With the increasing need for refrigeration, compressors are being developed in numbers and with better technology. However, the basics still stand. With the many refrigeration compressor types available, selecting one for your application can be overwhelming. Some factors to consider for the best unit are capital cost, operating efficiency, field serviceability, availability of spares and maintenance. Below, we compare screw compressors and centrifugal compressors. Each of them uses a different method to pressurize the refrigerant, but there are other differences. The choice of an ideal compressor type depends on factors such as fuel cost, energy market value, chiller characteristics and plant location. If you need help to make this decision, do not hesitate to contact ARANER’s team for expert advice.
What are refrigeration compressors?
Refrigerator compressors are used in refrigeration technologies, helping these systems maintain low temperatures. From condensers to evaporators, refrigeration systems rely on a number of elements to run smoothly and efficiently, as well as guaranteeing the right temperatures are maintained.
The role of refrigeration compressors is to circulate the refrigerant. In order to do so, a basic operation needs to take place: the refrigerant needs to change from a liquid state into gas, in the process known as evaporation. The rule is as follows: the more pressure applied to gas, the more temperature increases. This way, it’s possible to have a more precise control of this substance’s flow.
At this point, the compressor comes in. This piece of equipment can be considered as ‘the heart’ of a refrigeration system, as its role is to circulate the refrigerant by adding pressure to the circuit, thus increasing the temperature of the refrigerant.This step represents the first one in the refrigeration process, effectively starting ensuing operations that guarantee low temperatures are preserved.
As we’ll see below, there are many types of compressors, all of which come with their own pros and cons. Key differences between these systems include varying operational temperatures, as well as disparate internal components and operational processes. Depending on the type of compressors, some elements that can be found include impellers and rotors.
Additionally, depending on the type of compressor, this piece of equipment can be mounted in different parts of refrigerators.
In this context, the choice of compressor represents an important decision that must be guided by several factors. For instance, it’s important to understand that typical refrigeration compressors are run by electricity. This electricity is in charge of powering inner components within the compressor, such as pistons and crankshafts. Thus, their efficiency when it comes to energy consumption will have a direct impact on the equipment’s overall operational costs.
Types of compressors in refrigeration
Centrifugal compressors
Centrifugal compressors rely on kinetic energy developed by a rotating impeller to increase pressure of vaporized refrigerant. They belong to a larger umbrella of technologies — dynamic compressors—. During operation, the refrigerant enters the impeller wheel through an intake before flowing in the impellers. As the impellers rotate, they exert centrifugal force on refrigerant, thereby pressurizing it. Since the force created by an impeller is relatively low, for several cases centrifugal chillers come with several impeller in series.
Screw compressors
Screw compressors are positive displacement machines. Their operation is based on rotating rotors intermeshing and pressing the refrigerant along their length. There are two possible configurations of this compressor type: single-screw compressor and twin-screw compressor. You can find these compressors in both water-cooled and air cooled chiller configurations.
Fig 1: A Screw Compressor Unit Used For Industrial Refrigeration
Comparison of the two refrigeration compressor types
ARANER offers state-of-the-art centrifugal and screw compressors, for TIAC Turbine Inlet Air Cooling applications for processing plants, district cooling and so on. Our designs adhere to international standards for guaranteed optimum performance. Furthermore, the company can offer tailor made solutions on request. When it comes to choosing between centrifugal compressor or screw compressor, these are some factors to be considered.
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Speed
Centrifugal refrigeration compressor types normally operate at high speeds. Some units can have a spinning speed of up to 60,000 revs per minute! That is why you will find many of these in oil refineries and manufacturing plants, where continuous running and high gas flows are needed. Screw compressors are equally pivotal in manufacturing environments, but they may not run as fast.
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Pressure lift
In a centrifugal compressor, pressure head steepness depends on impeller design i.e. the curve steepness in direct correlation to the blades’ degree of slant from the true radial position. It may suffice it to say then that for any particular compressor, the pressure lift is quite constant. This is unlike screw compressors, which are very attractive in applications where ambient conditions or operating conditions change often.
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Efficiency
For full load and at design conditions the efficiency of screw and centrifugal chillers is quite similar. The main differences are coming when operating out of design conditions. As explained in point 2, pressure lift, screw compressors are able to adapt to fluctuating condensing and evaporating pressures taking advantage of this. For example, screw compressors can reduce their condensing pressure when ambient temperature is reduce. For the case of centrifugal compressors, this fluctuation is more complicated.
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Noise
Screw compressors can be quite loud, especially the dry subtype, during their operation, a problem that might even affect usage scope. Researchers have identified sources of this noise as system vibration, fluid flow and mechanical. Though any abnormally in the impeller can cause substantial noise, centrifugal compressors are comparatively usually less noisy than screw compressors. Anyhow, this difference in noise can always be surpassed with noise attenuation technologies.
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Life cycle cost
Life cycle cost of a machine covers maintenance cost, power cost and installation cost. For screw compressor, costs for maintenance are low. The machine can also have many service years. Most screw compressors are believed to last for more than thirty years, but this can be significantly longer with top-notch maintenance. With its higher rotational speeds, a centrifugal compressor requires special attention, especially on the bearings. Enhanced monitoring is also required for clearance and vibration. The most serious drawback of a centrifugal compressor is reduced operating efficiency at part load conditions. Below 25 % rated output, this machine suffers from surges that cause compressor damage if left unchecked. This may translate into higher life cycle costs.
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Compactness
Centrifugal compressors are generally smaller and lighter than screw compressors of similar capacities. This is very important for industries that are short of space. With the same amount of floor space, a centrifugal compressor can deliver twice as much as a screw compressor.
How to choose the type of compressor in refrigeration?
This depends on your needs. How much capacity do you need? Requirements will be very different depending on the application, location, ambient conditions, etc. ARANER can give all the guidelines to help you make the right decision regarding type and size of refrigeration compressor. Contact the experts for more regarding how to implement your options.
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