HVAC Refrigeration 101 Part 2

What is covered in Part 2 of “Refrigeration 101 - An Introduction to Refrigeration”:

  • Pressure as Related to Temperature
  • Boyle’s Law
  • Charles’s Law
  • Dalton’s Law
  • The Process of Refrigeration
  • The Refrigeration Cycle
  • The Refrigerant Charge Brief Summary
  • The Four Main Components of the Refrigeration Cycle
  • Safety and Regulations

Part 1 can be found here

Pressure as Related to Temperature - HVAC Refrigeration 101 Part 2

As stated previously, pressure affects temperature. It is vital to understand this, especially in refrigeration, as in the typical refrigeration system, as designed, the pressures fluctuate, and both high and low pressure is used to give us an outcome in temperature to give us refrigeration. At sea level, the atmospheric pressure is 14.696 PSI or as measured with a barometer 29.92 inches of mercury.

When we change the pressure, we can change the temperature at which water “changes state” from a liquid to a vapor. The same is true for various refrigerants. Watch this video where a vacuum pump is used to reduce the pressure on a glass of water. See what happens to the water when the pressure is drastically decreased.

Now you can see by manipulating the pressure, the boiling point of the water is changed to a lower temperature. In fact, in the video, there was no heat source whatsoever. The water boiled at room temperature. In refrigeration, a similar method is used to boil refrigerants. In refrigeration, as the refrigerant boils, it is absorbing energy or heat.

It is essential to understand this theory as you will be required, as a technician, to make adjustments to the refrigerant charge in a refrigeration system such as a refrigeration unit, air conditioner, or heat pump system. These principles will help you in troubleshooting problems and providing the condensing unit(s) and evaporator coil with the initial refrigerant charge when they are first installed.

When checking the system or adjusting the charge, you will use compound gauges that will give you the pressures of both the high side (head pressure) of the system and the low side (suction pressure) of the system. Aside from the newer state-of-the-art digital gauges, the older reliable gauges use a bourbon tube (as illustrated in the textbook) to measure the pressure in the refrigeration system for both the high side and the low side of the system.

Basic Physics and Gas Laws - HVAC Refrigeration 101 Part 2

In your textbook study and learn the basics of Matter, Mass and Weight, Density, Specific Gravity, and Specific Volume. Plus, you never know if they will be on a test question, so make sure you prepare yourself for these basic terms and principles. These basic terms and principles in physics are necessary to fully grasp as you move through this course and the future as you ply the trade of Refrigeration and HVAC.

Once you understand these terms of physics, it is time to move on to Gas Laws. Gas Laws are important to understand, especially in refrigeration, because we are dealing with gases and how these gases react to temperature and pressure in a hermetic system. Learning these terms will help grasp a deeper understanding of the refrigeration process and the nuts and bolts of the refrigeration cycle. As you progress in learning refrigeration, it will all come together as you apply these principles to the trade.

Boyle’s Law - HVAC Refrigeration 101 Part 2

Boyles Law

Boyle’s Law states that the pressure of a gas tends to increase as the volume of a gas decreases. The law was named after chemist and physicist Robert Boyle, who published the original law in 1662 after experimenting with air.

The law itself can be stated as follows:

For a fixed amount of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional.

Or Boyle’s law is a gas law, stating that the pressure and volume of a gas have an inverse relationship when the temperature is held constant. If volume increases, then pressure decreases and vice versa, when the temperature is held constant.

Using math and remembering that the temperature is constant we can see this law applied here:

Pressure x Volume = A constant number.

This is applied to refrigeration because during the refrigeration process the refrigerant in vapor form (liquids cannot be compressed) is compressed and it is important to understand the behavior of gases (in this case refrigerants) and how the refrigerants behave under certain conditions.

You can see from the animation that as the volume decreases the pressure increases and vice versa when the pressure decreases the volume increases. This is true as long as the temperature remains constant and the gas is enclosed in a closed system.

In the real world for an HVAC technician it is important to understand Boyle’s law because you will be working with gases (vapor) in the form of refrigerants. Understanding the behavior of gases is important to properly do your job. It does require critical thinking skills so make sure to study these laws and completely understand them.

Charles’s Law - HVAC Refrigeration 101 Part 2

In refrigeration, we are adding heat to the gas in the compression cycle as a result of mechanical energy and friction. That is a part of the heat produced by the electric motor that powers most hermetically sealed compressors. Therefore, Boyle’s Law is only relevant when we combine it with the second Gas Law. Charles’s Law is the second of gas laws after the first which is Boyle’s Law described above. The law was named after scientist Jacques Charles, who formulated the original law in his unpublished work from the 1780s. Simply put the law states that:

The volume of gas will increase with a temperature increase and decrease with a temperature decrease.

Charles law

A good example of this is the tire pressure in your car. In the winter the tires will have less volume because air has less volume. And those with low tire pressure indicators will have the indicator come on in the winter because of this law. The temperature outside is cold so the air has less volume.

Another example is a gas can left out in the sun on a hot day. As the sun heats the can the gasoline vapor expands and the safety vent, if closed, will pop open relieving the excess pressure in the can. If the can of gasoline is stored in a cooler environment such as a shed or basement and the pressure relief safety vent is left closed the volume of vapor will decrease as the can cool.

This often results in the can caving in a little. In refrigeration Charles’s Law is important to understand because the refrigeration process involves adding heat to refrigerant and taking heat away from the refrigerant. This means the refrigerant vapor will expand and contract depending on the amount of heat added or taken away. These examples are proven by Charles’s Law.

Dalton’s Law - HVAC Refrigeration 101 Part 2

Dalton’s Law is applicable to refrigeration because many of the refrigerants are mixtures of various chemicals in liquid and vapor form and it is possible for a refrigeration system, if not properly installed, can have other inert gases in the hermetic system other than the refrigerant which is very bad for the system.

I say that because I have run across systems installed by people who really did not know what they were doing and in the end, they gave in and called the company I worked for to solve the problem and correctly finish the job for them. You can nearly buy anything on the internet these days and some people purchase their own equipment over the internet and trying to save a dime install their own equipment. While I do not doubt some of these people are successful,

I have run across too many instances where these people got into trouble and threw in the towel and finally called a professional HVACR company to remedy their problem. Some people are honest about it, and some are not. They let you figure it out. In other occurrences, it is painfully apparent they (whoever was installing the system) did not know what they were doing and gave your company a call to finish it properly.

Understanding these principles will help you solve problems quickly and efficiently when face with a similar issue. Dalton’s Law is one of these principles.

Dalton’s law (also called Dalton’s law of partial pressures) states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

The textbook reference also gives an excellent example of Dalton’s Law by explaining it using nitrogen and oxygen. Make sure to read it and understand it because it is a possible test question.

The Process of Refrigeration - HVAC Refrigeration 101 Part 2

Refrigeration is nearly a necessity in modern times as it is needed to keep food fresh and people comfortable, especially in large multi-story buildings where the windows cannot be opened. In the old days, people used rudimentary refrigeration to keep food fresh without any comfort cooling for hot summer days.

Air conditioning for comfort in the old days was opening the windows to allow a fresh breeze to cool you down. As time and technology progressed, mechanical refrigeration systems came into existence to keep the food fresh, and eventually comfort refrigeration systems were invented and improved on enough so that enclosed buildings could be designed.

Today, refrigeration is a much-needed necessity for every household, industry, and commercial buildings. From the refrigerator to the air conditioner to processes for industrial refrigeration is a part of modern times that has become a necessity.

We all know the definition of refrigeration, as defined in the first module of this course. We also learned about heat and how heat behaves in various substances and in nature. Now we come to the part of the course where we learn the refrigeration cycle and how we technically move the heat to make temperatures ideal according to the set point of the desired temperature.

In the refrigeration process, the refrigeration circuit can be broken down into two parts, the high side, and the low side. On the high side, it is classified as high temperature and high pressure while the low side is classified as low temperature and low pressure. A detailed lesson on the components of the refrigeration cycle and how they work will be done in a later segment of this course. For now, focus on the process. Additionally, for the description of the process of refrigeration, this will be a description of a basic air conditioning system for a house.

Refrigeration High and Low Side

The Refrigeration Cycle - Metering Device - HVAC Refrigeration 101 Part 2

We can begin to describe the process anywhere in the refrigeration circuit, but for this course, we will begin at the metering device. The metering device receives liquid refrigerant from the condenser. As the refrigerant goes through the metering device, which restricts the flow, the pressure of the refrigerant drops as does the temperature. The metering device is best described “like” the nozzle on a water hose.

There is plenty of pressure in the hose itself, but the nozzle restricts the flow of water based on the manual setting of whoever is using the hose and the flow they desire. The metering device does much the same except it is specifically engineered for that particular refrigeration system, whether it is a refrigerator or an air conditioner for a house. The refrigerant is still a liquid aside from some flash vapor that changes as it passes through the metering device.

The Refrigeration Cycle - Evaporator Coil - HVAC Refrigeration 101 Part 2

air conditioner evaporator coilThe evaporator coil, located in the air handler or ductwork, has air passing over it from the blower. The evaporator coil receives the liquid (and flash gas) from the metering device. The air passing over the coil contains moisture and heat, while the evaporator coil has cold refrigerant inside it that came from the metering device. We know from Lesson One in this refrigeration course that heat is always attracted to cold.

We also know that air has moisture and will condense when it is in an environment where the dew point is low. So the cold evaporator coil is doing two things: 1) It is absorbing heat from the air 2), and it is condensing the moisture from the air or changing water vapor to a liquid.

The refrigerant moves through the evaporator coil absorbing the heat from the air. As the refrigerant absorbs heat, it begins to boil. As the refrigerant boils, it is changing state from a liquid to a vapor. By the end of the process, provided everything is working well, all of the refrigerants should be vapor-containing heat absorbed from the air.

The saturated refrigerant travels out of the evaporator and into the suction line on its way to the compressor. The refrigerant should be above its saturation temperature. Saturation of refrigerant in the evaporator means all of the liquid has turned to vapor. The vapor will continue to absorb more heat. The heat absorbed above the saturation temperature is referred to as superheating. For example, we all know that water boils at 212° F. at sea level.

Once the water boils away, and it is a complete vapor, we can still add heat to the steam. So any temperature above 212° F. with steam is considered “superheat”. So if the steam was 220° F. then you would have 8° F. of superheat in the steam. The same occurs in refrigeration when the refrigerant is in the evaporator coil, in the suction line, and at the compressor. More heat is added to the refrigerant (being absorbed) and the heat above the saturation point is called superheat. Saturation can only occur with 100% vapor therefore superheat will not be added to the vapor until complete saturation has occurred.

The Refrigeration Cycle - The Compressor - HVAC Refrigeration 101 Part 2

HVAC Rotary Compressors

Inside View of a Rotary Compressor

The refrigerant leaves the suction and is compressed in the compressor. Going back to a previous lesson, Temperature as Related to Pressure (from the previous module), we know that if we change the pressure we can also change the temperature. We observed in the video where the pressure was lowered and water was boiled at room temperature.

In this case, we are raising the pressure to do the opposite. Instead of raising the pressure to boil the refrigerant (which, at this point is already a vapor), we are raising the pressure to increase the temperature so that we can condense the refrigerant and change it back to a liquid.

That happens later in the condenser but as the refrigerant goes through the compressor the pressure is increased. Depending on the type of system and the type of refrigerant will depend on the pressure so nearly every system is different based on those factors.

As the refrigerant is compressed in the compressor and pressure rises so does the temperature. This is important to note because the temperature of the refrigerant needs to be higher than the outside air temperature after it leaves the compressor otherwise we will not condense the vapor back to a liquid. As the refrigerant is in the compressor it mixes with some of the oil needed to lubricate the compressor.

This oil will migrate throughout the refrigerant circuit and eventually make its way back to the compressor. Everything is working well and the vapor refrigerant has been compressed and the temperature and pressure raised as designed. Now the high pressure, the high-temperature refrigerant in vapor form leaves the compressor via the discharge line on its way to the condenser coils.

The Refrigeration Cycle - The Condenser - HVAC Refrigeration 101 Part 2

heat pump condensing unitThe condenser receives the refrigerant from the discharge line of the compressor. The refrigerant is a superheated vapor. The condenser fan is running and pulling air through the condenser coils. Let us say it is a 90° F. day outside so the air conditioner is running. Let’s say the vapor is 100° F. That is a difference of 10° F.

So as the vapor refrigerant moves through the condenser inside of the coils and the air is pulled through the outside of the coils a heat exchange process takes place and 10°F. of heat is rejected from the refrigerant in the condenser.

As the heat is rejected the vapor refrigerant begins to condense. As it condenses it begins to change state. It is like changing steam or water vapor back to liquid form.

The same happens to the vapor refrigerant except the pressure is much higher than atmospheric. Think of the video from Lesson One where water was boiled at room temperature.

That happened because the pressure was far below atmospheric pressure. When we raise or lower the pressure we change the temperature at which a substance changes state. The vapor refrigerant is now changing to a liquid refrigerant using this same process except at a higher pressure than atmospheric pressure.

The whole thing happened because of pressure and temperature. In the condenser, the pressure was high as was the temperature. The temperature in the outside air was cooler than the temperature of the vapor refrigerant and when the heat was removed from the outside air the vapor changed to a liquid. The heat that was absorbed in the evaporator is now rejected to the outside air. Once the refrigerant has completely changed to a liquid that is referred to as the saturation temperature.

Any other heat lost from the refrigerant is considered subcooling. In other words, if the saturation temperature is 90° F. and the refrigerant is cooled to 85° F. by the time it gets to the metering device the 5° F difference is considered subcooling. The liquid refrigerant leaves the condenser and enters the liquid line and is delivered to the metering device where the process begins all over again.

The Refrigerant Charge Brief Summary - HVAC Refrigeration 101 Part 2

r-410-a refrigerant charging chart

Before checking any charge in a refrigeration system, it is important to check the airflow. A dirty air filter or anything that causes restricted airflow will give you inaccurate readings and an improper charge for the refrigeration system. The creed of an HVAC tech before hooking up the refrigeration gauges should be airflow, airflow, airflow.

Whenever a technician is checking the charge on any refrigeration system the temperatures mentioned in this lesson are critically important to get right. An accurate superheat or subcool temperature is essential to making sure you have the proper charge. As mentioned before in a previous lesson, it is important that you have accurate, calibrated tools especially temperature measurement tools. To properly check a refrigerant charge in any refrigeration system you will need to know the pressures and temperatures of the evaporator and the condenser, or the high side and low side pressures and temperatures.

Many manufacturers offer a charging chart to make sure you get the charge correct. These charts are accurate as long as the temperatures and pressures you read are accurate. Other manufacturers offer other types of charts based on the high/low pressures and the superheat and subcooling temperatures. If you do not get the refrigerant charge correct problems can develop and the unit performance and efficiency will be affected.

There are a few ways to charge a system where there is no manufacturer’s chart available to make sure you have the correct charge. Probably the most reliable way to charge a system is to do a weigh-in where you weigh in the amount of refrigerant using a scale. This weight is usually on the nameplate of the condenser where the model number and serial number are located. The weigh-in is not always the most practical way to charge a system as some refrigeration systems have variables such as a long line set (over 25 feet) between the condenser and the evaporator.

The weigh-in is only practical when all of the “refrigerant” has been recovered and the system has been properly evacuated. Again, if you ever use this method make sure your scale is calibrated properly. This method always works best on package units where the amount of refrigerant in weight is specified on the manufacturer’s nomenclature label on the side of the unit. The same label where the model number and serial number are located. This label will have helpful technical data including electrical information and refrigerant information including the type of refrigerant.

Other methods for charging a system include the subcool or superheat methods and whether you use the subcooling method or superheat method will depend on the type of metering device that is used in the system. You cannot get an accurate charge on the system using the superheat method when the system is equipped with a TXV metering device (TXV - Thermostatic Expansion Valve). Other factors involved in superheat and subcool methods are collecting the temperature data for outside air temperature, indoor air temperature in both dry bulb and wet bulb temperatures.

Using a sling psychrometer or a digital psychrometer to get the wet bulb and dry bulb temperatures. This tool will tell you how much humidity is in the air and is needed for the charts. The dry bulb temperature is the temperature of the air as indicated on any thermometer while the wet bulb temperature will give you the temperature at which the moisture in the air will evaporate and change state to a liquid.

Essentially, when you get a wet-bulb temperature you are measuring the amount of latent heat that is contained in the air. Charging a system correctly requires knowledge of how the refrigeration system works along with several tools including some charts to give you accurate results of charging the system.

It is not in the scope of this course to teach these methods of charging. That will be done in an advanced refrigeration course later. We simply describe these methods here to familiarize you with the charging methods used in refrigeration. It is important that you gain some experience in the field before applying yourself to charging a system. The reasoning is because you need to know what to look for if and when there are problems with the system. A problem with the system needs to be corrected before charging can be successful. That comes with experience and a solid knowledge base of refrigeration and how it works including principles and theory.

The Four Main Components of the Refrigeration Cycle - HVAC Refrigeration 101 Part 2

As noted above the metering is a necessary component in the refrigeration cycle. Without a metering device to meter the refrigerant the refrigeration cycle will not work. There are several different types of metering devices but only two are primarily used in the average air conditioner and heat pump systems.

The least common types of metering devices a technician will encounter while working in residential or light commercial HVAC systems are the electronic expansion valve, automatic expansion valve, and float-type metering devices. Since this is a basic course those types will not be covered here. Instead, we will focus on the most common types of metering devices you will encounter when working on the typical HVAC system for residential and light commercial systems.

As a service technician or installation technician, you will be required to install and service these metering devices at times. This includes troubleshooting metering devices if a problem arises. So as an HVAC technician you will need to know exactly how these devices function in the refrigeration cycle and how to make proper decisions in their operation in the system. To be able to identify the problem if and when a problem arises you need to know the functionality of the device including when it is working properly and when it is not working properly.

Types of Metering Devices

Fixed Bore or Piston Type Devices - In many heat pumps and air conditioning systems you will find the fixed bore or the capillary tube metering device. Some refer to the fixed bore metering device as a piston-type metering device. Essentially, it is a small round piece of metal with a precisely drilled hole in the middle of it. In a new air conditioner (before installation for a split system) the piece of metal will typically be in a small plastic or paper bag and it will be the exact size needed for the tonnage of the air conditioner or heat pump system.

This piece of metal will fit into a brass fitting that is threaded so when the installation technician is finished brazing the copper line set the piston or fixed bore will be inserted into the brass and then tightened so no refrigerant will leak out of the system. In package air conditioners or heat pumps, the fixed bore or piston is already installed in the system.

For either the split system type of air conditioner/heat pump or the package systems that when the piston or fixed bore is installed that everything is very clean with no dirt or debris in the refrigerant lines. It is also important to make sure that once installed, refrigerant does not leak from any part of the system including where the metering device is located. Best practices when working with the refrigerant lines will be covered in a later module and it is important to follow the best practices to prevent problems such as contaminating the refrigerant or getting foreign matter in the metering device or filter driers.

Frozen Evaporator Coils Troubleshooting

Frozen Coil

Capillary Tube Metering Device - Commonly referred to as cap tubes capillary tube metering device is simply a very small copper tube the engineer of the systems matched up to the tonnage of the air conditioner or heat pump system. Coming from the condensing unit the refrigerant runs through the liquid line on its way to the metering device. In many instances where the cap tube metering device is used the liquid line will run into a distributor and coming from the distributor will be the very small copper lines that are the capillary tube metering device.

The small-cap tubes will run from the distributor to the evaporator coil where the refrigerant absorbs heat. The capillary tubes meter a precise engineered amount of refrigerant to the evaporator coil according to the size or tonnage of the system.

Capillary tubes are pre-installed at the factory so there really is no advice given for installation. Capillary tubes, like any metering device, can be plugged up with foreign matter if care is not taken and best practices not used during the installation process for split systems or for repairs to other types of HVAC systems with cap tube metering devices. Both the cap tube and the fixed bore piston metering devices can be successfully charged using the superheat method of charging the air conditioner or heat pump system.

Thermostatic Expansion Device or TXV

Thermostatic Expansion Valve PHT

TXV’s or Thermostatic Expansion Valves are commonly used in heating and cooling systems including air conditioners and heat pumps. A TXV modulates a piston based on the “superheat” of the refrigerant. In this way, a TXV can precisely maintain the flow of refrigerant into the evaporator coil. For this reason, TXV’s are used on more efficient equipment or higher SEER air conditioners and heat pumps. The bulb in the graphic above is attached to the suction line leaving the evaporator coil.

proper position of a TXV

The bulb has refrigerant inside it that reacts to heat or in this case, the superheat in the refrigerant leaving the evaporator coil. The refrigerant gas inside the bulb expands and contracts to depend on the amount of heat it senses. This expansion and contraction cause the bellows in the TXV to move the piston regulating the amount of refrigerant it meters or allows into the evaporator coil.

More superheat and the TXV needle opens more allowing more refrigerant into the evaporator. Less superheat and the needle closes off some to limit the amount of refrigerant that gets into the evaporator coil. In this way, the evaporator coil can get a precisely controlled amount of refrigerant into the evaporator for increased efficiency based on demand.

Problems that can develop with TXV’s include hunting where the TXV will open and give too much refrigerant and then close off. All this is done in rapid succession until something changes and the TXV stops hunting. Additional problems include trash getting into the needle and restricting refrigerant flow.

This can be determined by reading the pressures in the system and inspecting the TXV. It will be common to have high head pressure and a low suction pressure with icing of the TXV. Troubleshooting refrigeration systems is not really in the scope of this course however I thought I would throw that in to give you some advanced pointers.

There are a few other types of metering devices you can learn about by reading further in the textbook you purchased for this course. One to pay attention to is the electronic metering device or electronic expansion valve. More systems are using the types of metering devices in residential and commercial systems.

Coils: Evaporators and Condensers - Copper and Aluminum Tubing Used in HVACR - HVAC Refrigeration 101 Part 2

spine-fin-aluminum coilsThere are a few manufacturers that have always had aluminum coils for their condensing units. Trane and American Standard use what they call a spine fin coil. These are all aluminum coils with spine-fins projecting from the tubing. What does this mean for an HVAC technician?

Actually, not really much unless you need to make a repair in the aluminum coil. All the bonding of aluminum to copper is done at the factory so connecting to the condenser, with an aluminum condenser coil, to a line set is not difficult making the connection through brazing techniques.

For the majority of manufacturers, their condenser coils are made of copper with aluminum fins. Why? We have to look at the conductivity of copper and aluminum to understand why copper is preferred over aluminum and factor in the cost of materials for the competitive HVAC industry. According to ASHRAE (American Society of Refrigerating and Air Conditioning Engineers), copper has 1.7 more times the heat conductivity of aluminum. This makes copper more efficient when we are talking about heat transfer.

Efficiency is really important for the HVAC system because of energy use. The more efficient the system is the less energy it uses. However, manufacturers use a cost-to-benefit ratio in their sales and marketing of all aluminum coils over the copper/aluminum hybrids.

A line set is the copper tubing that runs from the condensing unit to the evaporator coil which is located in either the air handler or a case installed in the ductwork. It is comprised of two copper tubes one larger than the other. The smaller tube is the liquid line and the larger tube is the suction line. Refer to the refrigeration cycle for further details and a diagram for these lines.

Repairing copper in the field is much easier for most technicians than repairing aluminum. It happens where a coil, either through accident or mechanical failure, or vibration, the coil will get a hole in it. In that case, the system suffers a catastrophic loss of refrigerant and stops heating (for a heat pump) or cooling. I’ve repaired multiple copper coils using brazing techniques but never repaired an aluminum coil, only replaced them.

There are kits you can purchase to repair aluminum but I cannot tell you how confident I am in those kits because I’ve never used them. Some of the problems I’ve found with aluminum coils are the bonding point from the aluminum to the copper. Those were the early days and manufacturers have improved their bonding techniques, however, if you are ever doing a refrigerant leak check the bonding point is always high on the list of first checks for leaks.

Types of Evaporator Coils

Evaporator coils come in different types and sizes depending on the capacity of the system. Evaporator coils are sized for the capacity of the condenser and are either integral to the air handler or in a case located in the ductwork or attached to the air handler or gas furnace. It is important to familiarize yourself with the different types of coils that can be found in HVAC for air conditioners and heat pumps.

Coils can be used in either an up-flow, down-flow, or horizontal flow air handler system. It is important to understand that any type of cooling coil is located on the positive airflow side of the air handler blower. The purpose is because the air leaving the coil is saturated air with lots of moisture. If the coil is located on the negative pressure side of the air handler/furnace the saturated air will cause the air handler/furnace to sweat. 
  • “A” Coil - the “A” Coil looks like an “A”
  • “N” Coil - the “N” Coil looks like an “N”
  • Slab Coil - the “Slab” Coil looks like a slab

N-Coil Evaporator Diagram

Further subcategories of evaporator coils can be broken down into cased coils and uncased coils. For an HVAC technician, it is very important to protect the evaporator coil. The first line of defense in protecting the evaporator coil is an air filter. If no air filter has been used in the system the evaporator will become the filter and will quickly clog up with dust, dirt, and debris. Either a dirty air filter or a clogged-up air filter will cause problems with the refrigeration part of the HVAC system whether it is a heat pump or an air conditioner.

Cased Evaporator A-Coil

As an HVAC technician, never forget to check the airflow before checking any of the refrigeration components. Remember, airflow, airflow, airflow should always be your first check before checking the refrigeration side. Speaking of that, never forget the supply vents of the duct system. Make sure they are all open and blowing air. Good airflow is key to a properly working refrigeration system in an air conditioner or heat pump system.

Condensing Units - HVAC Refrigeration 101 Part 2

For an HVAC Technician, the condensing unit is where the compressor, basic controls, and condenser coils are located. There are some condensing units that do not house the compressor but these systems are mainly commercial HVAC systems. This course is for basic theory and an intro to basic refrigeration. Baby steps before getting into commercial refrigeration.

Coils and coil materials are described above. An extra note about condenser coils is the coils in high-efficiency condensers can be double-layered coils. In other words, in order to increase the area for heat exchange, the coils are sandwiched together. This increases the heat exchange area and the efficiency of the condenser.

heat pump condensing unit

Compressors are described below. Compressors can be single-stage in basic systems to two-stage and modulating in high-efficiency units.

Condenser fan motors can be basic multi-speed permanent-split capacitor (PSC) fan motors to ECM condenser fan motors to condenser fan motors designed to modulate.

  • PSC motors are the most common and typically use a single-speed even though the motor may be multi-speed. Capacitors are typically dual capacitors that serve the compressor and the condenser fan motor.
  • ECM condenser fan motors can be single-stage, two-stage, or modulate depending on the ECM controls and staging of the condenser.
  • Modulating condenser fan motors use inverter control to modulate the condenser fan motor with the compressor.

Basic condenser controls will depend on the type of HVAC system you are working with. Heat pumps and air conditioners work off the same principle of refrigeration and are similar however they both have controls that differ. The similar controls in a condenser for both types of systems can include:

  • A compressor contactor -the compressor contactor is the basic switching mechanism that turns the system on and off. It is controlled by the thermostat inside the home. The compressor contactor (normally open) feeds power to the compressor and condenser fan motor. In some cases, when the system a crankcase heater, the compressor contactor is a single-pole contactor so that the crankcase heater can receive power even when the contactor is open. The control voltage for the average compressor contactor in a residential HVAC system is 24 volts and is typically wired in series with any safety devices/switches for the condenser.
  • Safety devices/switches can also include high-pressure and low-pressure refrigeration switches. These switches protect the system from too high-pressure conditions (high head pressure) and/or low-pressure conditions as result from either freezing (could be caused by low airflow as noted above in the evaporators section) or by a loss of charge.
  • Additional controls for heat pumps include the defrost controls for defrosting the heat pump when it is running at low ambient temperatures.
  • Staging controls for modulating and two-stage systems.
  • Inverter systems have electronic controls for modulating the compressor and the condenser fan motor.

Compressors for Refrigeration

  • Scroll Compressor - The most common compressor you will find in the residential condensing unit is the scroll compressor. These can be either single-stage or two-stage compressors with the two-stage compressors offering a higher level of efficiency and comfort. Of course, the two-stage scroll will have additional controls to shift the unit from the first stage to the second stage. The scroll compressors use two offset plates to compress the vapor refrigerant.
  • Reciprocating Compressor - the reciprocating compressor is the next most common compressor found in residential condensing units. This compressor uses a piston to compress the vapor refrigerant.
  • Rotary Compressors - these compressors are typically found in modulating systems and are controlled by an inverter. High-efficiency mini-split systems use rotary compressors.
  • There are other types of compressors for refrigeration that are used for commercial chillers and refrigeration systems. These are beyond the scope of this lesson. Additionally, there are absorption chillers that do not use compressors. Again, these are not in the scope of the lesson for basic refrigeration.
two-stage scroll compressor

Scroll Compressor Inside Heat Pump Condenser

Safety and Regulations - HVAC Refrigeration 101 Part 2

Safety is crucial to any occupation including HVAC and working with refrigerants and performing as an HVAC technician. As an HVAC technician, you will be required to get a license to handle refrigerants and to comply with all the safety guidelines in working with and handling refrigerants. This includes all the duties associated with that job including brazing refrigerant lines and working around electricity.

It is essential to practice safety including wearing PPE  (Personal Protective Equipment) and being aware of safety issues when working with electricity, torches, and refrigerants.

Electrical Safety –  Electricity is invisible and it will bite you. HVAC equipment has high voltage electricity running to it. It is important, as a safety measure, to ensure the power is turned off before working around the equipment. This can be accomplished at the circuit breaker and at the equipment itself. Always remember to kill the power before touching the equipment.

Additionally, even when the power is turned off, capacitors in HVAC equipment still hold a charge. Like a battery, a capacitor can shock you even when the power is turned off at the source. Whenever you are in doubt, double-check the breaker and any switches and use a multi-meter to make sure the power is turned off.

Brazing and Fire Safety - Whenever brazing it is important to observe fire safety and personal safety by wearing your personal protective equipment. Special goggles for eye protection should be worn if you are brazing or observing the brazing. Additionally, ensure the area where you are brazing is free of combustible materials and that you have a fire extinguisher to extinguish any fires that may arise while brazing.

Brazing is done in excess of 800° Fahrenheit so be aware of the high temperatures even when finished brazing and the torch is put away. Make sure you have leather gloves for the purpose of handling overheated piping.


From personal experience, fires have started when brazing. A new condenser was being installed because the old condenser had a bad compressor. The technicians did not clean the old leaves away from the area where they were soldering and the leaves inadvertently caught fire. They did not have a fire extinguisher but luckily one of them had a gallon jug of water and extinguished most of the flames with the water and stomped the rest out.

Refrigerant Safety – Refrigerants are special chemicals designed to “change state” at specific temperatures/pressures and absorb heat. In great quantities or closed environments, refrigerants can be asphyxiants. In other words, they will cause you to stave of oxygen deprivation. Always keep this in mind and never ignore any alarms in mechanical rooms that house refrigeration equipment. If you feel yourself growing lightheaded in such a place, depart the space immediately and inform your supervisor.

Furthermore, when using flames around refrigerants, refrigerant piping, and refrigerant oils, the fire may cause the refrigerant to change. This can be observed by the flame changing color. Some refrigerants will change to very harmful and noxious gases that can damage your lungs or kill you.

Phosgene gas is one of those hazards and you will know it when you see the flame change color from blue to green. It will completely take your breath away and cause harm to you. Inform your supervisor and find another way to braze or solder by removing excess oil or the refrigerant from the system. The oil used in refrigeration systems will hold a minimal amount of refrigerant. Protect yourself and your health by being aware of this potential health hazard.

Regulations - HVAC Refrigeration 101 Part 2

There are many different safety and code regulations you will have to comply with as an HVAC/R technician. Never take safety or regulations for granted. As an HVAC technician working with refrigerants will require a refrigeration license. This course is a pre-requisite course for the EPA Section 608 exam preparation.

That is a course that is on our agenda and we are at work on writing the course now. Handling refrigerants without a license can cost you thousands of dollars. Stay tuned for our course on EPA Section 608 or find another course to get get your license. Take the regulations seriously through education and experience.

HVAC Refrigeration 101 Part 2

Link to part 1 HVAC Refrigeration 101