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Posts tagged ‘Refrigeration Cycle’

Refrigeration Basics: Understanding the Refrigeration Cycle

         Don Gillis | Lead Technical Trainer

          Emerson’s Educational Services

Welcome to the fourth installment in our series of blogs intended to help not just beginning service technicians, but anyone who wants to learn more about the basics of refrigeration. In this blog, I explain the nuances of vapor injection along with the full refrigeration cycle. For this blog series, we have also created companion videos about each topic that you can cross-reference while accessing related information at Education.Emerson.com.

Comparing Refrigeration to a Baseball Diamond

The refrigeration cycle requires four main components. No matter how small or how large a cooling system might be, its design will include a compressor, a condenser, a metering device and an evaporator.

When I teach new technicians, I often compare the refrigeration cycle to the layout of the field for the game of baseball. I’ve found this analogy makes refrigeration equipment and processes easier for them to understand.

 

 

 

 

 

 

 

 

 

 

 

In my example, a compressor is located at home plate at the bottom of the baseball diamond (shown above). In a refrigeration or cooling system, compression is the first step:

  • Refrigerant enters as a low-pressure (LP), low-temperature (LT) superheated vapor and exits the compressor as a high-pressure (HP), high-temperature (HT) vapor.
  • The compressor mechanically compresses the refrigerant gas.
  • Under pressure, the refrigerant volume is reduced and the temperature is raised.

The second step involves a condenser, located at first base on the right side of the baseball diamond:

  • Hot, pressurized refrigerant gas arrives from the compressor into the condenser, which is designed to reject heat by lowering or returning the temperature of the refrigerant to its condensing temperature.
  • As it rejects heat, the condenser converts the vapor to a sub-cooled liquid.
  • In most condensers, the refrigerant gas enters at the top of the equipment and leaves at the bottom because the refrigerant in a liquid state is much heavier than the weight of refrigerant in a gas state.

In the third step, a metering device located at second base at the top of the baseball diamond regulates the amount of refrigerant released into the evaporator in response to the cooling load and causes a pressure drop.

The metering device also:

  • Measures the superheat at the evaporator outlet
  • Maintains a constant temperature by raising or lowering the amount of refrigerant flowing into the evaporator

At the fourth step, cold liquid refrigerant mixes with vapor causing the saturation temperature as it boils off or vaporizes in the evaporator, located at third base, on the left side of the baseball diamond:

  • The process allows the refrigerant to absorb heat through a series of metal coils.
  • The low-pressure superheated vapor refrigerant gas then returns to the compressor to continue the refrigeration process.

Here is the value of comparing the refrigeration process to a baseball diamond: If I draw a vertical line from home plate up to second base, everything in the system on the right side of that line is under high pressure; everything on the left side of that line is low pressure.

Likewise, if I draw a horizontal line from first base to third base, the refrigerant above the line is in a liquid state; below the line, the refrigerant is a vapor, regardless of whether it is under high or low pressure.

Liquid Injection Cools Compressor and Increases Capacity

A compressor is designed to operate at very high temperatures, so a liquid injection method has been developed to cool the compressor internally. How this works can be confusing; refrigerant is injected in a vapor state, not in a liquid state.

When necessary, liquid injection cools a compressor to enable it to run reliably under difficult high compression ratio conditions normally seen on low-temperature freezer applications.

  • Refrigerant is piped from the system liquid line, through an injector valve to the compressor; in scroll compressors, the refrigerant is injected directly into the scroll elements.
  • Without this cooling, the compression elements can get too hot and the oil breaks down, leading to compressor failures.

Another approach called enhanced vapor injection (EVI) increases refrigeration capacity and, in turn, the efficiency of the system:

  • A heat exchanger is utilized to provide subcooling to the refrigerant before it enters the evaporator.
  • A small amount of refrigerant is evaporated and superheated above its boiling point.
  • This superheated refrigerant is then injected mid-cycle into the scroll compressor and compressed to discharge pressure.

The diagram below shows how enhanced vapor injection (EVI) increases the efficiency of the system.

 

 

 

 

 

 

 

 

 

EVI increases the compression ratio and, in the process, boosts capacity for the refrigeration system. The greatest gains can be achieved during the summer months and other periods when warm ambient temperatures require more cooling.

View our new video series to learn more about the refrigeration cycle. For a deeper dive into all of our training content and access to our other educational resources, visit Education.Emerson.com.

Refrigeration Basics: A Look at Each Step of the Refrigeration Cycle

         Don Gillis | Lead Technical Trainer

          Emerson’s Educational Services

Welcome to the third installment in our blog series intended to help not just beginning service technicians, but anyone who wants to learn more about the basics of refrigeration. In this blog, I introduce some of a refrigerant system’s many basic components, explain what each is designed to do and discuss how they work together. For this blog series, we have also created companion videos that you can cross-reference while accessing other related information at Education.Emerson.com.

Step one: compression

In a compressor-based refrigeration or cooling system, refrigerant enters the compressor as a low-pressure, low-temperature superheated vapor and exits the compressor as a high-pressure, high-temperature superheated vapor. The compressor — which could be centrifugal, reciprocating, rotary, scroll or screw design — mechanically compresses the refrigerant gas. Under pressure, the refrigerant volume reduces, and the temperature rises.

The relationship between pressure and temperature is critical to how efficiently and effectively the system can achieve and maintain its intended setpoint.

Step two: condensing

This second step in the refrigeration process is necessary to convert the vapor to a liquid. As the compressor releases hot, pressurized refrigerant gas into a condenser, it rejects the heat by lowering or returning the temperature of the refrigerant back to its condensing temperature. Condensers utilize three primary cooling methods:

  • Air-cooled — often found in small systems and residential applications; air flows naturally or is forced by a fan over metal (typically copper or aluminum) coils, which carry the heated refrigerant
  • Water-cooled — utilized in commercial systems, large plants and when operating in higher ambient temperatures; cool water replaces natural or forced airflow around the coils carrying the heated refrigerant
  • Evaporative — combines air and water cooling in large-scale facilities such as those for making ice

In most condensers, the refrigerant gas enters at the top of the equipment and leaves at the bottom because the refrigerant in a liquid state is much heavier than the weight of refrigerant in a gas state.

Step three: thermal expansion valve (TXV)

In the third step, the liquid refrigerant is cooled further when pressure is suddenly decreased by the TXV. The valve regulates the amount of refrigerant released into the evaporator in response to the cooling load.

It also measures the superheat leaving the evaporator outlet and maintains a constant temperature by raising or lowering the amount of refrigerant flowing into the evaporator. A precisely controlled flow maximizes the efficiency of the evaporator and ensures that we only have vapor returning to the compressor.

The TXV has multiple ports with bulbs that read:

  • P1 — the temperature as refrigerant leaves the evaporator
  • P2 — the pressure inside the evaporator
  • P3 — the closing force to control superheat flow into the evaporator (if adjustable)
  • P4 — the opening force, liquid line pressure

Step four: cooling in the evaporator

The evaporator is the part of a refrigeration system where the actual cooling takes place. In this fourth step, sub-cooled liquid refrigerant begins boiling off or vaporizes in a process that allows the refrigerant to absorb heat through a series of metal coils. The refrigerant is saturated through this process and this low-pressure, superheated vapor and then returns to the compressor to continue the refrigeration process all over.

What does a suction line accumulator do?

The last system component you should be aware of is the suction line accumulator. This device protects the compressor from a sudden surge of liquid refrigerant and oil that could enter the compressor. Compressors are designed to compress refrigerant in its vapor state. If liquid refrigerant gets into the compressor, we refer to that as “liquid floodback,” resulting in a condition called slugging that can reduce efficiency and cause premature equipment failure.

View our new video series to learn more about the refrigeration cycle. For a deeper dive into all our training content and to access our other educational resources, please visit Education.Emerson.com.

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