System Design

Successful design of a compressed air system requires correct sizing all system components. This section provides information for designing the following system components:

• Air compressors [Go]
• Air dryers [Go]
• Air receivers [Go]
• Cooling towers [Go]
• Heat recovery systems [Go]

Air Compressors

Establishing required compressor capacity is challenging in existing plants and could be even more difficult in new installations where there isn’t a history of air consumption.

A comprehensive study should include the minimum pressure required for the system to operate all plant equipment, and the average, minimum and peak flow of compressed air for different production periods. A list of all uses should include this information for each end use with a load factor applied to determine if the use is continuous or intermittent. Some pneumatic equipment, such as air tools, may be operated only twenty-five percent of the time. Therefore the load factor would be different from a continuous automated process. There may be some operation, which consumes a great amount of air for a relatively short duration of a few minutes. This can often be handled with storage receivers at the point of use. High volume users, which consume a large amount of compressed air for longer periods, such as grit blasting will probably require that an additional air compressor be automatically started to maintain system pressure for the duration of that process.

When designing a new air compressor consideration must also be given to:

• The compressor locations and ambient conditions
• Type of control for each compressor and how to automate the compressors for optimum efficiency
• Space requirements
• Cooling and ventilation
• Primary storage receivers
• Centralized or decentralized multiple compressor systems
• Maintenance, service availability
• Expansion of the air usage within the next few years

General Rules

1. Generally, compressors are most efficient when operating at full load. Natural gas engine-driven compressors offer variable speed control down to perhaps sixty percent of capacity, which eliminate many of the inefficiencies associated with constant speed electric motor driven compressors.
   
   
2. Baselining the existing compressed air load is an important first step. This is best achieved by installing accurate mass flow recording meters to monitor the consumption during all hours of plant operation. It is possible that more compressors are operating than are necessary to sustain the required plant system pressure.
   
   
3. It is important to have an ongoing practice to replace inappropriate uses with other sources of power and to identify and repair leaks as a continuous process.
   
   
4. It is probable that many existing electric motor compressors are inefficient at below 80 percent of their rated output, but may be relatively efficient at full load. In a hybrid system using both electric and natural gas driven compressor, use the natural gas engine driven compressor to trim the base load and have the modulating electric driven units which are inefficient at part load, to always operate at full capacity.
   
   
5. Operate compressors at the lowest possible system pressure. You will reduce the input power approximately one percent per two psi reduction in pressure. Also the consumption of all unregulated uses will be reduced in direct proportion to the reduction in absolute pressure. Thus at 100 psig, reducing pressure ten psi to 90 psig will result in lowering consumption of unregulated uses to:
   
   
   
   This savings is in addition to the five percent reduction due to the lower input power required. (*At sea level)
   
   
6. Individual or a combination of compressors can be selected and controlled to operate efficiently at all conditions. When the peak consumption is caused by high intermittent short duration loads, adequate storage should be added which could allow operation of smaller compressors, which will save energy. (See section on sizing air receivers.)
   
   
7. If the minimum pressure requirement is dictated by a low volume user, it is usually beneficial to address this operation individually. Often selecting a different component within that operation (larger air cylinder for example) or a dedicated small compressor or booster will allow the main compressors to operate at a lower pressure resulting in significant energy savings.

Air Dryers

A good rule to save capital costs and energy is to dry compressed air only to the level required for the process. In other words, if a 38°F pressure dewpoint is acceptable, but a small user requires –40°F, do not dry all the air to –40°F. It may require less than 1kW per 100 cfm for a refrigerated dryer to produce a 35-38°F pressure dewpoint, but as much as 3 kW per 100 cfm for a regenerative dryer to provide a –40°F dewpoint.

Dryers are rated on the basis of the "three 100’s"

• Ambient temperature 100°F
• Inlet air temperature 100°F
• Inlet air pressure 100 psig

Any other conditions require correction factors, which are usually available in the manufacturer’s catalogs. For example, if the inlet air temperature to a refrigerated air dryer were 120°F, the dryer would be derated by approximately 30%.

Apply the same installation considerations to dryers as you would for air compressors.

Read the instruction book and pay particular attention to ensure all automatic drain traps are operating.

Consider minimum pressure drop when selecting air dryers.

Similar considerations should apply to any filters which are in the system.

Air Receivers

A liberal air receiver and storage capacity should be supplied for the system. The air receivers are intended to serve several important functions. Initially with reciprocating compressors, the receiver (or "tank") was used to damp pulsations and to collect condensate. With the advent of rotary screw compressors, the receiver continues to serve as an important part of compressor and system pressure controls and to address intermittent large demands, which may intermittently be in excess of compressor capacity.

In some cases of short duration high volume use, a large air receiver may allow a smaller air compressor to be used and allow the automated controls to improve overall system efficiency.

With reciprocating compressors, the rule of thumb was to size the air receiver at one gallon per cfm of compressor capacity. Today with most systems using electric motor driven rotary screw compressors, many of which operate load/unload, three to four gallons per cfm may be necessary to attain any part load efficiency.

With natural gas engine driven compressors used as efficient trim machines, or at full load, a rule of two gallons per cfm of compressor capacity may be adequate for most installations.

Calculating the actual size for the volume required for a receiver to supply a given cfm is as follows:

T = Time interval in minutes, during which a receiver can supply air without excessive drop in pressure
V = Volume of tank in cubic feet
C = Air requirement in cubic feet of free air per minute
Cap = Compressor capacity
P_b = Absolute atmospheric pressure, psia
P₁ = Initial tank pressure, psig (compressor discharge pressure)
P₂ = Minimum tank pressure, psig (pressure required to operate plant)

If Cap is greater than C, the resulting negative answer indicates that the air compressor will supply the required load. If the compressor is unloaded or shut down, Cap becomes zero and the receiver must supply the load for "T" minutes.

Notes:

1. Use experience and judgment to select the next largest standard size receiver.
2. *For some secondary point of use receivers, "Cap" may be zero.
3. For intermittent applications, consider placing a check valve and flow restrictor upstream of the receiver to reduce pressure fluctuations in the main distribution piping.

Cooling Towers

Involve the supplier of the engine-driven package.

Some of the information you should provide to the cooling tower representative:

1. Total heat load
   Engine
   Compressor
   Any auxiliary water-cooled equipment (refrigerated dryer etc.)
2. GPM and temperature of hot water to the tower
3. Cold water temperature required
4. Wet bulb temperature
5. Weight and dimension limitations

Other considerations:

• Type of tower – closed loop or evaporative
• Cooling tower pumps
• Bleed off – required to limit dissolved solids
• Make-up water including "drift", "bleed off" and evaporation
• Tower location (tower discharge air in same direction as prevailing summer wind)
• Noise levels
• Water treatment
• Other environmental concerns
• Energy

Heat Recovery Systems

Consider heat recovery for process applications.

Reciprocating Engines – Approximate heat balance

35 percent of input fuel converted to mechanical power
25% to 40% rejected to jacket water
5% to the ambient
Balance to the exhaust
Perhaps 50-65% is recoverable

Rotary Screw Compressors

80% of the input power is rejected as heat to the lubricant that is used for cooling the injected fluid in the compression chamber, 20% of the heat is in the compressed air and most of it is recoverable.