There is a partly true idea floating around some plant maintenance circles that “compressed air is free.” Readers of this journal know that isn’t true. But, what if non-compressed air could be seen as “free?” Is there something we can get for free from nature to reduce the cost of our compressed air? What if lower temperature intake air was nature’s gift? What if all we need is a bit of tin to duct air from a different source?
The answer is: “It depends.”
The goal of this article is to debunk a few misconceptions, and show how inlet air temperature actually affects compressor efficiency in three kinds of systems. In summary, inlet air temperature has a modest impact on compressor efficiency, depending on the situation. This article will discuss the following two factors that impact efficiency: (1) The Type of Compressor, and (2) The Compressor Controls.
Definitions
Before I launch into the article, let’s define several key terms:
Inlet cubic feet per minute (icfm): This is the airflow rate in actual volume per time units at the intake of a compressor. It is at actual conditions, uncorrected for density. This is what you could measure at the inlet of a compressor using a velocity measurement like a pitot tube.
Standard cubic feet per minute (scfm): This is the delivered airflow in volume, converted to a standard reference point that doesn’t exist in the actual compressed air system. It is essentially mass flow divided by a constant. This is what you would measure with a mass flow meter at the discharge of a compressor that compensates the measurement with pressure and temperature.
Isentropic (or Adiabatic) Compression: This is an ideal model of compression when no heat is transferred out of the compressor, as if the walls of the compression chamber are insulated, and temperature goes way up. It is reversible. Interestingly, it is not particularly “efficient” to compress this way. This is closer to the way a single-stage, oil-free screw compressor works. Two-stage, oil-free compressors with inter-cooling have good efficiency, but they behave like “adiabatic” compressors as far as the inlet temperature is concerned. Adiabatic efficiency is the ideal adiabatic power divided by the actual power.
Isothermal Compression: This is an ideal model of compression when all the heat is transferred out of the compressor, and temperature is constant through compression. Interestingly, it is quite “efficient” to compress this way. This is closer to how an oil-injected screw compressor works, particularly a multi-stage unit or a centrifugal with many stages and inter-cooling between stages. Isothermal efficiency is the ideal isothermal power divided by the actual power.
Volumetric Efficiency: This is the inlet volume flow (icfm) divided by the ideal flow with no slip or “displacement.”
How Inlet Air Temperature Impacts Different Types of Compressors
There are three common types of compressors in most plants, including positive-displacement, screw (oil-injected and oil-free), and dynamic (centrifugal). We will generalize their efficiencies for the purpose of understanding how inlet temperature affects it. I realize these are generalizations, and there are exceptions. The goal of this article is not to compare compressor efficiencies between these types of compressors, but to examine the impact that inlet air temperature has on the efficiency of each.
Oil-Injected Screw
This type of compressor is designed for general industrial applications where trace amounts of oil can be in the air stream. There is very little slip between the rotors, resulting in high (and constant) volumetric efficiency. Also, significant internal cooling occurs during compression. As a result, the temperature rise per stage is the lowest of the three, and the compressor behaves closest to the “isothermal” compression model. In a simplified sense, power and flow vary with inlet temperature in an oil-lubricated screw compressor as follows:
- Volume (icfm) is constant with inlet temperature.
- Delivered flow (scfm) is directly proportional to inlet temperature. A 10ºF decrease in inlet temperature will result in about a 1.9 percent increase in mass flow.
- Required power in isothermal compression is independent of inlet density, and only dependent on pressure ratio and speed. So, in an oil-injected screw compressor, which isn’t perfectly isothermal, power only goes up a small amount with lower inlet temperature.
NOTE: Some discussions in the literature attribute the insensitivity of power to inlet temperature in this type of compressor to the constant oil temperature control. They claim that inlet flow is essentially constant because of the constant oil temperature. In my view, that it not the case. It is unlikely that the inlet air reaches thermal equilibrium temperature with the oil in the very short time before cut-off. Oil temperature does have an impact on power, but it is due to viscosity. Low viscosity reduces drag, but increases slip between rotors. So higher ambient temperature in an air-cooled unit that had high oil temperature could actually reduce capacity, but it would reduce viscous drag at the same time (and vice versa). In a water-cooled unit, there would be no impact on volume flow.
Conclusions
- Efficiency (scfm/kW) increases with inlet air temperature reduction, because mass flow goes up while power barely does.
- Ambient temperature is primarily an oil-quality and maintenance concern.
Oil-Free Screw
This type of compressor is designed for pharmaceutical and high tech applications where no oil can be in the air stream. There is some slip between the rotors, and no cooling in the compression stage — just cooling between the two stages. As a result, the temperature rise per stage is the highest of the three, and the compressor behaves closest to the “adiabatic” compression model. At higher pressures, volumetric efficiency drops, also resulting in lower adiabatic efficiency. In a simplified sense, power and flow vary with inlet temperature in an oil-free screw compressor as follows:
- Volume (icfm) is constant with inlet temperature.
- Delivered flow (scfm) and inlet density (lb/min) are both directly proportional to inlet temperature. A 10ºF decrease in inlet temperature will result in about a 1.9 percent increase in mass flow.
- Required power in isentropic compression is also roughly proportional to inlet density and pressure ratio (at constant icfm).
- The compressor is temperature-sensitive in two ways:
- High inlet temperatures cause outlet temperature of the first stage to rise uncontrolled, which could result in shutdowns.
- Low inlet temperatures cause the outlet temperature of the first stage to drop uncontrolled, requiring temperature control at the intercooler to keep from condensing moisture and hurting the second stage.
Conclusions:
- Efficiency (scfm/kW) doesn’t change with inlet air temperature, all other things being equal.
- Temperature rise considerations make it best to run with a cool inlet, as long as the intercooler is controlled.
Multi-Stage Centrifugal
This type of compressor is designed for general industrial applications where no amounts of oil can be in the air stream and higher airflows are needed. Inlet air temperature (and thus density) affects the compression significantly. Flow changes with pressure like a centrifugal pump — low flow at low pressure and low flow at high pressure. In addition, compressor efficiency changes based on the point on the curve. It behaves like an adiabatic compressor, but with varying flow due to intake changes. In a simplified sense, power and flow vary with inlet temperature in a centrifugal compressor as follows:
- Available head (pressure) changes with inlet density. Lower intake temperatures allow the compressor to increase pressure, pushing out an uncontrolled compressor’s curve (wide open inlet) almost as if the speed increased, and vice versa.
- Since the pressure changes with inlet temperature, the inlet guide vanes (IGVs) will have to cut back to keep the pressure from rising (see controls). However, the inlet flow still rises.
- Since the curve rises, the range between the full open and minimum throttled “surge line” increases at lower intake temperature, allowing more turndown before surge (if controls are set up properly).
- Power increases with inlet mass about the same as the flow increases. The compressor motor current needs to be controlled at a maximum amp level to keep it from overloading in cool intake scenarios.
- This type of compressor is very inlet temperature sensitive. Power increases and flow increases, more so than in an oil-free screw. However, power and flow change by about the same amount.
Conclusions:
- Efficiency (scfm/kW) is fairly constant. Based on a recent analysis we performed with several models and makes, changing the inlet temperature by 20ºF only changed efficiency by 0.2 percent.
- Ambient temperature is primarily an air delivery, motor load and surge protection issue.
Leveraging Compressor Controls to Take Advantage of Inlet Air Temperature
The above discussion is mostly theoretical. Unless the real system controls allow at least one compressor in the system to respond efficiently to the increased flow that the lower intake temperature provides, there will be no energy savings. In fact, it can go backwards. We will briefly discuss how controls in the three types of compressors can take advantage of the intake air temperature effect, or make it worse.
Oil-injected Screw
These are modulation, load-unload or variable speed drive (VSD) compressors. If a higher flow from a lower intake temperature occurs, the following will happen in the two types of control (I will assume a two-compressor system, including base and trim):
- Modulating Trim Compressor: Increasing flow from a lower inlet air temperature is roughly like increasing the speed of the compressors. Increasing flow rate of compressors will cause the pressure to rise in the system and leaks to consume more. The trim compressor will respond to the increased pressure by modulating more, which reduces its efficiency. In addition, the increased pressure will raise the power of the compressors. The increased efficiency from inlet air temperature reduction can be all given away by these increased losses.
- Load-Unload Trim: The trim compressor will then have to unload more often, reducing loaded. However, its idle time will increase, reducing the potential increase in overall system efficiency.
- VSD Trim: The trim compressor will reduce speed, reducing loaded power without increasing the base-load compressor power. Efficiency will be better, similar to the ideal calculations.
Oil-Free Screw
These are load-unload or variable speed drive (VSD) compressors. If a higher flow from a lower intake temperature occurs, the following will happen in the two types of control (I will assume a two-compressor system, including base and trim):
- Load-Unload Trim: Increasing flow from a lower inlet temperature is roughly like increasing the speed of the compressors. The trim compressor will then have to unload more often, reducing loaded kWh by about the same as the increased kWh of the base-load compressor. However, its idle time will increase, reducing overall system efficiency.
- VSD Trim: The trim compressor will reduce speed, reducing loaded kWh by about the same as the increased kWh of the base-load compressor. Efficiency will be about the same as with a warmer inlet.
Centrifugal
These are typically inlet modulation with blow-off compressors. Inlet modulation can be for constant pressure or constant mass. Constant pressure is most common. If a higher flow from a lower intake temperature occurs, the following will happen (I will assume a two-compressor system, including base and trim):
- Increasing flow from a lower inlet temperature is roughly like increasing the speed of the base-load compressor(s). Increasing the flow rate of compressors will cause the pressure to rise in the system and leaks to consume more. The trim compressor will respond to the increased pressure by modulating more, which reduces its efficiency slightly. In addition, the increased pressure will raise the power of the compressors. In the worst case, the trim compressor will start to blow off (or increase blow off, if it already is doing so sometimes). System efficiency will probably stay the same, or decrease.
Will Inlet Temperature Drop Increase Energy Efficiency?
I’m not trying to rain on anyone’s parade — I am just trying to provide a dose of reality. If you have an oil-injected rotary screw compressor system with a VFD running as trim, then inlet temperature drop will definitely save energy. It won’t provide huge energy savings, but it will be enough to make the project worthwhile — if it’s just ducting changes. In all other cases, savings are questionable, and can actually go negative.
If you’re considering inlet temperature reduction, or inlet filtration improvement, I recommend doing it from a maintenance and air/oil quality perspective. However, keep in mind the increased motor current that can result, and the potential over-cooling and condensation that can occur.
For more information, contact Tim Dugan, P.E., President, Compression Engineering Corporation, tel: (503) 520-0700 or visit www.comp-eng.com.
To read similar System Assessment articles, please visit www.airbestpractices.com/system-assessments.
