce wear and tear on compression module, inlet control
system, and motor
-
Eliminate pulsing air flow
-
Avoid overloading purification system with surges in air
demand
-
Damp out the dew point and temperature spikes that follow
regeneration.
Reprinted from Art Foundry Journal,
August, 1996
By Richard E. Porri
Ingersoll-Rand
Company