If it’s a typical mid-Atlantic summer, you’re probably spending as many hours of the day as possible in the cool part of the refrigeration cycle. In our climate, it’s the most comfortable place to be this time of year. Moving heat out of your home is a complex process. In a typical Baltimore residence, the familiar central air conditioner actually incorporates the most elaborate engineering in the house. That’s probably why, during summer, it’s also the single largest energy consumer. When everything works right, the refrigeration cycle is a marvel of chemistry, mechanics and technology that extracts heat energy and humidity from the house, leaving behind only the blessings of cool, conditioned comfort.
Willis Carrier didn’t plan it that way. He mistaffght just as well have called his device a “humidity remover” when he invented the first modern air conditioner in 1902 and installed it in a New York City printing factory. He was primarily concerned with lowering humidity inside the building to improve print quality control. To accomplish that, the system he designed utilized the refrigeration cycle principally to extract water vapor from the air. Fortunately for all of us, it also had a pleasant side effect of cooling the air, as well.
Modes of Refrigerant
While Carrier’s patented device was a complicated behemoth that circulated poisonous ammonia as the refrigerant, the basic theory behind the process would sound familiar to anyone learning about air conditioning and today. The term “refrigerant” describes any one of a number of chemicals that handle heat in unique, efficient ways, depending on the temperature and pressure at a particular moment:
- Refrigerant evaporates (or boils, to be exact) at room temperatures.
- As refrigerant evaporates from a liquid into a frigid vaporous state, it readily absorbs heat energy.
- Refrigerant vapor pressurized to very high temperatures condenses from a hot gas back to a liquid, releasing its load of heat energy.
- When rapidly depressurized, hot liquid refrigerant morphs yet again into a cold vapor and absorbs more heat.
Once you have a single chemical that assumes all those identities, all you need are the mechanics to make it multi-task effectively. A modern air conditioner combines evaporation, air circulation, pressurization and heat transfer into the closed-loop refrigeration cycle that keeps us cool all summer. Here’s a round trip around that circular cycle, starting from cool and winding up there, too.
The Evaporator Coil
Installed inside the indoor air handler, the evaporator coil incorporates rows of copper tubing that circulate cool refrigerant vapor at low pressure and a temperature of about 40 degrees. The coil also features air passages that allow warm household airflow to pass through the coil by the force of the system blower. Heat energy naturally flows from a hotter zone into a colder zone. Heat in the airflow readily transfers through the highly conductive copper coil surfaces and into the absorbent refrigerant vapor within. With the heat energy extracted, household air leaves the air handler in a cool state and is dispersed by the blower into the ductwork to keep the home comfortable on a hot day.
One more helpful thing happens at the evaporator coil. Exploiting yet another convenient fact of physics, the cold surfaces of the evaporator cause water vapor in warm household air to instantly condense to liquid. This condensate — a busy central A/C can produce over 20 gallons on a humid day — flows into a collection pan and down a drain.
Dry air doesn’t hang on to heat like moist air does, so a dryer indoor environment cools easier and feels more comfortable. That’s why Carrier called his device an “air conditioner” instead of merely a “cooler.” When it comes to effective indoor comfort, it’s both the heat and the humidity.
The Compressor Cycle
Still a vapor but warmer from its load of heat energy, refrigerant flows through an insulated conduit to the outdoor half of the central air conditioner. It’s in the cabinet located immediately behind or to one side of your house. Inside the enclosure is a powerful electric compressor. This component is one of the reasons why your A/C bill is higher in summer; a compressor requires 240 volts of electricity to perform the heavy work of compressing the refrigerant flow.
As the refrigerant is compressed, the molecules of heat energy are squeezed together tightly, causing the discharge temperature of the refrigerant flow to soar up to over 200 degrees. In this superheated, high-pressure state, refrigerant exits the compressor and heads for the condenser coil.
The Condensing Process
Think of it as the reverse of the evaporator coil. In fact, the condenser employs a very similar design to circulate refrigerant, as well as allow airflow passage through the coil. Yet it performs an exactly opposite function from the evaporator. As the very hot refrigerant gas enters the condenser coil, it rapidly condenses to a liquid state.
A basic principle of physics states that the condensation process always releases heat energy. Refrigerant does so super efficiently, “rejecting” its load of concentrated heat as it circulates through the coil. This released heat energy is conducted through the copper coil tubes and radiated into the air by coil fins attached to the tubing. The condenser fan located inside the unit pulls air through the coil and disperses that heat into the outdoor air.
If you’ve ever stood beside the outside half of your central A/C system while it’s running, you’ve probably noticed the temperature of air blowing out of the unit. Feel familiar? That heat now exhausting into your backyard used to be inside your house.
The Return Trip
After leaving the condenser coil, the refrigerant cycle loops back to the indoor air handler through another insulated pipe. Just before refrigerant re-enters the evaporator, however, one more vital process takes place. In its warm, pressurized liquid state, refrigerant is in no condition to absorb heat inside the evaporator coil. The pressurized refrigerant passes through an expansion valve installed at the evaporator and is forced through a very small orifice. As the refrigerant exits the valve, the instantaneous expansion and rapid pressure drop causes the concentrated molecules of heat energy to spread far apart. The refrigerant temperature plunges and the flow converts to a vaporous state again.
As the closed-loop refrigerant cycle begins anew, the cold refrigerant vapor circulating through the evaporator coil absorbs still another load of heat from your household air, as well as extracting humidity. Meanwhile, you relax in cool comfort and think kind thoughts about Willis Carrier.
Issues With the A/C
So it’s just as simple as that? Actually, no: All the above is A/C 101, the (very) short version. To a qualified HVAC technician, the refrigeration cycle is a highly technical topic that requires professional training and continuous study to stay abreast of improvements in the technology that utilizes it. Nor is any system foolproof. Issues in the refrigeration cycle can have a major impact on the efficiency and performance of your air conditioner.
Some of the most common problems you may encounter include:
- Dirty coil surfaces – The evaporator coil and condenser coil transfer heat efficiently. Not so much, however, if they’re coated with dust and dirt from the airflow passing through the coil. The efficiency of the heat transfer process through the copper coil tubing is severely diminished by dirt and dust accumulations and system performance plunges while operating costs increase. Coil cleaning is a standard part of annual A/C maintenance you should schedule every year.
- Insufficient refrigerant – Because it’s the lifeblood of the refrigeration cycle, low refrigerant levels can open a Pandora’s box of system shortfalls and mysterious malfunctions. If your refrigerant is low, you’ve got a leak somewhere in the system and simply adding more refrigerant is, at best, a very short-term solution. An HVAC service technician will measure the refrigerant level. If it’s below specs, he’ll use instruments to track down any leak, then repair it to return the system to full functionality and efficiency.
For more information on the refrigeration cycle as well as professional service to keep it flowing free in your Baltimore, MD home, check out Griffith Energy’s air conditioning solutions, or call 888-474-3391.