What Kind of Compressors are used in Industrial Chillers

In the HVAC sector, any chilled water refrigeration system is categorized as a chiller. Chillers operate based on condensation or absorption refrigeration principles. Compression chillers utilize electromechanical energy to initiate the cooling cycle, while absorption chillers rely on heat energy to carry out the refrigeration cycle. A compression chiller system, commonly found in commercial and residential structures, encompasses four main components:

1. Compressor
2. Evaporator
3. Condenser
4. Expansion Valve

The compressor is the crucial component of any compression chiller, with classifications typically based on the type of compressor integrated within:

• Reciprocating Chiller
• Screw Chiller (ChillerScrew)
• Centrifugal Chiller (ChillerCentrifugal)
• Scroll Chiller (ChillerScroll)

Moreover, compression chillers are significant energy consumers within buildings. Gaining insight into the various chiller types, their characteristics, benefits, and limitations is vital for making informed technical, engineering, and economic choices when selecting, purchasing, and deploying chillers.

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Compressor Types in Compression Chillers

Most refrigeration systems, from household air conditioners to large commercial chillers, utilize the compression cycle for refrigeration. The compressor is central to this cycle, performing two essential tasks: a) condensing the gaseous refrigerant back into a liquid and b) absorbing heat from the air or water being cooled. The compressor also pumps the cooled liquid refrigerant through the system.

Given that refrigeration systems have varying capacities, chillers are categorized according to the compressor type:

1. Positive-Displacement Compressor
2. Dynamic Compressor

Each compressor type employs distinct methods to elevate the vapor pressure of the refrigerant:

1. Positive displacement compressors mechanically compress the evaporated refrigerant to higher pressure and reduce its volume, divided into reciprocating, rotary, and scroll varieties that deliver a constant gas volume at a steady rate.
2. Dynamic compressors, like centrifugal compressors, enhance the pressure of vaporized refrigerant via a rotating impeller's kinetic energy.

Compression Chillers by Compressor Type

As mentioned previously, compression chillers are classified according to the compressor type integrated into their structure. To better understand compression chillers, the following sections cover different chiller types, their advantages, disadvantages, limitations, and applications.

Reciprocating Compressors

Reciprocating compression chillers are fitted with reciprocating compressors that utilize cylinders and pistons for refrigerant gas condensation. A piston’s downward motion creates a vacuum within the cylinder, allowing refrigerant entry. Upon reaching its lowest point, the piston ascends, sealing the refrigerant before compressing it. Compressed refrigerant is expelled when the pressure exceeds the discharge valve's threshold. Reciprocating compressors come in three configurations: a) Hermetic, b) Semi-Hermetic, and c) Open.

1. In hermetic compressors, the motor and compressor exist within a single sealed unit, necessitating complete replacement upon failure as repairs are not feasible.
2. Semi-hermetic compressors feature accessible motors, allowing for repairs and potentially longer lifespans.
3. Open compressors have separate motors connected via couplings, relying on outdated technology.

Reciprocating chillers are often inexpensive and efficient at lower capacities. Additionally, these systems can employ multiple compressors to match building loads, are easy to control, and offer adjustable speeds. With solid construction, they can operate effectively in harsh conditions, such as those in southern Iran. Reciprocating chillers come as air-cooled or water-cooled variants, capable of capacities between 5 to 150 tons of refrigeration.

However, their main drawback is the higher repair and maintenance needs compared to other newer chiller types. The complexity of these systems leads to greater noise and vibrations, requiring careful consideration for vibration isolation.

Reciprocating chillers become impractical for loads exceeding 200 tons. Increased capacity may lead to higher purchase costs compared to other chiller types, alongside higher energy consumption.

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Screw Compression Chillers

Screw compression chillers feature screw compressors characterized by spiral rotors for refrigerant condensation. The engagement of these rotors creates sealed spaces that capture the refrigerant, and as the rotors rotate, they progressively compress the refrigerant until it is expelled. Screw compressors can be categorized as:

• Single-Screw Compressor
• Twin-Screw Compressor

The single-screw version utilizes a main rotor synced with two gate rotors. In contrast, the twin-screw design consists of precisely aligned rotors that rotate in opposite directions, allowing for better operational control with minimal efficiency impact under fluctuating suction pressures. Screw chillers are available in both air-cooled and water-cooled configurations.

Due to their compact design and limited moving parts, screw chillers are lighter than reciprocating and centrifugal variants but still provide excellent reliability and minimal vibration. These machines are built for continuous operation and require less maintenance. Screw compressors can efficiently manage high lift forces, promoting energy savings without surge complications during load reduction.

However, screw compressors typically carry a higher initial acquisition cost and may not be the best choice for low cooling loads compared to reciprocating chillers. Their cooling capacity usually spans from 80 to 250 tons, extending up to 750 tons with multiple compressors.

Centrifugal Chillers

Centrifugal chiller systems feature centrifugal compressors that harness rotary impeller movement to exert centrifugal force on the refrigerant within a chamber. As the refrigerant enters through a wide inlet and circulates among the impellers, pressure increases due to forced movement. These chilling systems are adept at condensing large refrigerant volumes at relatively low pressures. Often, multiple impellers are used in series due to limited condensing force from a single impeller. Centrifugal systems are favored for their simplified design and fewer moving components.

Centrifugal chillers can operate at both positive and negative pressures, depending on refrigerant type. They share characteristics with reciprocating chillers, available in Hermetic Sealed and Open structures. Generally, sealed centrifugal compressors achieve broader usage despite slightly lower operating efficiency.

Pros of Centrifugal Chillers

The greatest advantage of centrifugal chillers is their high performance under full-load conditions. Unlike fixed displacement machines, centrifugal compressors can accommodate a broad capacity range across a limited pressure range, serving large refrigerant volumes effectively. By adjusting design aspects like the number of stages, impeller diameter, and refrigerant type, centrifugal compressors can optimize both refrigerant and design temperatures.

Centrifugal chillers excel in capacities of 200 tons of refrigeration or more and often initiate operation using electric motors, though steam turbines or gas engines may also be utilized.

Cons of Centrifugal Chillers

The primary challenge associated with centrifugal chillers involves their performance during partial loads. When demand declines, the chiller limits refrigerant flow by partially shutting the inlet valves, which while useful at larger capacities (up to 20% nominal output), can inhibit efficiency. Moreover, when a load drops below 25% of nominal output, these systems risk surging. To mitigate this risk, manufacturers implement specialized controls, typically compromising partial load efficiency.

Additionally, centrifugal chillers tend to have higher initial price tags, particularly for units under 200 tons, compared to screw and reciprocating types.

Scroll Chillers

Scroll chillers incorporate compressors utilizing spiral disks to facilitate refrigerant compression. Within a scroll compressor, one disk remains stationary while the other moves in orbit, creating variable volume sealed spaces that compress the refrigerant. The gaseous refrigerant is introduced at one end and progressively moves towards the center as the volume decreases, ultimately leading to its expulsion.

Scroll chillers have made considerable advancements and are rapidly overtaking reciprocating models. Their compact size, low noise, minimal vibrations, and efficiency make them attractive for small to medium building applications and residential structures, especially in <30 tons capacities.

However, scroll compressors are typically non-repairable and prone to issues of oil loss at lower temperatures.

Comparative Analysis of Chillers Based on Cooling Capacity

The compressor sizes for compression chillers are quantified by engine horsepower (HP), kilowatt power (kW), refrigeration capacities (kW), tons of refrigeration (TR), or BTU/hr. Ultimately, both the refrigerant capacity and pressure dictate the performance and applicable scenarios for each chiller type.

Reciprocating chillers generally range from 5 to 150 tons of refrigeration and consume about 1 to 1.3 kW/TR. They typically achieve greater efficiency in water-cooled versions at around 0.7 kW/TR. For screw chillers, capacities range from 70 to 500 tons (air-cooled) and 70 to 750 tons (water-cooled), requiring between 1.1 to 1.5 kW/TR. Centrifugal chillers typically possess capacities of 90 tons and greater, with optimal efficiency ranging from 0.5 to 0.6 kW/TR.

Guidelines for Optimal Chiller Selection

For peak loads reaching 80-100 tons, reciprocating chillers are a suitable choice. For loads between 100-200 tons, utilizing multiple reciprocating chillers can be beneficial. Chillers with screw compressors become economically favorable for loads beyond 200 tons, accommodating applications requiring as high as 750 tons. Beyond this point, centrifugal chillers are the most cost-effective option, provided water is accessible for heat removal.

Centrifugal chillers cater primarily to applications exceeding 750-ton capacities, providing unrivaled reliability and efficiency. It is recommended to utilize water-cooled condensers for their optimal performance.

It is reiterated that water chillers outperform air chillers in efficiency.

Capital Cost Analysis of Compression Chillers

Investment costs are comparably low for reciprocating chillers but higher for screw variations. Generally, reciprocating chillers are less expensive and more widely used, particularly at lower capacities. Conversely, screw chillers are the most expensive initial option.

Centrifugal chillers tend to be 10-15% less expensive than screw chillers under similar operating conditions but have a higher initial cost under 200-ton capacities compared to reciprocating models.

Efficiency Evaluation of Compression Chillers

Chiller efficiency is measured by kilowatts per ton of refrigeration (kW/TR) or coefficient of performance (COP), defining the ratio of thermal energy output (BTU) to thermal energy input (BTU).

• Reciprocating chillers consume between 1 to 1.3 kW/TR, achieving peak performance factors between 2.7 to 3.52.
• Water-cooled screw chillers generally require 0.6 to 0.8 kW/TR, attaining performance factors of 5 to 7, with both screw and reciprocating types achieving equal efficiency ratios under similar conditions.
• Centrifugal chillers boast outstanding performance at peak loads, consuming around 0.53 kW/TR for capacities over 300 tons, and 0.6-0.7 kW/TR for lesser capacities, with minimal power requirements improving over time due to technological advancements.

The table below provides approximate power consumption values across various compression chillers operating at full load.

Partial Load Performance Comparison of Compression Chillers

In cooling applications, peak loads only occur during specific periods of the year. Due to varying patterns of occupancy and environmental influences, chillers operate below peak capacity for the majority of their lifespan. Data indicates that chillers operate at full capacity only 1% of the time, 42% at 75%, 45% at 50%, and 12% at 25% loads. Therefore, partial load efficiency is crucial in selecting a chiller:

• Screw chillers demonstrate impressive efficiency across all loads.
• Centrifugal models rely on inlet valve manipulation to manage flow, decreasing efficiency as loads fall beneath nominal output thresholds.
• Reciprocating compressors tend to outperform screw models at minimal loads, allowing for customizable configurations with multiple units to match a building's needs efficiently.
• Choosing scroll compressors can yield high efficiency for smaller buildings.

Economic Life of Chillers

The economic life of a chiller estimates the timeframe in which it remains beneficial for the user. Reciprocating chillers have an estimated lifespan of 15 years, while chiller systems with rotary and centrifugal compressors can last up to 25 years.

Conclusion

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