By: Cas | Posted on: 07-06-2024

When it comes to industrial compressed air systems, rotary screw air compressors play an important role. These compressors fall into the category of positive displacement options and are known for their efficiency and reliability.

In comparison to reciprocating air compressors, rotary screw compressors offer several advantages. They are highly durable and often outlast other types of compressors, making them a cost-effective choice in the long run. Additionally, rotary screw compressors are equipped with an internal cooling system that allows them to operate continuously without the need for frequent maintenance.

The benefits of using rotary screw compressors in industrial applications are significant. These compressors are capable of generating energy ranging from 5 to 350 horsepower, making them suitable for large-scale jobs that demand continuous airflow. Whether it's powering tools like jackhammers or supporting sandblasting operations, rotary screw air compressors are the go-to choice for many professionals in the industry.

But (!), they are (often) not suitable for smaller applications. In fact, the most troublesome compressor installations I have seen were most of the time under-used rotary screw compressors.

General Advantages and Disadvantages of Rotary Screw Air Compressors

When considering the utilization of rotary screw air compressors in industrial settings, it is essential to weigh both the advantages and disadvantages of these machines. This comprehensive overview will delve into the benefits of higher CFM and consistent airflow, as well as the drawbacks related to maintenance complexity and cost implications.

Advantages:

1. Higher CFM: Rotary screw air compressors provide higher CFM (Cubic Feet per Minute) compared to other types, ensuring a steady and reliable airflow for various industrial processes.

2. Consistent Airflow: The continuous operation of rotary screw compressors results in a consistent airflow, crucial for maintaining productivity in industrial settings where compressed air is a primary power source.

3. Lower Total Cost of Ownership: Despite the higher initial cost of rotary screw compressors, their efficiency and durability often lead to a lower total cost of ownership over time, making them a cost-effective investment for long-term industrial use.

Disadvantages:

1. Higher Initial Cost: One of the main drawbacks of rotary screw compressors is their higher initial cost compared to other types of air compressors, which can be a barrier for smaller applications with budget constraints.

2. Maintenance Complexity: Rotary screw compressors require more specialized maintenance compared to other types, involving regular inspections, lubrication checks, and filter replacements to ensure optimal performance.

3. Need a minimum usage: Rotary screw compressors require what I call a 'minimum usage'. Both in terms of air flow needed throughout the day and the number of hours per day the machine is used.

Working Principle of Rotary Screw Air Compressors and how it affect use in small scale operations

Rotary screw compressors, falling under the positive displacement category, offer a consistent flow of compressed air regardless of outlet pressure. The mechanism behind rotary screw compressors involves a pair of screws rotating in opposite directions within a sealed chamber to compress air efficiently.

Oil-Injected and Oil-Free Compressors

Oil-injected rotary screw compressors utilize oil for sealing, cooling, and lubricating purposes within the compression chamber. This design ensures efficient operation and higher compression ratios. Conversely, oil-free rotary screw compressors eliminate the use of oil in the compression process, making them suitable for environments that require contaminant-free air, such as pharmaceutical or food production facilities.

In small scale operations, you will never find an oil-free screw compressor. Why? Because they are expensive, only available for larger air flows and if oil-free air is available, there are better solutions available (like the right compressed air filters or scroll compressors).

So we can focus on oil-injected screw compressors. They come in two flavors: fixed speed and variable speed (VSD).

Fixed-Speed vs. Variable-Speed Compressors - and what to watch out for

Fixed-Speed Compressors

Fixed-speed rotary screw compressors operate at a constant speed, delivering a consistent flow of compressed air regardless of the demand. While they are simpler in design and often more affordable upfront, fixed-speed compressors may consume more energy during idle periods when air demand is low.

For small applications, the main problem with fixed-speed screw compressors is the fact that you are going to be wasting a lot of energy from it running unloaded.

In small applications, you'll often see the screw compressor run for a minute to fill up the air receiver, then run unloaded for a few minutes (which is normal for a screw compressor as it can't start and stop too many times per hour), and then either stop, or load again to fill the air receiver again (depending on receiver size and air consumption).

Variable-Speed Compressors

Variable-speed rotary screw compressors adjust their motor speed to match the required air output, resulting in energy savings during low-demand periods. These compressors are highly efficient and offer flexibility in managing varying air requirements. Although they come at a higher initial cost, the long-term energy savings can outweigh the investment.

The problem with small applications (low air consumption) is that the VSD compressor never really speeds up above minimum speed. This way, the compressor never really heats up. This results in a lot of condensation inside the compressor.

The Two Main Problems

The main problem with screw compressors in small-scale applications is that the low air demand does not sufficiently heat up the air compressor and the imbalance between compressor size and air demand makes for a very inefficient (electricity/cost wise) compressed air system.

Heating up

If a screw compressor does not heat up to it's intended running temperature (around 80 degrees Celsius), a lot of problems with condensation occur. The air that is being compressed contains a lot of moisture, at low temperatures it will condense inside the compressor, diluting the oil and causing oxidation problems (rust) in valves and other internal parts.

Energy Usage

As said before, a too low air consumption will result in long periods of unload running compared to the amount of running hours per day or week. Since the compressor still uses about 1/3 of full power energy during unload running, this is all wasted energy.

Fixing existing troublesome low-usage compressor setups

There are two ways to fix existing setups where a screw compressor was selected in an application that in reality demands a too-low air consumption of said air compressor.

1. Install Big Air Receiver

Upgrade the existing air receiver to a bigger one, or simply install a second air receiver. What this does is it makes the whole system 'slower'. It takes more time for the compressor to fill up the big air receiver, and it also takes more time for the consumers to 'drain' the stored air in the air receiver.

The longer fill-up times makes sure that the compressor has time to heat up and clear itself from water.

The longer drain time of the air receiver makes sure that the compressor will actually stop in between loading cycles (reducing energy consumption from 30% to 0 at those times).

2. Manually full-load the compressor every week

If installing a bigger air receiver is not an option because of cost, size or simply because air usage is so low that it still does not solve the problem, the only real solution is to make sure the compressor runs full-power for at least once a week.

We can do this by simply opening a valve to purge air from the system (use a sound muffler). (there is some technique involved here, let me know if anyone wants to know and I will write a short article about it).

Let the compressor run for 20 minutes to an hour at full load. This is actual standard practice after maintenance.

This will make the compressor heat up to it's designed operating temperature, which will evaporate and expel all condensation water from the compressor.

Alternative to screw compressors for Small Scale Air Compression Needs

When it comes to smaller scale air compression needs, alternative solutions might be more suitable due to the challenges faced with rotary screw compressors in such settings.

As we saw, while rotary screw compressors are highly efficient and reliable for large-scale industrial applications, they may not always be the best choice for smaller operations.

There is really only one alternative: the reciprocating piston compressor

Reciprocating compressors, also known as piston compressors, are a popular and cost-effective choice for smaller work sites such as garages and home construction projects. They achieve high pressure through pistons driven by a crankshaft and are suitable for applications that do not require continuous heavy-duty use.

They do not suffer from the same problems that screw compressors do. They can start/stop many times without and problem (they are smaller and have no unload running cycle typically) and they heat up quickly so condensation is less of a problem.

Since piston compressors are generally smaller capacity compared to screw compressors, you might find yourself in the situation that a screw compressor is actually too big for your setup, but a piston compressor is too small.

In this situation, simply opt for two piston compressors. Or even three!

Seriously! The most troublesome, money wasting machines (both in terms of wasted investment, extra maintenance costs and energy usage) that I have seen, have always been rotary screw compressors that were too big for the job.