Several HVAC systems require field refrigeration piping to be designed and installed on-site. Examples include Condensing units, Direct expansion (DX) coil in air handlers, Remote evaporators with air-cooled chillers and Chiller with a remote air-cooled condensers. This Guide covers R-22, R-407C, R-410A, and R-134a used in commercial air conditioning systems. It does not apply to industrial refrigeration and/or Variable Refrigerant Volume (VRV) systems.
The information contained in this Application Guide is based on Chapter 2 of ASHRAE's Refrigeration Handbook and McQuay's good experience with this type of equipments.
A properly designed and installed refrigerant piping system should:
- Provide adequate refrigerant flow to the evaporators, using practical refrigerant line sizes that limit pressure drop.
- Avoid trapping excessive oil so that the compressor has enough oil to operate properly at all times.
- Avoid liquid refrigerant slugging.
- Be clean and dry.
Refrigerant Piping Design Check List
The first step in refrigerant piping design is to gather product and jobsite information. A checklist for each is provided below. How this information is used will be explained throughout the rest of this guide.
- Model number of unit components (condensing section, evaporator, etc.)
- Maximum capacity per refrigeration circuit
- Minimum capacity per refrigeration circuit
- Unit operating charge
- Unit pump down capacity
- Refrigerant type
- Unit options (Hot Gas Bypass, etc.)
- Does equipment include isolation valves and charging ports
- Does the unit have pump down?
- Sketch of how piping will be run, including:
- Elevation changes
- Equipment layout
- Specific details for evaporator piping connections
- Ambient conditions where piping will be run
- Ambient operating range (will the system operate during the winter?)
- Type of cooling load (comfort or process)
- Unit isolation (spring isolators, rubber-in-shear, etc.)
Typical Refrigerant Piping Layouts
This Figure shows a condensing unit mounted on grade connected to a DX coil installed in a roofmounted air-handling unit.
- A liquid line supplies liquid refrigerant from the condenser to a thermal expansion (TX) valve adjacent to the coil.
- A suction line provides refrigerant gas to the suction connection of the compressor.
This Figure shows a roof-mounted air-cooled chiller with a remote evaporator inside the building.
- 1. There are two refrigeration circuits, each with a liquid line supplying liquid refrigerant from the condenser to a TX valve adjacent to the evaporator, and a suction line returning refrigerant gas from the evaporator to the suction connections of the compressor.
- 2. There is a double suction riser on one of the circuits.
This Figure shows an indoor chiller with a remote air-cooled condenser on the roof.
- 1. The discharge gas line runs from the discharge side of the compressor to the inlet of the condenser.
- 2. The liquid line connects the outlet of the condenser to a TX valve at the evaporator.
- 3. The hot gas bypass line on the circuit runs from the discharge line of the compressor to the liquid line connection at the evaporator.
Piping Design Basics
Good piping design results in a balance between the initial cost, pressure drop, and system reliability. The initial cost is impacted by the diameter and layout of the piping. The pressure drop in the piping must be minimized to avoid adversely affecting performance and capacity. Because almost all field-piped systems have compressor oil passing through the refrigeration circuit and back to the compressor, a minimum velocity must be maintained in the piping so that sufficient oil is returned to the compressor sump at full and part load conditions.
A good rule of thumb is a minimum of:
- 500 feet per minute (fpm) or 2.54 meters per second (mps) for horizontal suction and hot gas lines.
- 1000 fpm (5.08 mps) for suction and hot gas risers.
- Less than 300 fpm (1.54 mps) to avoid liquid hammering from occurring when the solenoid closes on liquid lines.
Hard drawn copper tubing is used for halocarbon refrigeration systems. Types L and K are approved for air conditioning and refrigeration (ACR) applications. Type M is not used because the wall is too thin. The nominal size is based on the outside diameter (OD). Typical sizes include 5/8 inch, 7/8 inch, 1-1/8 inch, etc.
Copper tubing intended for ACR applications is dehydrated, charged with nitrogen, and plugged by the manufacturer (see Figure below).
Formed fittings, such as elbows and tees, are used with the hard drawn copper tubing. All joints are brazed with oxy-acetylene torches by a qualified technician. As mentioned before, refrigerant line sizes are selected to balance pressure drop with initial cost, in this case of the copper tubing while also maintaining enough refrigerant velocity to carry oil back to the compressor. Pressure drops are calculated by adding the length of tubing required to the equivalent feet (meters) of all fittings in the line. This is then converted to PSI (kPa).
Pressure Drop and Temperature Change
As refrigerant flows through pipes the pressure drops and changes the refrigerant saturation temperature. Decreases in both pressure and saturation temperature adversely affect compressor performance. Proper refrigeration system design attempts to minimize this change to less than 2°F (1.1°C) per line. Therefore, it is common to hear pressure drop referred to as “2°F” versus PSI (kPa) when matching refrigeration system components.
For example, a condensing unit may produce 25 tons (87.9 kW) of cooling at 45°F (7.2°C) saturated suction temperature. Assuming a 2°F (1.1°C) line loss, the evaporator would have to be sized to deliver 25 tons (87.9 kW) cooling at 47°F (7.2°C) saturated suction temperature.
Table below compares pressure drops in temperatures and pressures for several common refrigerants. Note that the refrigerants have different pressure drops for the same change in temperature. For example, many documents refer to acceptable pressure drop being 2°F (1.1°C) or about 3 PSI (20.7 kPa) for R-22. The same 3 PSI change in R-410A, results in a 1.2°F (0.7°C) change in temperature.
Note Suction and discharge pressure drops based on 100 equivalent feet (30.5 m) and 40°F (4.4°C) saturated temperature.
Liquid lines connect the condenser to the evaporator and carry liquid refrigerant to the TX valve. If the refrigerant in the liquid line flashes to a gas because the pressure drops too low or because of an increase in elevation, then the refrigeration system will operate poorly. Liquid sub-cooling is the only method that prevents refrigerant flashing to gas due to pressure drops in the line.
The actual line size should provide no more than a 2 to 3°F (1.1 to 1.7°C) pressure drop. The actual pressure drop in PSI (kPa) will depend on the refrigerant.
Oversizing liquid lines is discouraged because it will significantly increase the system refrigerant charge. This, in turn, affects the oil charge.
As the liquid refrigerant is lifted from the condenser to the evaporator, the refrigerant pressure is lowered. Different refrigerants will have different pressure changes based on elevation. Refer Table 2 to for specific refrigerants. The total pressure drop in the liquid line is the sum of the friction loss, plus the weight of the liquid refrigerant column in the riser.
Pressure Drop PSI/ft (kPa/m) Riser
Only sub-cooled liquid refrigerant will avoid flashing at the TX valve in this situation. If the condenser had been installed above the evaporator, the pressure increase from the weight of the liquid refrigerant in the line would have prevented the refrigerant from flashing in a properly sized line without sub-cooling.
It is important to have some sub-cooling at the TX valve so that the valve will operate properly and not fail prematurely. Follow the manufacturer’s recommendations. If none are available, then provide 4 to 6°F (2.2 to 3.3°C) of sub-cooling at the TX valve.
Liquid lines require several refrigerant line components and/or accessories to be field selected and installed (Figure below). Isolation valves and charging ports are required. Generally, it is desirable to have isolation valves for servicing the basic system components, such as a condensing unit or condenser. In many cases, manufacturers supply isolating valves with their product, so be sure to check what is included. Isolating valves come in several types and shapes.
Referring to this Figure :
- Working from the condenser, there is a liquid line filter-drier. The filter drier removes debris from the liquid refrigerant and contains a desiccant to absorb moisture in the system. Filter driers are either disposable or a permanent with replaceable cores.
- Next there is a sight glass that allows technicians to view the condition of the refrigerant in the liquid line. Many sight glasses include a moisture indicator that changes color if moisture is present in the refrigerant.
- Following the sight glass is the TX valve.
Possible accessories for this system include:
- A hot gas bypass port. This is a specialty fitting that integrates with the distributor – an auxiliary side connector (ASC).
- A pump down solenoid valve. If a pump down is utilized, the solenoid valve will be located just before the TX valve, as close to the evaporator as possible.
- Receivers in the liquid line. These are used to store excess refrigerant for either pump down or service (if the condenser has inadequate volume to hold the system charge), or as part of a flooded low ambient control approach. Receivers are usually avoided because they remove sub-cooling from the condenser, increase the initial cost, and increase the refrigerant charge.
Liquid lines should be sloped 1/8 inch per foot (10.4 mm/m) in the direction of refrigerant flow. Trapping is unnecessary.
Suction gas lines allow refrigerant gas from the evaporator to flow into the inlet of the compressor. Undersizing the suction line reduces compressor capacity by forcing it to operate at a lower suction pressure to maintain the desired evaporator temperature. Oversizing the suction line increases initial project costs and may result in insufficient refrigerant gas velocity to move oil from the evaporator to the compressor. This is particularly important when vertical suction risers are used.
Suction lines should be sized for a maximum of 2 to 3°F (1.1 to 1.7°C) pressure loss. The actual pressure drop in PSI (kPa) will depend on the refrigerant.
Suction Line Piping Details
While operating, the suction line is filled with superheated refrigerant vapor and oil. The oil flows on the bottom of the pipe and is moved along by the refrigerant gas flowing above it. When the system stops, the refrigerant may condense in the pipe depending on the ambient conditions. This may result in slugging if the liquid refrigerant is drawn into the compressor when the system restarts.
To promote good oil return, suction lines should be pitched 1/8 inch per foot (10.4 mm/m) in the direction of refrigerant flow. Evaporator connections require special care because the evaporator has the potential to contain a large volume of condensed refrigerant during off cycles. To minimize slugging of condensed refrigerant, the evaporators should be isolated from the suction line with an inverted trap as shown in Figures below:
The trap should extend above the top of the evaporator before leading to the compressor.
- With multiple evaporators, the suction piping should be designed so that the pressure drops are equal and the refrigerant and oil from one coil cannot flow into another coil.
- Traps may be used at the bottom of risers to catch condensed refrigerant before it flows to the compressor. Intermediate traps are unnecessary in a properly sized riser as they contribute to pressure drop.
- Usually with commercially produced air conditioning equipment, the compressors are “prepiped” to a common connection on the side of the unit.
- Suction line filter driers are available to help clean the refrigerant before it enters the compressor. Because they represent a significant pressure drop, they should only be added if circumstances require them, such as after compressor burnout. In this instance, the suction filter drier is often removed after the break-in period for the replacement compressor. Suction filter driers catch significant amounts of oil, so they should be installed per the manufacturer’s specifications to promote oil drainage.
Discharge gas lines (often referred to as hot gas lines) allow refrigerant to flow from the discharge of the compressor to the inlet of the condenser. Undersizing discharge lines will reduce compressor capacity and increase compressor work. Over sizing discharge lines increases the initial cost of the project and may result in insufficient refrigerant gas velocity to carry oil back to the compressor. Figures below.
Discharge Line Piping Details
Discharge lines carry both refrigerant vapor and oil. Since refrigerant may condense during the off cycle, the piping should be designed to avoid liquid refrigerant and oil from flowing back into the compressor. Traps can be added to the bottom of risers to catch oil and condensed refrigerant during off cycles, before it flows backward into the compressor. Intermediate traps in the risers are unnecessary in a properly sized riser as they increase the pressure drop. Discharge lines should be pitched 1/8 inch per foot (10.4 mm/m) in the direction of refrigerant flow towards the condenser.
Whenever a condenser is located above the compressor, an inverted trap or check valve should be installed at the condenser inlet to prevent liquid refrigerant from flowing backwards into the compressor during off cycles. In some cases (i.e. with reciprocating compressors), a discharge muffler is installed in the discharge line to minimize pulsations (that cause vibration). Oil is easily trapped in a discharge muffler, so it should be placed in the horizontal or downflow portion of the piping, as close to the compressor as possible.
Multiple Refrigeration Circuits
For control and redundancy, many refrigeration systems include two or more refrigeration circuits. Each circuit must be kept separate and designed as if it were a single system. In some cases, a single refrigeration circuit serves multiple evaporators, but multiple refrigeration circuits should never be connected to a single evaporator. A common mistake is to install a two circuit condensing units with a single circuit evaporator coil.
This Figure shows common DX coils that include multiple circuits. Interlaced is the most common. It is possible to have individual coils, each with a single circuit, installed in the same system and connected to a dedicated refrigeration circuit.
While most common air conditioning applications have one evaporator for each circuit, it is possible to connect multiple evaporators to a single refrigeration circuit.
Figure below shows a single refrigeration circuit serving two DX coils. Note that each coil has its own solenoid and thermal expansion valve. There should be one TX valve for each distributor. Individual solenoids should be used if the evaporators will be operated independently (i.e. for capacity control). If both evaporators will operate at the same time, then a single solenoid valve in a common pipe may be used.