Depending on the type of HVACair conditioning or heat pump system it is and the efficiency range of the system will depend on the type of metering device the system has installed by the HVAC manufacturer of the air conditioner or heat pump system. Lower efficiency HVAC air conditioner and heat pump models have fixed orifice types while higher efficiency systems have thermostatic expansion valves installed in the HVAC air conditioner or heat pump system. The thermostatic expansion valve is far more complex than the fixed orifice metering device as the thermostatic expansion valve metering device modulates the refrigerant flow based on the temperature of the refrigerant temperature in the evaporator coil. This allows a specific amount of refrigerant, based on demand, to be metered into the evaporator coil while the fixed orifice metering device allows the same amount of refrigerant to enter the enter the coil no matter the conditions or the demand. As the liquid refrigerant enters the metering device it changes temperature and pressure. A partial amount of the liquid refrigerant flashes into a refrigerant gas or vapor as it leaves the metering device and enters the evaporator coil.
Thermostatic Expansion Valve – Also referred to as the TEV or TXV for short the thermostatic expansion is used in many air conditioning and heat pump applications including use in chillers for chilled water systems. TXVs respond to the temperature of the refrigerant leaving the evaporator coil or evaporator barrel. The TXV has a sensing bulb that holds a slight refrigerant charge inside the bulb. The TXV bulb is remote from the TXV and is attached to the TXV via a capillary tube or cap tube. As the temperature increases and decreases the refrigerant inside the bulb responds by expanding and contracting based on the temperature pressure relationship of refrigerants. As the refrigerant expands and contracts causes a bellows to move in and out which causes a piston to open and close precisely based on the leaving temperature of the refrigerant in the evaporator coil. This allows the TXV metering device to feed the evaporator coil the precise amount of refrigerant it needs to maintain a specific superheat. Because the TXV precisely meters the refrigerant the TXV is used in many air conditioner and heat pump systems that need a higher efficiency. A properly engineered and installed system that uses a TXV will only give the evaporator coil what it demands and nothing more or nothing less.
The other type of expansion valve used in HVAC is the fixed orifice. The fixed orifice is simple and can be either a piston type or utilize a distributor that feeds capillary tubes which terminate in the evaporator coil. The piston type comes in various sizes and is interchangeable with the capacity or tonnage of the condensing unit. Always follow the manufacturers instructions for piston size if using the fixed orifice for HVAC refrigeration.
Capillary tubes (cap tubes) are also used to meter refrigerant to the evaporator coil. These are considered fixed orifice since they hole in the tube(s) is a fixed size.
There are other non-conventional types of metering devices used for HVAC refrigeration including the float type and the electronic type however these are not commonly used in standard HVAC equipment.
There are other types of metering devices used in HVAC vapor compression refrigeration but these are the most common types are used in HVACR in both the commercial market and residential markets. As systems grow more sophisticated and complex that is likely to change. Electronic types are the next big step in refrigeration used for air conditioning and heat pump systems.
Evaporator Coil – the evaporator coil is responsible for absorbing heat into the coils and the refrigerant. The evaporator coil is an essential component of vapor compression refrigeration. This is usually done as result of passing air or water over the evaporator coil where a heat exchange process takes place. Heat leaves the air or water (whatever medium is used) and is absorbed into the coil and the liquid refrigerant. As more and more heat is absorbed by the evaporator coil and the refrigerant again changes state from a liquid to a refrigerant vapor. By the time the refrigerant leaves the evaporator coil the refrigerant should be all vapor and ready to be received again by the refrigeration compressor.
A new evaporator coil replacing an old leaking coil
The evaporator coil is typically inside the air handling unit or it can be installed in the duct work near the air handling unit. When the air handler blower turns on and the condenser is running refrigerant flows from the compressor through the condenser coils and then through the metering device. and into the evaporator coil. The refrigerant changes state in the evaporator coil and again in the condensing unit. In the evaporator coil the change of state is from a liquid to a vapor while in the condenser the change of state is from a vapor to a liquid. This is all necessary for the process of refrigeration to work properly. The typical evaporator coil is constructed of copper with aluminum fins surrounding the copper coils. The aluminum adds surface area to the copper coils and enhances the heat exchange rate making the coils more efficient.
Components of Vapor Compression Refrigeration – The Evaporator Coil
The condenser coil receives the high pressure high temperature refrigerant vapor from the compressor and immediately begins to remove heat from the refrigerant vapor. As the refrigerant vapor makes its way through the condenser coil more and more heat is removed and the refrigerant vapor changes state from a refrigerant vapor to a refrigerant liquid. While the liquid refrigerant changes temperatures from a higher temperature to a slightly lower temperature the pressure remains constant. As the refrigerant vapor leaves the condenser coil it makes it way to the metering device.
Most condensing units utilized in HVAC whether it is the commercial HVAC field or the residential HVAC field are constructed of copper with aluminum fins. The aluminum fins are mechanically attach to the copper tubes that make up the condenser coils or the piping that channels the refrigerant from the beginning of the condenser coils all the way to the end or exit of the condensing unit where the refrigerant makes its way to the metering device. The aluminum fins attached to the copper pipe enhance the heat exchange process in the air source condensing units. A few manufacturers use only aluminum coils in their condensing units to take advantage of the efficient heat exchange properties of aluminum. The aluminum coils also have fins on them and these are usually referred to as spine fin coils. The spine fins extend the surface area of the aluminum coil enhancing the heat exchange process for efficiency purposes.
Many condensing units also house the compressor and condensing unit controls. The condensing typically utilizes its own dedicated circuit from the main circuit breaker panel. In a split system the condensing unit will have a line set or refrigeration lines that run from the condensing unit to the evaporator coil inside the building. The condenser is an essential component of vapor compression refrigeration.
Components of Vapor Compression Refrigeration – The Condenser
In a typical HVAC air conditioner or heat pump there are four major components that make the process of refrigeration work. Basically, this is how air conditioners work. And how heat pumps work. The first component in the process is the compressor or the heart of any air conditioner or heat pump system.
HVAC Refrigeration Compressor – The HVAC refrigeration compressor receives the refrigerant vapor from the evaporator coil and compresses the refrigerant vapor. The vapor enters the refrigeration compressor at a low pressure and a low temperature and leaves the refrigeration compressor at a higher temperature and a higher pressure. The lower temperature refrigerant vapor is responsible for providing cooling for the refrigeration compressor and it also carries with it a small amount of oil which migrates from the compressor throughout the system. After the refrigerant vapor is compressed it is discharged to the condenser coil. The compressor is an essential component in the vapor compression process of refrigeration.
It is necessary for the evaporator to supply the compressor with vapor refrigerant as it is impossible to compress a liquid. Compressors that receive liquid refrigerant can fail mechanically. In heat pump applications the system is equipped with a refrigerant accumulator. The accumulator allows liquid to boil into a vapor before entering the compressor thereby preventing any liquid refrigerant from entering the compressor. The term liquid slugging is used for compressors that receive liquid refrigerant. A properly installed, maintained, and engineered air conditioner or heat pump system will never experience liquid slugging so it is especially important that qualified personnel install and service air conditioning and heat pump systems.
Components of Vapor Compression Refrigeration – The Compressor
A new evaporator coil replacing an old leaking coil
You had the new air conditioner or heat pump system installed some years ago and suddenly it stopped heating or cooling properly so you call the HVAC contractor to check it out. The HVAC technician arrives and spends about thirty minutes troubleshooting and then he comes to you with the news. The unit needs a charge of refrigerant. It is low on freon (R-22) or R-410A. You ask the HVAC technician if it is normal for this to occur and he informs that, no, it is not normal for the HVAC system to lose refrigerant that you have a leak. So you advance to the next question and ask, “can you find the leak”. The HVAC technician says yes he can. Here are the options for you and depending on which option you choose will depend on the cost.
This freshly rebuilt reciprocating compressor is destined for a large commercial chiller. Chillers chill water for air conditioning. Compressors compress refrigerant vapor and pump the compressed refrigerant to the condenser to be condensed into a liquid. The compressor provides compression which is an integral part of the refrigeration process. The old compressor was condemned for electrical burn out. The windings in the old compressor burned up and caused the circuit breaker to trip. Continue reading “Refrigeration – HVAC Compressor Rebuilt” »
Centrifugal compressors are used in many applications including pipeline transport of natural gas along with many other related applications including vapor compression refrigeration. Centrifugal compressors have different operating limitations than reciprocating compressors, scroll compressors, and rotary compressors. The centrifugal compressor has an operating range where it operates efficiently however outside of this range, either the over or under that operating, the centrifugal compressor is basically useless. Other issues with centrifugal compressors make them not ideal for use in certain compressor applications because of the operating range issues. Continue reading “HVAC Centrifugal Compressors” »
This refrigerant alarm serves two chiller rooms and alerts operators and other staff that refrigerant has been released into the mechanical room. Chillers have a built in pressure relief valve that will release the refrigerant in the event too much pressure builds up in the system. This prevents a possible explosion from too much pressure and also protects the equipment from a catastrophic failure that would make the chiller unusable. Other failure scenarios that set off the refrigerant alarm includes a refrigerant leak from the chiller so the refrigerant alarm would alert the maintenance staff of a chiller refrigerant leak condition. Refrigerant will asphyxiate humans and the refrigerant alarm is required by code for obvious safety reasons. The refrigerant alarm control automatically sets off audible horns, sirens, and automatically activates exhaust fans to evacuate the refrigerant from the mechanical room. Continue reading “Refrigerant Alarm Control for Chiller Room” »
R-22 is a refrigerant that has been widely used in air conditioners and heat pumps for many years. It has been deemed hazardous to the environment and it will begin phase-out by law in the year 2010. The replacement refrigerant for R-22, R-410A, is being produced to meet the rising demand for new air conditioner and heat pumps systems. Why do consumers need to understand the impact this law is having on the HVAC market? Because it will determine what HVAC consumers pat for future repairs made to their equipment. Everything boils down to the new law, refrigerant and HVAC equipment manufacturers retrofitting factories for the new HVAC equipment, and supply and demand. The changeover has already begun and there is some apprehension about this changeover.
HVACrefrigeration leaks (also referred to as freon leaks) can cause serious issues with HVAC heat pump equipment. HVAC refrigeration circuits are designed and built to hold the refrigerant. It is a sealed hermetic system. The HVAC refrigeration circuit should never need refrigerant. An HVAC refrigeration system does not burn refrigerant. The refrigerant is used over and over again to move heat from one place to another. Aside from a leak, once the refrigeration system is properly charged, the system should never need refrigerant for the life of the equipment. Continue reading “HVAC Refrigerant Leaks – Freon Leaks – Old Leaking Condenser Coils” »