FSP0007 – Refrigerant – Facility Science Podcast #7

By | June 11, 2019

 

Notes for FSP0007 – Refrigerant
What is refrigerant
A refrigerant is a substance (sometimes a mixture of substances) that is used in a refrigerant cycle to move heat from one place to another. A substance that we use as refrigerant has certain properties that make it suitable for this use which I will get to… And, while there are multiple classes of substance that might be considered refrigerant under certain circumstances, here I’m specifically talking about a refrigerant cycle in which the refrigerant undergoes phase change to facilitate the heat transfer. This is widely used for, cooling/heating, air conditioning, cold storage, dehumidification.
Motivation
  • So first, to motivate this, imagine that you have a building in a very cold place covered with ice and snow. Inside the building you have a workshop that creates an unbearable amount of heat. You would like to take advantage of the cold outside to cool down your workshop. One way you could do this is by taking some tubing (hose), running it all around inside your building then running it outside and coiling it all around in the ice and snow, so you have a long loop of hose that is half inside and half outside. You could then pump water through the hose. When the water is in the part of the hose that is inside, it will heat up, then when the hot water reaches the part of the hose that is outside, it will cool off. Since there is effectively infinity “cold” outside relative to the heat that can be generated inside, the temperature inside will cool down. In this case you are using the water inside the hose to transport the heat outside.
  • The problem with the story I just told you is that, in real life, when we want to make the inside of something colder ((or move heat out of something) like a building or a refrigerator), it’s usually hotter on the outside that on the inside (So we want to make the refrigerator colder than than room it’s in, but we have to move the heat from inside the refrigerator into the room.  In this case,  pumping the water through the hose to move the heat from a hot place to a cold place isn’t going to do the trick. So we use what is called a refrigerant cycle, which requires refrigerant, combined with some energy in the form of mechanical work to move heat from cold place to a hot place.
  • Based on the story I told before, we really want a magical kind of substance that is colder than the air inside so the inside air can heat it up, but then is somehow hotter than the air outside so that the outside air can cool it down.
  • Unfortunately, no such substance exists, so we have to be clever.

Thermodynamics

  • Latent heat – in this context is the heat required for a phase change (say from liquid to (vapor) gas…i’m going to use both vapor and gas….they are technically slightly different things but the difference doesn’t really matter here, so i’ll probably use them interchangeably). I’ll use water as an example. If you add heat to water, the temperature of the water will increase to the boiling point. Once the water reaches its boiling point, it will start changing phase from liquid to gas (it will boil – steam). If you were able to capture all of the steam so none of the heat is able to escape the system, and you continued to add heat to the water/steam mixture, the temperature would not continue to increase until all of the liquid water turned to steam. When you had only steam, and no more water, the temperature of the steam would start to increase. That extra heat that goes into the phase change, but that doesn’t increase the temperature is called latent heat. If you remove heat from the steam, the same thing happens. The temperature of the steam will decrease until the condensation temperature (that’s the temperature where the steam turns into liquid water). At the condensation temperature, continued removal of energy will not cause a decrease in temperature, but will instead cause the phase change. When all the water is liquid, the temperature will start decreasing again. Side note, in contrast to latent heat, the name for the heat that results in a change in temperature is called sensible heat.
  • boiling/condensing point depends on pressure. You might know that water boils at 100 deg C (or 212 F). You might also know that that’s only true at sea level or 1 atmosphere of pressure. IF you have ever lived at a high altitude you might know that water boils at a lower temperature at high altitude because there is less pressure from the atmosphere holding the water vapor down in the container. So, if you can control the pressure being exerted on a substance, you can alter the boiling point of the substance
  • Under controlled circumstance, increasing pressure of a gas or liquid increases temperature, decreasing pressure of a gas or liquid decreases temperature. 

Refrigerant Cycle

refrigerant cycle – For simplicity, I describe the refrigerant cycle in a typical air conditioning scenario where we want to cool the inside and put the heat outside. In general, in this scenario, it is hotter outside than it is inside.
  • 4 components in the cycle – compressor, condenser, expansion/metering device, evaporator
  • The cycle is a closed loop, so there is no real “starting point.” The refrigerant goes around and around, so I’ll start at an arbitrary place.
  • step 1: the refrigerant as vapor (gas) enters the compressor. The compressor compresses the gaseous refrigerant, increasing its pressure and consequently its temperature (see thermodynamics concepts). Our refrigerant is now a hot, high pressure gas.
  • Step 2: the hot, high pressure gas enters the condenser. The system is set up so that  by traveling through the condenser, the refrigerant will change from a hot, high pressure gas to a hot high pressure liquid (in other words, the refrigerant condenses in the condenser…which is why it is called a condenser). So the condenser removes the latent heat from the refrigerant (and any additional heat that went into increasing the temperature above the boiling point). In the typical air conditioning scenario, the condenser is outside and the refrigerant has been heated by compression to be much hotter than the air outside. The condenser is a coil of copper pipe arranged to maximize surface area, and a fan that blows air across the coil. Since the air is cooler than the refrigerant (by design), the air removes heat from the refrigerant and takes it away. Air cooled condensers are typical, but using water to cool the condenser coil in order to remove heat is also common.
  • Step 3: The hot, high pressure liquid hits the expansion valve (more generally, metering device…there are several types) which presents a resistance to the flow of refrigerant. In the simplest case you can think of this as just a spring loaded valve. The refrigerant has to force its way through the expansion valve which means there will be a lower pressure refrigerant on the other side of the metering device. The drop in pressure causes a decrease in temperature, so in the  section of the loop beyond the expansion device, we have a cold liquid (or more likely a mixture of cold liquid and vapor, so the refrigerant is near the boiling point at the new temperature).
  • Step 4: The cold liquid enters the evaporator. While traveling through the evaporator, the refrigerant heats up and (surprise, surprise) evaporates. So, in this step, the refrigerant takes on the latent heat of vaporization and then possibly heats up a bit past the boiling point. The cool vapor leaves the evaporator and continues toward the compressor to restart the cycle. Again, in the typical air conditioning situation, the evaporator is a coil of copper tubing and a fan that blows the air inside the the building across the coil. Heat from the air is transferred to the refrigerant causing the air to cool down.
  • A note about the compression step and expansion step:
    • In the compression step we do work on the refrigerant to increase its pressure. We’re adding energy to the system with the compressor.
    • In the expansion step the refrigerant does work on the metering device to decrease its pressure (for example by pushing against the spring).
    • Ideally, the energy added to the refrigerant by the compressor and the energy removed from the refrigerant by the expansion valve are equivalent, which means that the heat removed from the refrigerant in the condenser is equivalent to the heat added to the refrigerant by the evaporator. Ideally. So the compression step and the expansion step undo each other. They are there simply to change the boiling point of the refrigerant to suit our needs by manipulating the pressure of the refrigerant.
  • Two more related terms – superheat + subcool
    • superheat – The refrigerant absorbs heat when it travels through the evaporator. The main function of the evaporator is to convert the refrigerant from a liquid to a gas, the heat that causes the conversion, the latent heat, doesn’t increase the temperature of the refrigerant (like I said before). Any additional heat the refrigerant picks up, beyond the latent heat will increase the temperature of the refrigerant vapor. The temperature increase above the boiling point (above the latent heat) is called superheat and is measured in degrees. So, if the refrigerant leaves the evaporator at a temperature 10 degrees above the boiling point, we say that the there is 10 degrees of superheat. Some amount of superheat is desirable as it ensures that no liquid refrigerant will enter the compressor (liquid refrigerant in the compressor is a bad thing). The wrong amount of superheat indicates a problem with the system. The proper amount depends on the exact parameters of the system.
    • subcool – This is similar to superheat except that it references the heat removed from the refrigerant in the condensor. In the condensor, heat is removed from the refrigerant, the latent heat is removed during the phase change from gas to liquid. Any removal of heat beyond the latent heat goes to reducing the temperature of the liquid refrigerant. This is called subcooling, and like superheat, subcool is measured in degrees. So if the refrigerant leaves the condensor 10 degrees colder than the condensing temperature, we say there is 10 degrees of subcool. Just like with superheat, there is a “right amount” of subcool for any system.
  • Manifold gauges – gauges with hoses hanging off – one of the primary tools used by refrigeration service personnel is the manifold gauge set.
    • Allows measurement of refrigerant pressure in the refrigerant loop. Normally, there are 2 different places to connect the gauges to the system
      • “Suction line” is the section between the evaporator and the compressor. The refrigerant is a low pressure gas in this section. The pressure is relatively low here because the compressor is pushing gas from this section into the higher pressure condenser section. It’s called the “suction line” because the low pressure “pulls” refrigerant through the evaporator.
      • “Liquid line” is the section between the condenser and the metering device. The refrigerant is a warm liquid in this section.
    • In a closed system of fixed volume (like the typical refrigerant loop), the pressure and the temperature of a substance are directly related. In other words, knowing one tells you the other which means the pressure gauges tell the refrigerant technician the temperature of the refrigerant as well as the pressure.
    • The information provided by the gauges, along with other supporting observations, allows the technician to determine if there is enough refrigerant in the system, or if there is too much, or if there is the right amount of heat exchange in the evaporator or the compressor.
  • multistage refrigeration
    • A typical refrigeration system like I have been describing can only create so much temperature difference between the evaporator and the condenser. When the temperature difference gets too great, the pressure differential becomes unmanageable.
    • Some industrial processes or scientific or medical applications require evaporator temperatures much colder than a typical refrigerant cycle can manage.
    • The solution is to use multiple refrigeration stages. Depending on the required temperature, this can mean adding an additional compressor along with some other gas and liquid management mechanisms in the loop or it can mean using multiple separate refrigeration loops. In the case of multiple loops, you would have one loop cool the refrigerant in a second loop while the second loop cools the space that you’re trying to cool. This can be nested several stages deep.
  • Use this to heat instead of cool – heat pump cycle
    • Put the condenser in the place you want to heat up, same exact cycle.
    • Many systems are designed to be reversible. they have a mechanism to reverse the flow of refrigerant so that the evaporator becomes the condenser and the condenser become the evaporator.
Types of refrigerant, Regulations, Environment
  • properties of refrigerant, now that we know how it works, we can decide what are desirable properties in a refrigerant.
    • thermodynamic properties
      • high heat of vaporization (that’s the latent heat associated with the liquid to gas phase change).
      • Boiling point in the desired temperature range. We need the refrigerant to boil in the evaporator and condense in the condenser so we need a substance that can be made to do so at reasonable pressures.
    • Not corrosive to the system components – copper tubing, compressor, seal materials, etc.
    • Non-flammable (though some substances used as refrigerants or proposed to be used are flammable)
    • Not harmful to the environment – There have been concerns about the environmental effects (ozone layer depletion and global warming potential) of some refrigerants leading to regulation of these substances.
  • Types of refrigerants – Many types of substances used as refrigerant.
    • R-notation – refrigeration types are often referred to using R-notation. This is an R followed by some numbers and possible some letters. Originally, the numbers referred to the structure of the refrigerant molecules. The numbers indicate the number of carbon, hydrogen, and fluorine atoms while the letters indicate particular atom arrangements or other added elements. The system has since been expanded to include other refrigerants that don’t conform to that coding system. Examples you might have heard of are R-12, R-22, R-134a or  R-410a
    • The right type of refrigerant to use in a particular piece of equipment is the refrigerant it was designed for.
    • Some substance used as refrigerant have been determined to have unreasonably high environmental impact resulting in regulation of these substances. Notably CFC and HCFC refrigerants that are widely used have been almost completely phased out of production in many countries  largely due to their negative effect on the ozone layer. This phase out was initiated by the Montreal Protocol, an international treaty that was initially agreed to in 1987.
    • An example of an extremely common HCFC refrigerant you might have heard of is R-22, commonly know by the brand name Freon-22. This refrigerant and others like it are still used in a lot of equipment all over the world, although use of the refrigerant has been increasingly restricted since the Montreal Protocol.
    • covers “What supermarkets and property and facility managers need to know”
    • Section 608 of the clean air act prohibits knowing release of refrigerant…
    • Mainly concerned with so-called ODS (Ozone depleting substances), which are CFC, HCFC and HFC refrigerants.
    • Gives an overview of requirements to document and repair refrigerant leaks
    • Indicates types of records that have to be kept of the service and replacement of this equipment.

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