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Site Conditions to Consider When Sizing your Compressor

Site Conditions to Consider When Sizing your Compressor
Why Site Conditions are Critical When Selecting Your Compressor
Centrifugal compressors supply compressed air for a variety of applications in diverse industries ranging from food and beverage processing to petrochemical plants and oil refineries. Centrifugal compressors cover a range of air flow rates and discharge pressures, causing different levels of power consumption. When selecting a compressor, it is imperative that you consider the proper site conditions, which include:
1. Ambient Temperature
2. Ambient Pressure
3. Relative Humidity
Why does this matter? Choosing the correct size compressor will lead to savings in capital and operating costs as well as a reduction in power consumption. Choosing the wrong compressor can be a costly mistake.
Temperature Effect
Temperature has a large impact on the rate at which mass is passed through the machine. The illustration below represents the typical pressure vs flow in multiple temperatures. When the temperature decreases the max capacity of the compressor increases, conversely higher temperature leads to a decrease in the max capacity. This means that compressors must be able to run in multiple operating conditions. The Inlet Guide Vane (IGV) is used to turn down the compressor and reduce the flow in colder conditions. For example, your specific site may have high temperatures in the summer and low temperatures in the winter, but if the compressor is sized properly then it will be able to turn down depending on the temperature and still save greatly in power consumption.
Pressure vs Flow at Various Ambient Air Temperatures
So why not provide a very conservative high temperature as the worst-case condition? Increasing the size of the compressor leads to high-power consumption, larger motors, and larger coolers. All these additional components increase the capital investment of the machine, which causes higher operating costs. Providing realistic operating temperatures can result in huge savings. 
Relative Humidity Effect
In addition to temperature, another key factor that will affect mass flow is relative humidity. In the figure below, the effect of humidity on pressure vs flow is highlighted. Higher humidity means the mass flow is decreased, resulting in a lower compressor capacity. Although, the relative humidity does not have as much of an effect as temperature, it will also affect the selection of a compressor. For example, in the graph below, if 540 lb.,/min is required, the compressor would not be able to meet that requirement if the relative humidity is 80% or greater. Once again, an overly conservative relative humidity value can result in an oversized compressor.
Pressure vs Flow at Various Relative Humidity Values
Coolant Supply Temperature Effect
The last key consideration for selecting a compressor is the coolant supply temperature. The intercooler performance can be given by the approach temperature, which is the difference between the outlet air temperature and the inlet coolant temperature. For a cooler with a known approach temperature value, if coolant is supplied at a higher temperature, the air discharged from the cooler and into the next stage will be hotter. Similar to high ambient temperature and humidity, as the coolant temperature increases the mass flow capacity decreases. The figure below shows the effect that coolant supply temperature has on pressure vs flow for various coolant temperatures. As a result, it is important to provide accurate coolant supply temperatures, so the compressor is properly sized.
Pressure vs Flow at Various Coolant Temperatures
Pressure vs Flow at Various Coolant Supply Temperatures
A brief case study (which can be accessed here) shows potential increase in power consumption by oversizing the compressor for unrealistic max ambient conditions. It also shows how reducing required flow at max ambient conditions can reduce overall power consumption. Your compressor should meet ambient requirements all year and be able to turn up when hot and humid or turn down when cold. Providing unrealistic or uncommon conditions will result in an oversized or undersized compressor, leading to cost inefficiency. All these factors affect overall power consumption, capital costs, and longevity of your compressor.

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