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Refrigeration Technologies Overview

Vapor/Compression

The refrigerant vapor is compressed and pumped into the condenser (heat exchanger) by a compressor. Compressing the gas causes the temperature of the gas to increase.  The gas is now at a temperature higher than that of the surrounding air, the heat flows from the condenser and the gas becomes cooler.  Removing the heat from the condenser allows the gas to turn into a liquid.

The liquid refrigerant is still under pressure as it flows to the expansion valve (or capillary tube).  The expansion valve meters the liquid refrigerant into the evaporator (heat exchanger) to maintain a low-pressure condition in the evaporator.  The low pressure in the evaporator causes the liquid to boil into a gas.  As the liquid changes into a gas (boils) it absorbs heat (refrigerating affect).

The heat absorbed by the refrigerant gas is carried to compressor to start the whole cycle again.  This is the manner by which heat is pumped from the cold area (i.e. your refrigerator) to the warm area (outside the refrigerator).

 

 

Absorption System

With the application of heat at the generator, ammonia vapor is driven from the solution. This hot vapor rises into the separator and a portion of the water condenses and flows by gravity into the absorber. The hot ammonia vapor continues to rise into the condenser where it gives up its heat to the surrounding air and condenses into a liquid. The liquid ammonia enters by gravity into the evaporator, where it is mixed with hydrogen gas. Circulation of hydrogen gas causes a reduction in pressure within the evaporator.  The low pressure causes the ammonia liquid to boil into a gas (evaporating) and absorbing heat in the process (refrigerating effect).  The mixture of hydrogen/ammonia vapor that’s carrying the absorbed heat is now drawn by gravity into-the absorber.  Because the water from the separator has a greater affinity for ammonia, it separates from the hydrogen gas.  The hydrogen gas being very light rises and returns to the generator to start the cycle again.  

  


 

Any technology that moves heat from a cooler area to warmer area can be referred to as a “Heat Pump”.   A popular concept of a heat pump is an appliance that cools in the summer and heats in the winter. In summertime heat is pumped from the cooler indoors to the warmer outdoors, and in the winter time heat is pumped from the cooler outdoors to the warmer indoors. (Think of pumping water up hill, energy is required to power a device to lift the water). 

Air conditioners and refrigerators are appliances that are technically heat pumps.

In order to heat and cool the same area a four-way valve is required to reverse the flow of refrigerant through the heat exchangers.

Any technology that moves heat from a warmer area to a cooler area is referred to as a heat exchanger. (Think of water flowing downhill, no energy is required to flow water down hill, just a tube to contain the water)  Heat exchangers create larger surfaces areas to minimize the resistance to heat flow.  (Just like water flowing in a pipe, the larger the pipe the better the water flows, smaller pipes require higher pressure and smaller heat exchangers require higher temperature)  Heat exchangers use air, water, and other fluids to transport the heat from one area to another or as an efficient means of spreading the heat over larger surface areas for dissipation.  A car engine is a good example, it uses water/glycol to remove heat from the cylinder block then transport the heat to the radiator.  The radiator uses the water/glycol to spread the heat over a large surface area so the air flowing through the radiator can efficiently pick up the heat and carry it away.  Even though a lot of energy is used on the engine’s water pump and fan to move both the water/glycol and air, there is no heat pump or refrigeration effect; heat is flowing from warm to cold.

A heat pipe is also a heat exchanger.

Heat sink and heat exchanger describe the same process, transferring heat from a warmer area to a cooler area.  The term heat sink is more popular describing an all-metal heat exchanger used to absorb and conduct heat without the use of fluids. Fans are sometimes used to convect (disburse) heat off the heat exchanger.

Heat is a form of energy just like electricity, solar (light), and wind (pressure).  Heat can be converted into electricity, light, and pressure (force).  Heat describes the amount of energy and is popularly expressed in BTU or Calories.  Temperature describes the “quality” or “level” of the heat.  One Btu is defined as the energy required to raise one pound of water one degree Fahrenheit. Consider this analogy, heat is analogous to a volume of water, temperature is analogous to the water pressure, and water flow rate, gallons per hour is analogous to heat flow rate, BTUs per hour.  In effect a water pump is similar to a heat pump. To pump a larger volume of heat or water, more energy is required, pumping water against higher pressures and pumping heat against higher temperatures require more energy.

Cooling and heating can also be accomplished by absorbing heat through a chemical process.  Ice is use to cool a beverage or food by absorbing heat as it melts (a solid absorbs heat to become a liquid).  A liquid also absorbs heat to become a gas.  The reverse is also true; to convert gas to liquid will release heat, and liquid to solid releases heat.  Changing the phases from liquid to vapor and from vapor to liquid is how your typical heat pumps operate (vapor/compression technology). 

 

To sum up:

  1. Energy is required to power a device so that heat can be “pumped” from a cold area to a warm area.
  2. To cool a room (air condition) or a beverage (refrigeration) requires that you remove heat or absorb it.  Conversely to heat something requires that you add heat or energy.
  3. Energy is not required to move heat from a warm area to a cold area.  Energy can be used to help the heat transfer process by powering fans and/or water pumps to assist in the conduction and convection of heat.
  4. Ice can be used as a cooling process but it takes energy to produce the ice (there is no free ride).
  5. Technically cold does not escape your refrigerator, heat gets in.
  6. An electrical or fuel heater is not a heat pump because its not pumping heat.  Heat is created through energy conversion or chemical reaction.

A thermoelectric device is created when two dissimilar materials form a junction. If there is a temperature difference between the two dissimilar materials, electricity will flow (thermocouple effect); conversely if a voltage is applied, heat will flow from one end of the junction to the other, resulting in one side becoming colder and the other side warmer (Peltier effect and electron hole theory).

A unique feature is that by simply reversing the polarity (reversing the plug), you can reverse the flow of heat, thereby changing from a refrigerating mode to an efficient heating mode.  For example, a power input of 24 watts can give you 59 watts of heating.  These modules will operate on a low voltage, high current DC source.  Through the use of a simple power supply, the modules can operate on any AC source.        

There are no moving parts that will wear or break, nor gases or corrosive liquids that can leak.  Thermoelectric modules offer a very reliable means of refrigeration for very small heat loads, however efficiency in the refrigeration mode is very low.  Thermoelectric is not appropriate for solar cooling applications.     

 

 

 

Vapor / Compression System

Advantages

  1. Capable of large refrigerating loads at lower initial purchase and operating cost.

  2. Very efficient

  3. Very compact system for small to very large heat loads

  4. Cycle can be reversed for heat pump operation

Disadvantages

  1. Parts can wear out 

  2. Noise

  3. Potential refrigerant leaks

  4. Operates in limited orientations 

 

Thermoelectric Refrigeration

Advantages

  1. Light weight and compact for very small heat loads.

  2. No moving parts, eliminating vibration, noise, and problems of wear.

  3. Reversing the direction of current transforms the cooling unit into a heater.

  4. Operates in any orientation.  Not affected by gravity or vibration

  5. Very low cost device for cooling in small appliances

  6. Precision temperature control capability

Disadvantages

  1. Limited to very small refrigeration loads due to cost.

  2. Not suitable for solar due to poor efficiency

  3. Heat sinks required to conduct heat to and from the thermoelectric modules become very heavy and bulky as the refrigeration capacity increases.

 

 

Absorption System

Advantages

  1. No moving parts

  2. No vibration or noise on small systems

  3. Small systems can operate without electricity using only heat, large systems require power for chemical pumps

  4. Can make use of waste heat

Disadvantages

  1. Potential refrigerant leaks

  2. Operates under limited vibration and orientations

  3. Complicated and difficult to service and repair.

  4. Stalls in hot ambient.

  5. Very bulky

  6. Poor efficiency


 

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Revised: December 13, 1999 .