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The compressor is the most vital part of the HVAC unit. Without a compressor, the HVAC unit couldn&#;t change the temperature of anything. Each type of compressor has different qualities that change its functionality. Compressors can be a little confusing at first, so here&#;s a brief explanation of how they work, plus some numbers to help you decide which HVAC system is the best fit for your home.
How do HVAC compressors work?
Each type of compressor works a little differently, but the idea is generally the same. Each HVAC system contains an evaporator coil placed below a compressor. This evaporator coil (see the image below) contains refrigerant, which it evaporates over time. The vaporized refrigerant is guided into the compressor, which increases its pressure in order to increase the temperature of the vapor refrigerant. (For any nerds out there like me, this is due to the Ideal Gas Law: PV = nRT.)
The warmed vapor refrigerant then travels from the higher pressure of the compressor back to the lower pressure of the evaporator coil. Once the vapor refrigerant exits the compressor, it is in a lower pressure state and so condenses, cools, and becomes a liquid once again. The heat expelled from the refrigerant during this decompression process is transferred to outside the home, and the process repeats.
What are the different types of HVAC compressors, and what are the benefits of each?
Reciprocating
Rotary-vane
Rotary-scroll
Rotary-screw
Centrifugal
Cost per kW
Low
Medium
Medium
High
High
Efficiency
Low
Medium
High
Very high
Very high
Vibrations
High
Moderate
Moderate
Lowest
Lowest
Manufacturing accuracy
Easy
Difficult
Difficult
Very difficult
Very difficult
Input power per unit
Low-high
Low
Low
High
High
Other
Run on oil and gas, low cost of installation.
Constant performance, minimal wear. Oil.
Are you interested in learning more about compressor open type? Contact us today to secure an expert consultation!
Small, quiet, no metal or lubrication. Oil-free.
High reliability, quiet. Oil.
No valves, pistons, cylinders. Oil-free.
Which compressor is the most environmentally efficient?
Based on the data above, the rotary-screw and centrifugal compressors are the most efficient; they are able to produce the most cool air. However, they each also require the most &#;input power per unit,&#; or the most power to run their compression processes.
The rotary-scroll compressor requires a low input power and has a high efficiency. This difference in power input to power output is the largest of any of the compressors. Additionally, it has the option of being oil-free, thus making it one of the cleanest options. The rotary-scroll compressor is therefore the most environmentally efficient.
Which HVAC compressor is the most cost-efficient?
The table above shows that the least expensive option is the reciprocating HVAC compressor; however, this compressor also has a low efficiency and high amounts of vibration and so isn&#;t the best choice overall. The type of compressor with the lowest cost, highest efficiency, and lowest power input is also the rotary-scroll.
Which HVAC compressor would therefore be best for my home?
Based on the table above, the rotary-scroll is the best option; it has an average cost, high efficiency, and requires a low power input, thus making it the most efficient compressor both in terms of the environment and cost.
However, this answer will depend on how much money you&#;re willing to spend on your compressor. Generally, more expensive means that the compressor lasts longer, has a higher efficiency, or has a longer warranty, so it&#;s usually worth it to get more expensive.
The compressor isn&#;t the only part of your HVAC system that matters, so be sure to research how the other components affect the efficiency, cost, and input power of your unit. But as you choose an HVAC system, be sure to take into account the compressor it utilizes.
At Industrial Air Systems, although we only stock and supply rotary screw and scroll air compressors, we realise that these two types of compressors may not always be the best solution and in some cases, a reciprocating compressor may be better suited to your needs.
Below we cover seven key points to consider on the two most common air compressor types (reciprocating and rotary screw). This will help you understand the main benefits of each compressor before you purchase, which will give you a better idea of which one is right for you.
Table of contents:
1. Power consumption
2. Cubic feet per minute (CFM)
3. Duty cycle
4. Air pressure
5. Upfront cost
6. Servicing cost
7. Longevity
1. Power consumption
Reciprocating compressors
Reciprocating (piston) compressors are reasonably cheap to run if you require a small amount of compressed air off and on throughout the day. They generally have a small inbuilt air receiver that can store compressed air for use and will only turn on to refill that air receiver when the compressed air is being used. However, they become less energy efficient when they are running continuously throughout the day as they are designed to work in short increments and will perform much better when they can take a break.
Rotary screw compressors
Rotary screw compressors are much higher capacity machines that are designed for continuous use up to 24 hours a day, 7 days a week. This means that most rotary screw compressors operate at full power all the time (fixed-speed drive) even when the compressed air is not being used. The intake valve is simply held closed meaning that no compressed air is produced. This means that they are often consuming power that is not being used to make any compressed air which could waste electricity.
There is, however, a solution to this problem which is a rotary screw compressor that uses a variable-speed drive (VSD) motor. The VSD motor can vary the amount of air it produces by varying the amount of electricity it uses. This means that it will only produce just enough compressed air to keep up with the demand and will slow down or even stop when the demand decreases. This type of operation makes a rotary screw compressor a very energy efficient and power saving machine.
Summary
Reciprocating compressors are reasonably cheap to run when compared to a fixed-speed rotary screw compressor. However, a variable-speed rotary screw compressor of the same size will be the most energy efficient solution and sizes up to 20 HP can potentially save $500 to $3,000 a year or more in electricity where the other two machines would not.
2. Cubic feet per minute (CFM)
Cubic feet per minute or CFM is the amount of compressed air the compressor can output per minute.
Reciprocating compressors
Reciprocating compressors have low CFM ratings and typically have about 3.5cfm per horsepower. These compressors are generally available in anything from 2 HP to 20 HP. These are only suitable for intermittent use where they can use the compressed air tank to help produce the required amount of airflow for a short period of time. Once the air tank has been emptied, however, the compressor will need time to refill this tank and because the CFM is low, refilling the tank will take longer and airflow will be limited during this cycle.
Rotary screw compressors
Rotary screw compressors, as mentioned earlier, are designed to run continuously and have much higher CFM ratings than a reciprocating compressor, typically around 4-5cfm per horsepower. On top of this, rotary screw compressors are available in much bigger sizes, anything from 5 HP to 500 HP and beyond. This means that they are better suited when continuous airflow is required over a longer period of time and are also better suited when a large amount of compressed air is required per minute.
Summary
Reciprocating compressors will do well in small workshops where small amounts of compressed air is required intermittently and the compressor has time to refill its air receiver tank. However, if compressed air is needed in greater volume and/or continuous flow, the reciprocating compressor will become very inefficient as opposed to a rotary screw which is designed for environments where compressed air is in high demand, continuously.
3. Duty cycle
The duty cycle of an air compressor is the amount of time the compressor can produce compressed air in the total cycle period. For example, a compressor with a 25 percent duty cycle will require 45 seconds downtime for every operating minute meaning it can only be active for 15 seconds every minute or 15 minutes every hour.
As discussed earlier, reciprocating compressors are designed to operate intermittently so will have a lower duty cycle of around 50 percent. On the other hand, rotary screw compressors are designed and built to run continuously and supply continuous compressed air, which means most, if not all rotary screw compressors have a 100 percent duty cycle.
Summary
If you only require a small amount of compressed air intermittently, then a duty cycle of 25 to 50 percent will be suitable. If you have either a higher demand for compressed air or you need compressed air on and off frequently, then a duty cycle of 75 to 100 percent will be more practical and reliable for you. And if you require continuous compressed air, a compressor with a 100 percent duty cycle is really the only option.
4. Air pressure
PSI (pounds per square inch) is the standard unit of measurement for air pressure in the compressed air industry. Most standard reciprocating and rotary screw air compressors operate at around 7-8 Bar which is around 100-120 PSI. This is generally enough for most air tools and factory machines to run on and often it is just the CFM requirements that change when running more air tools or bigger machines.
In certain applications when higher air pressure is required, specialised rotary or reciprocating compressors are used that can provide pressures of up to 11 Bar or 160 PSI.
When extreme PSI is needed for applications like a high-pressure nitrogen (or other gas) generator, a booster compressor is run alongside a standard compressor to &#;boost&#; the compressed gas to pressures of up to 350 Bar or 5,000 PSI.
Summary
Most compressors can produce more than enough PSI for your workshop or factory and in cases where higher pressure is needed, both rotary screw and reciprocating compressors are available for this. The deciding factor might be the running efficiencies or the CFM demand.
5. Upfront cost
When it comes to comparing the upfront cost, there is no denying that a reciprocating compressor is almost always cheaper than a similar-sized rotary screw. At the lower horsepower range, a decent 5 HP reciprocating compressor can be purchased for as low as $2,000 to $3,000 NZD, however, a rotary screw compressor of the same horsepower would be around $7,000 to $8,000 NZD. This upfront difference in cost would hold true up until the 15 HP size where reciprocating compressors become too inefficient for their size and are seldom produced by manufacturers in any size larger than this.
Summary
If you are looking for an air compressor in the 2 to 15 HP range, with low or intermittent use requirements and are mainly concerned with the upfront cost rather than running efficiencies, then a reciprocating compressor might be the better solution for you. However, if you require a compressor larger than 15 HP, or you want greater performance, a rotary screw compressor would be the best long-term cost option.
6. Servicing cost
Reciprocating compressors
A reciprocating compressor is a lower performance and while they are supposed to be serviced every 500 hours, they can arguably keep going for a much longer time without being serviced. They are generally used on and off throughout each day and would not clock nearly as many running hours each year as a rotary screw.
Rotary screw compressors
Rotary screw compressors are generally put into factories where there is a high demand for compressed air volume or continuous use, as such they are typically running for much longer hours than a reciprocating compressor. Because of the continuous use rate and high performance, a rotary screw compressor needs maintenance and servicing every 1,000 to 2,000 hours to keep it in perfect working order.
Summary
While a rotary screw compressor may need more frequent servicing than a reciprocating compressor, it&#;s always recommended that this is done at the appropriate times, to avoid unnecessary wear and tear. No matter what compressor you use, you should always book in regular servicing of your machine and equipment so that it continues to perform at peak efficiency.
7. Longevity
Reciprocating compressors
In its lifetime, a good quality reciprocating compressor will clock a total of 8,000 to 10,000 running hours before it requires a full replacement. Now if we compare that to a rotary screw compressor, you start to see why they have become the compressor of choice and the industry standard in terms of longevity.
Rotary screw compressors
A rotary screw compressor will clock anywhere from 30,000 to 50,000 running hours before it requires an overhaul or full replacement and in some current cases, there are rotary compressors out there with well over 50,000 hours and still in perfect working order!
Summary
If longevity and high performance are critical for your workshop or business then we will always recommend a rotary screw compressor. They are built to clock a very high number of hours before they would need to be replaced.
Reciprocating compressor or rotary screw compressor?
While we have outlined the many differences between these two popular air compressors, ultimately, this decision is yours. You will know what will work best for your factory or workshop whether that be a reciprocating compressor that's better for shorter running times or a rotary screw compressor that&#;s built for maximum efficiency.
If you would like further advice on which compressor to choose, our team is more than happy to help! As industrial compressed air experts we can guide you through the different options available and also explain the benefits of our PneuTech range of rotary screw compressors. Just get in touch with us here.
For more information about our range and our popular variable-speed rotary screw compressors, head to the product page by clicking below.
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