Industrial Utility Efficiency    

Choosing The Right Blower Rental Technology for Low-pressure Air Applications

Blower systems designed to deliver continuous airflow at pressures of 50 psig or below are critical to the operation of many processes including wastewater treatment, pneumatic conveying, fluid catalytic cracking, and fermentation to name a few. Many circumstances could arise that require an operator to require a rental blower for a period; a few examples include:

  • Upgrade project: a plant needs to maintain operations throughout a project to remove their existing blowers and install new ones.
  • Rebuild:  a plant needs to replace the functionality of a machine taken out of service for repairs (proactively or reactively).
  • Supplemental needs: a plant experiences a shortage in blower capacity during summer months, or during a specialized production run and requires additional blowers to supplement the capacity of the existing system.
  • Capital avoidance: a plant needs additional blowers but cannot get capital funding for a project, so they rent units.

The rental industry for compressed air was developed in response to the needs of many industries including petrochemical, road construction, and industrial manufacturing. Many rental air compressors built to serve these industries are rotary screw machines designed to deliver compressed air in the 1,600 cfm range with pressures at 90 to 150 psig. And because there is an abundance of these machines available, they are often used to fulfill temporary needs in applications that require the air at far lower pressures. However, there are numerous factors to consider when considering the use of rotary screw air compressors as rentals.

Rental blowers

Decision-makers can come out ahead by carefully evaluating available technologies when renting blowers that deliver continuous airflow at pressures of 50 psig or below.

Cost Analyses A Must

The biggest drawback of applying rotary screw air compressors in applications that require lower operating pressures is the inefficiency in compressing the air to 90-plus psig and then regulating it down to the needed pressure. Whether it’s a diesel- or electric-drive unit, it is by far the least energy efficient method within the rental industry to deliver air at less than 50 psig as shown in Figure 1. Cost analyses are a must.

Technology

Application – 1,600 cfm

Wastewater Treatment @ 7 psig

Pneumatic Conveying @ 29 psig

Pneumatic Conveying @ 50 psig

Gear Driven Centrifugal Blower

(Electric drive)

Flows too low for a typical rental

Multi-stage Turbo Blower

(Electric drive)

$1,700

n/a

n/a

Positive Displacement Blower

(Electric drive)

$1,100

n/a

n/a

Single Stage Oil Free Screw Blower (Electric drive)

n/a

$2,500

$3,700

Rotary Screw Air Compressor

(Oil-free or oil-flooded, electric)

$5,500

$5,500

$5,500

Rotary Screw Air Compressor

(Oil-free or oil-flooded, diesel)

$8,000

$8,000

$8,000

Figure 1: Outlined are energy costs for three different types of technologies during one week of operation at three different pressure levels and the same 1,600 cfm flow requirement. Calculations assume the cost of electricity is $0.11 per kWh and cost of diesel fuel is $2.75 per gallon. Additionally, the energy efficiency levels achieved with a multi-stage turbo blower, positive displacement blower, or single-stage oil-free screw blower are only possible when the units are equipped with a Variable Frequency Drive (VFD) or the rental provider has the perfectly geared machine in stock.

The need to examine the true costs for any given technology can be further illustrated using a hypothetical example involving a water reclamation plant that relies on 14,000 diffusers, pushing air through nines zones in four different aeration basins.

In this example, the plant requires 5,500 cfm at eight psig, but it needs to take the blowers offline to carry out urgent maintenance, and it doesn’t have the ability to provide electrical power to rental machines. In order to continue processing water, the plant requires rental equipment to supply air during the maintenance project.

If the plant chooses to rent four diesel drive rotary screw air compressors with a regulator to reduce the discharge pressure down to eight psig, this option would be expected to consume around 12,000 gallons of diesel fuel per week. However, if the plant chooses to rent one or two positive displacement blowers powered by a diesel generator it can expect to consume around 2,400 gallons of diesel fuel per week. A rough comparison of the total cost of the two solutions shows the rotary screw air compressors would cost more than three times as much as positive displacement blowers powered by a generator.

 

 A Closer Look at Technologies for Rent

There are numerous examples of plants choosing rotary screw units in situations like the one at this hypothetical wastewater treatment plant. Why would they do that is the question? The answer lies in a lack of awareness of more appropriate rental technologies.

Here’s an overview of various technologies available that deliver air at 50 psig and below in the rental market – and how the right choice in blower technologies can have a tremendous impact on the total cost of a blower rental project.

Rotary Screw Air Compressors

A positive displacement rotary screw machine is designed to deliver airflow at 90-150 psig. These machines can be Class 0 oil-free air (two-stage), or oil flooded (single stage) units. Within the rental industry, these machines are air-cooled, portable, and typically include an aftercooler, which allows for reduced discharge temperatures. A standalone regulator is usually required to adjust the discharge pressure below 50 psig. Rental rotary screw compressors are easily installed and started using local resources. 

Often when a rotary screw air compressor is being applied in an application that only requires airflow at 50 psig or below, the power requirements of the unit are so much greater than the equipment being replaced that adequate power is not available. This causes the rental provider to offer diesel drive units as an alternative. 

Gear Driven Centrifugal Blowers

A gear driven centrifugal blower is a dynamic machine designed to deliver Class 0 oil-free air. These machines can be built to operate across a wide range of flows and pressures including 50 psig and below.  

This technology can be the most energy efficient option if the pressure and flow requirements are very close to the design point of the specific rental machine. However, these machines have a limited range of efficient operation once set up and commissioned. If the plant has needs that vary from the rental machine design point, throttling the inlet valve and/or opening the bypass valve to discharge compressed air to atmosphere will prevent surge. Yet either of these flow-regulating actions will impact the efficiency of the machine considerably. It’s why gear driven centrifugal blowers are a viable option when the system pressure is stable, but the units may experience operational issues in an application like pneumatic conveying where pressures typically fluctuate. 

There are also complexities involved in renting gear driven centrifugal blowers. They typically require permitted loads for shipping and specialized technicians to be on site for commissioning and start-up. In addition, many are water-cooled, which could necessitate the rental of a cooling tower. The likelihood that gear driven centrifugal blowers are the best technology for a rental project increases as the expected project length increases to approximately three to six months or longer. The same is true if the necessary flows increase in the range of 10,000 cfm and higher.

Multi-stage Turbo Blowers

A multi-stage turbo blower is a dynamic machine designed to deliver Class 0 oil-free air across a wide range of flows and pressures in the range of 14 psig and below. This technology has limited ability to deliver airflow at pressures between approximately 14 and 22 psig.

These machines are typically air-cooled and without an aftercooler. Multi-stage turbo blowers are viable options when system pressure is stable, but may have operational issues in an application like pneumatic conveying. The rental units are easily installed, commissioned, and started using a plant’s local resources. In most applications, a multi-stage turbo blower will not be the most energy efficient rental option available and the efficiency can degrade considerably if the machine is not outfitted with a VFD.   

Positive Displacement Blowers

A positive displacement blower is a machine designed to deliver Class 0 oil-free air up to approximately 15 psig and it typically utilizes rotary lobe technology. These machines are typically air-cooled and without an aftercooler. Positive displacement blowers are resilient to a wide range of flows and operating conditions. Rental machines are easily installed, commissioned, and started using a plant’s local resources. In many rental applications, a positive displacement blower equipped with a VFD will offer the best combination of versatility, energy efficiency, ease of installation and operation for oil-free applications below around 15 psig.

 Aerzen DV038

Shown is an Aerzen DVO38 positive displacement rental blower rated to deliver 2,200 scfm at 14.5 psig.

Single Stage Oil-free Screw Blowers

A single stage oil-free screw blower is a machine designed to deliver Class 0 oil-free air between approximately 10 and 50 psig and utilizes oil-free rotary screw technology. These machines are typically air-cooled without an aftercooler. The machines are resilient to a wide range of flows and operating conditions. Rental single stage oil-free screw blowers systems are easily installed, commissioned, and started using a plant’s local resources. In many rental applications, this type of unit outfitted with a VFD will offer the best combination of versatility, energy efficiency, and ease of installation and operation for oil-free applications between 10 and 50 psig.

Factor in Overall Project Costs

For any rental project to commence, the technology selected must do the job reliably and has to be commercially viable for the plant. The best solution is the one that delivers the needed flows at the needed pressures and with the right combination of speed and total project cost. Yet it’s also important to examine other factors the impact costs. For example, large freight costs coupled with long shipping times for the most energy efficient solution could wipe out any potential energy savings for a short duration rental project. Here are additional factors to consider when determining what technology to utilize: 

  • Cost of downtime while implementing the rental project.
  • Freight costs (both ways).
  • Installation and commissioning/decommissioning costs, such as specialty technicians, contractors, electricians, riggers, etc.
  • Risks if the technology selected for the rental does not operate reliably across the range of needed flows and pressures.
  • Energy costs (electricity or diesel fuel).
  • Expected length of the rental.

Selecting the right rental solution can be challenging, especially if the decision is made while under pressure in an emergency situation without adequate time to study the various technologies. Selecting the wrong technology could dramatically increase the total cost and effectiveness of the rental project.  The best solution may not be the one that delivers the quickest, is the cheapest to rent, is the most energy efficient, or the easiest to install. 

The best advice is to carefully consider each aspect of a rental project to determine the overall project cost and how that rental system can help the plant achieve its objectives in a timely and fiscally responsible manner.

 

About Aerzen Rental USA, LLC.

Aerzen Rental USA, LLC is a wholly-owned division of the German manufacturer, Aerzener Maschinenfabrik GmbH, and has been a recognized world leader in the production of rotary positive displacement machines since 1868. Established in 2018, Aerzen Rental USA, LLC is based in Atlanta, Georgia. For more information About Aerzen Rental USA, LLC, contact Matthew Piedmonte, Director, Aerzen Rental; email mpiedmonte@aerzenusa.com; tel: 404-673-1826, or visit www.aerzenrental.com.

 

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