Refrigeration: Technology Appendix edit
This section of the refrigeration assessment guide will answer common questions that may have arisen during previous reading. Along with a few new topics, much of the following expands on previous subjects.
- Refrigeration properties
- Simple refrigeration cycle
- Multistage systems
- Refrigerant properties
Working fluids and their properties are key to the operation of a refrigeration system. The thermodynamic characteristics of the working fluids define the conditions that must be met in the design of a successful system. A short list of commonly used refrigerants include:
- Chlorofluorcarbons - now mostly banned because of their destructive impact on the Earth's protective Ozone layer. R-11, R-12, R-113, R-114 and R-115
- Other Halocarbons; R22, R134a, R502
- Inorganix Ammonia (R717), water (R718), carbon dioxide (R744)
- Hydrocarbons methane (R50), ethane (R170), propane (R290)
Important refrigerant properties include boiling point, latent heat of vaporization, and specific volume. The system designer must consider these properties when designing and sizing equipment for a specific application. Equipment is generally designed to work with one specific refrigerant.
- Boiling Point
Phase change (boiling/condensing temperature for a given refrigerant is constant for any given pressure. Refrigeration system designer use constant temperature phase change as the primary method of heat transfer in most systems. System pressures are chosen to correspond to a phase change temperature that will drive heat transfer in the desired direction.
For example, the suction (low-side) pressure is chosen so that the liquid refrigerant evaporates at the required temperature; the saturation temperature is below that of the cooled space or medium. the condensing pressure is chosen such that gaseous refrigerant will condense. rejecting energy to the outside, at a saturation temperature above that of the outside air.
- Latent Heat of Vaporization (hfg)
Latent heat represents the amount of energy per unit mass of refrigerant that is absorbed during evaporation at a specific pressure. Refrigeration absorbs energy while evapor at low pressure and expels energy condensing at high pressure. refrigerant with a lower latent heat of vaporization will absorb less energy during evaporation than those with higher hfg. The required refrigerant mass flow rate depends on hfg, and required cooling load.
- Specific Volume
Specific volume is the volume occupied by a unit mass of the refrigerant at a specific pressure. Mass flow of refrigerant and specific volume at the compressor inlet determine the required compressor displacement fora particular refrigerant.
- Saturation Temperature
The temperature for a given pressure at which the gaseous refrigerant begins to condense into a liquid.
Performance calculations require knowledge of other thermodynamic properties that you can find in appropriate tables or charts.
- Mollier diagram
A pressure-enthalpy (p-h) diagram that describes the behavior of a refrigerant under conditions that range from saturated to super heated. A drawback of the diagram is the difficulty of reading data accurately. A table or equation yields more accurate results.
- Simple Refrigeration Cycle
Refrigeration in the simple cycle passes through 4 distinct states. More complex systems are always a combination of the simple cycle. The diagrams below show a schematic of a simple system and thermodynamic characteristics for the 4 operating states.
- Description and Assumptions
Following are descriptions of the 4 states that make up the simple refrigeration cycl and assumptions that were made in drawing the cycle on the Mollier Diagram. The refrigeration states are label 1 to 4 in the diagram.
- 1-2 Condenser
A heat exchanger that rejects heat from high pressure refrigerant as it condenses. We assume no pressure drop across the condenser. In most cases, this is a reasonable estimate. If not, condenser manufacturers adjust their heat transfer ratings.
- 2-3 Expansion
High-pressure liquid (often sub-cooled passes through an expansion device. refrigerant pressure drops because the passage through the device is physically very small. Some liquid flashes to gas, absorbs heat, and cools the refrigerant to the lower saturation temperature. Heat transfer is small. the vertical line 2-3 in the Mollier diagram assumes that the device is isenthalpixc.
- 3-4 Evaporator
Cold, lower pressure liquid-gas mixture evaporates absorbing heat from the cold environment. The Mollier diagram assumes no pressure drop.
- 4-1 Compressor
Refrigerant gas is compressed to higher-pressure (often superheated). The refrigerant must be completely gaseous because liquid can destroy a compressor. The Mollier diagram assumes no friction or heat transfer (constant-entropy). The assumption is a good initial approximation and can be adjusted with an efficiency rating.
All vapor compression refrigeration systems include the four states described in the previous section. Following is further detail on theses four states and some of the equipment involved.
High-pressure, super-heated gas cools to its saturation point then condenses as energy is removed. Air and water are two common heat "sinks".
Air has a lower specific heat than water. The effectiveness of the condenser can be quantified by measuring the temperature difference between the refrigerant as it leaves the condenser and the dry-bulb temperature for an air-cooled unit.
Energy from the condenser is transferred to water. The water in turn may be cooled through a cooling tower and possibly recycle in a closed system or used once passing on to another process or disposal. scaling is a concern that can reduce efficiency in water-cooled condensers. refrigerant tubing must be cleaned often.
These condensers use air flow across tubes that are sprayed with water, to take advantage of evaporative cooling effects. Some water evaporates or is blown from the condenser and must be made up. This type of condenser should be used where water is available and the cooling load justifies the added complexity of the system and the water treatment that it requires. The effectiveness of the condenser, the condensing approach temperature, can be measured as the temperature difference between the refrigerant as it leaves the condenser and the wet-bulb temperature.
Condensing "capacity" determines the minimum condensing pressure for a system. The condensing pressure will remain at its pressure setting, unless available heat transfer cannot fully condense the refrigerant. In such a case, condensing pressure and temperature rise until the refrigerant can fully condense. This occurs commonly during hot weather. When condensing temperature and pressure rise, it is referred to as "floating" above ambient temperature. For much of the year, ambient temperature will be low enough for the condensing pressure to stay at its minimum.
When the ambient temperature is near the minimum condensing temperature, condensing temperature will float higher to ensure that all refrigerant condenses. the condensing temperature is always above the ambient temperature by no less than the condenser' sminimum approach temperature difference (MATD). Factors the influence condenser MATD include: heat rejection rate, condensing surface area, and heat transfer coefficient of the condenser. MATD is often available in manufacturer or vendor literature, but is usually 10-15 degrees F at capacity.
Energy is used at the condenser:
- Fans modulate the overall heat transfer coefficient to control condensing pressure. Increasing airflow across the coils improves heat transfer, reduces approach temperature difference, and decreases condensing temperature and pressure.
- Pumps circulate water on evaporative or shell-in tube condensers.
- Purge control: Purge units are located near the condensers. They remove non-condensable gases such as air that may leak into the system when suction pressure at the compressor drops below atmospheric pressure. These gases must be removed or system performance falls. Non-condensable gases effectively reduce available condensing heat exchange area; condensing pressure rises, heat transfer falls, and corrosion may occur.
- Expansion Device
Refrigerant pressure drops in an expansion device. Expansion cools the refrigerant as some evaporates and the mixture cools to the lower saturation temperature. The expansion device can be as simple as a orifice or capillary tube. It is often more sophisticated, including controls to actively modulate flow.
An active flow control valve senses refrigerant temperature at the evaporator exit. The refrigerant gas is typically superheated several degrees above the saturation temperature so that the controls can sense complete evaporation. This also protects the compressor from liquid-slugging damage. active flow control senses refrigerant conditions entering and exiting the evaporator, adjusting the mass flow through the device accordingly. Mechanical or electronic pressure and temperature sensors feed information to the flow control device to adjust the refrigerant flow. Float valves sense the liquid refrigerant flow. Float valves sense the liquid refrigerant level ini a receiver feeding the evaporator refrigerant flow is adjusted to maintain the desired conditions.
- Expansion valves use little or no energy. Electronic expansion valves require minimal power to monitor and control the valve.
Liquid refrigerant evaporates as energy is transferred to the refrigerant in the evaporator (cold side). A refrigeration system commonly removes energy from air, water, antifreeze liquid, or product. The compressor controls the suction pressure in the evaporator.
Evaporator fans or pumps use energy. Cross-flow coils use fans to maximize heat transfer. Pumps circulate refrigerant on liquid overfeed systems and antifreeze solutions through chillers.
- Defrost Control
Defrost removes ice that forms when an evaporator cools moist air below freezing. Defrost systems generally are needed for the low-stage evaporator on a multi-stage system. Ice reduces heat transfer. Several defrost methods are available, but all require that the evaporator be heated. Ice is melted by either pumping warm water or hot refrigerant gas through it, or by energizing heating elements.
Choice of defrost type depends on available energy. The most energy efficient method is to use war, wastewater to defrost the evaporators, but this is not always possible. Any other type of defrost will require extra energy. Electric heating element defrost is next in efficiency, followed by hot gas defrost. Hot gas defrost uses the compressor to circulate hot refrigerant gas. All can be programmed to operate in timed cycles or on power.
The compressor uses energy to compress the refrigerant to higher pressure. The required suction pressure is determined by the temperature of the refrigerated space or product and the MATD of the evaporator.
- Condensing pressure is determined so that condensing temperature is higher than the temperature of the cooling medium. For steady operation, all energy added must be removed from the system. Energy absorbed from the cold environment and energy added to drive the cycle is rejected at the condenser.
Suction and discharge pressures depend on
- Refrigerant Type
- Desired cold temperature
- Outside temperature (wet or dry bulb)
- Evaporator MATD
- Condenser MATD
Energy transfer from a low to a high temperature medium will not occur without adding work to the system (compressing refrigerant). The majority of the work added to are refrigeration system is at the compressor.
Some compressors have the ability to operate at part load by unloading cylinders (in reciprocating compressors) or moving a slide valve (in a screw compressor). Part load power is not directly proportional to refrigerant flow, however, because pressure drop depends on control type and load. Performance curves are available from som compressor manufacturers that show performance under different loads.
- Compressor Types
Compressors can be positive displacement or dynamic. Centrifugal compressors are dynamic, and are not included in this guide. Descriptions follow for positive displacement compressors. Note that compressor-lubricating oil is entrapped in the refrigerant during compression, particularly with screw compressors. Separators are typically required to remove the oil from the refrigerant.
- Reciprocating compressor
A motor turns the crankshaft of the compressor, reciprocating the pistons in their cylinders. Each piston moves down, creating a low-pressure volume and refrigerant gas flows into the cylinder. Then the piston moves back up and compresses the refrigerant until it is discharged at high pressure.
Reciprocating compressors use reed or plate valves that operate on pressure differences. When the cylinder pressure drops below suction line pressure, the suction valve opens allowing refrigerant in while the discharge valve remains closed. When cylinder pressure rises above discharge line pressure, the discharge valve opens and refrigerant is forced out while the suction valve remains closed.
Clearance volume is required in compressor cylinders to allow for valve operation and equipment stretch overtime. Because of the extra clearance, refrigerant is never completely expelled out of the compressor. As the compressor executes its suction stroke, trapped refrigerant re-expands before pressure drops enough to let new refrigerant in. This necessary clearance volume reduces the compressor's volumetric efficiency.
- Screw Compressors
Refrigerant flow is smoother than from a reciprocating unit. [This is really a lame description. Copy one from elsewhere if it exists in this or previous versions]
- Rotary vane
Rotary vane compressors have an eccentrically mounted cylinder on stationary bearings. Vanes mounted in slots on the cylinder seal against the wall of the housing. Rotary vanes are sometimes used as the low-stage compressor in a two-stage system, due to good volumetric efficiency. In general, rotary vane compressors are not used often, require high maintenance, and have poor part load efficiency. Consider replacing with a different type of compressor when they are encountered.
- Mulit-stage Systems
The simple refrigeration cycle is limited by the maximum pressure difference a single compressor can efficiently sustain. The condenser and heat sink (outside air, water, etc.) help determine discharge pressure. For simple systems with a single compressor, suction pressure is limited. A single stage system generally cannot efficiently achieve temperature below -15 degrees F.
A multistage system must be used to achieve lower refrigerant temperatures. There are two common types of multistage systems: compound or cascade. A compound system has a single continuous loop. The following p-h diagram shows thermodynamic properties at different points.
- Two-stage Compound System
The main advantage of two-stage systems is that they can reach lower temperatures than single-stage systems. Multistage equipment is more expansive than a single-stage system, but it is more efficient.
- Two-stage Cascade System
A cascade system is actually two separate single stage systems that are joined the cascade condenser. The cascade condenser serves as the condenser for the low-pressure system and the evaporator for the high-pressure system. A cascade system is actually two separate single-stage systems that are joined at the cascade heat exchanger. The cascade head exchanger serves as the condenser for the low-stage system and the evaporator for the high-stage system.
- Economized Single-Stage System
An economized single-stage screw compressor is less expensive than a two-stage system, but can be used to achieve temperatures lower than a simple cycle. An economizer can also increase refrigeration capacity 10% to 40% at a given suction pressure. The following figures show the schematic and p-h diagram for an economized single-stage system.
In an economized cycle, a valve expands high pressure liquid refrigerant to an intermediate pressure in an economizer vessel, from which refrigerant gas is drawn through an intermediate suction port to the compressor. The remaining liquid refrigerant goes through a second expansion valve to an evaporator.
The intermediate pressure saturated gas mixes with superheated gas in the compressor and decreases the overall superheat of the mixture. The temperature decreases with flow through the low-stage compressor, reducing total compressor power.
- Heat Transfer
The following equations describe the heat transfer across a heat transfer across a heat exchanger.
- Q = U x A x delta T = m x ha-b
- Q = heat transfer
- U = Overall heat transfer coefficient
- A = exchanger surface area
- Delta T = Temperature difference
- m = refrigerant mass flow
- delta ha-b = Change in specific enthalpy between states a and b
Heat transfer increases with an increase in any of the above listed controlling variables. For example, heat transfer (Q) will increase with a larger temperature difference, then refrigerant mass flow or enthalpy change must also increase to remove the additional energy. Otherwise, fans will cycle to slide valve, or increasing compressor motor speed.
When a refrigeration load increases, the system must adjust to greater heat transfer. .The surface area of an evaporator does not change, but fans can affect heat transfer. Fans require less energy than a compressor, so it is more efficient to maximize evaporator heat transfer with the fans and allow suction pressure to rise to reduce compressor load.
Refrigeration loads and outdoor temperatures vary with time. A refrigeration system must react to these changes properly to maintain the temperature of the cooled medium or space. Several types of control are available to react to load variation. The most common methods for controlling a refrigeration system are slide valve, cylinder unloading, on-off, compressor speed, compressor unloading, and hot-gas bypass.
- Slide valve
Most screw compressors have a slide valve for part load control. The slide valve allows some refrigerant to exit the rotors without compressed for part-load operation.
- Cylinder Unloading
Individual cylinders in a reciprocating compressor can unload to vary refrigeration flow through the compressor. For example, a two-cylinder reciprocating compressor may reduce power 40% with a 50% reduction in flow. The power reduction is not direct, due to friction in the unloaded cylinders.
Suction pressures determines when the compressor will turn on and off. When suction pressure increases due to the refrigeration load, the compressor turns on until the suction pressure drops below the low pressure set-point. This control is used primarily for small systems (<5 hp) where it doesn't result in short-cycling the motor. On-off control is the simplest on reciprocating compressors.
- Compressor Speed Control
By controlling the speed of the compressor motor, the refrigerant mass flow rate can be modulated. Variable Speed Drives (VSD) can change compressor speed. VSD reduces the power requirements of a compressor at part-load, but is not common because it is expensive, results in minimal efficiency improvement, and may be unreliable.
- Compressor Unloading
Multiple compressor systems include several compressor that share a common header. Mass flow is controlled by turning individual compressors on and off. In this type of system, compressors may be staged so that the most efficient compressor turns on first.
Hot gas bypass. The diagram depicts hot gas bypass. This method is the least desirable because there are no part-load compressor savings. The compressor always runs at full load. the bypass valve senses compressor suction pressure (refrigeration load) and bypasses hot gas to the evaporator. This method is not efficient.
- 1) Reduce Discharge Pressure
Work done by the compressor increases with compression ratio. Suction pressure is usually fixed, but condensing pressure depends on load, outdoor air temperature, and condenser fan controls.
The minimum condensing pressure is controlled by fans on air-cooled and evaporative condensers. Fan control changes the overall heat transfer to maintain condensing pressure. The condensing approach temperature difference is commonly designed to be around 10 degree F with all fans at full load.
To transfer heat to warm summer air, refrigerant discharge pressure often is higher than during winter or nights when the air is colder. However, the minimum condensing pressure settings are commonly left at summer levels year-round. Higher pressure settings that are acceptable during summertime are inefficient on cooler days. A condensing temperature set point higher than 80 degrees F throughout the entire year is probably excessive. Condensing temperature set points over 100 degrees F are common for halocarbon systems. When the minimum pressure settings are reduced, fans will operate more to reduce condensing temperature. Compressor energy savings at a lower discharge pressure usually exceed increased fan energy use.
Strategies for controlling discharge pressure may include multiple fans cycling, 2 -speed or VSD fans. When ambient temperature rises, condensing pressure will tend to rise. If the fans are not already at full capacity, their use factor or speed will increase to maintain the condensing pressure. Once the fans reach full capacity, any further increase in ambient temperature will cause the condensing pressure and temperature to increase as well. Condensing temperature will remain above the ambient temperature by the minimum approach temperature difference. This condition is called "floating" discharge pressure.
When reducing condensing pressure, take care that it does not fall below the minimum required to circulate the refrigerant through the system to avoid flash gas and to cool the compressor if liquid injection is used. One way to determine the MATD is to manually operate all of the fans on the condenser. Watch the system response for potential problems as discharge pressure falls. This must be done slowly as it usually takes a while to stabilize. Check compressor oil temperature and monitor discharge pressure. Compare to ambient temperature. Minimum condensing temperature will depend on ambient temperature at the time of the test.
If refrigerant flow is inadequate at low pressures, identify and adjust or replace needle valves or orifices that may be restricting flow. A small, magnetically - coupled, sealed centrifugal pump can be installed in the liquid line after the condenser to sub-cool the liquid by increasing its pressure, thereby reducing flash gas. another small pump might ensure adequate liquid injection cooling at low pressures. Ammonia systems require special impellers to avoid corrosion.
With a drop in condensing pressure, the mass flow required to obtain a desired refrigeration effect decreases. Compressor power decreases. Compressor power decreases with decreases in both compressor work and mass flow. Net heat rejection also decreases with decreased mass flow. System efficiency represented by coefficient of performance (COP), increases with the increased refrigeration effect and decreased compressor work.
LOOK FOR: Higher condensing pressure. If you are assessing a plant in the summer, the condensing pressure will typically be floating. You'll be floating. You'll have to ask or record pressure settings and whether they have changed.
- 2) Increase Suction Pressure
Compressor controls maintain suction pressure. Plant personnel often set compressor controls at lower temperatures than needed for efficient operation. A large temperature difference is maintained between the evaporator and cooled medium to ensure everything is kept cool.
With an increase in suction pressure, the refrigeration flow rate can be decreased while maintaining the same system cooling. reducing both refrigeration flow and compression ratio decreases compressor power and total heat rejected from the condenser. COP increases with the increased refrigeration effect and reduced compressor work.
LOOK FOR: Low refrigeration suction pressure. Low refrigerant temperature may be required because the evaporator coils are dirty or scaled. It may be cost effective to buy more evaporators. Also look for back pressure regulators reducing suction pressure.
- 3) Thermosyphon and Liquid Injection Cooling
Oil absorbs some of the heat of compression. Oil must be cooled to avoid compressor damage. Oil cooling applies to screw compressors.
Liquid injection cooling is one method of cooling the compressor oil. High-pressure liquid refrigerant is expanded (cooling it ) and injected into an intermediate port in the compressor. The disadvantage of liquid-injection cooling is that the re-injected refrigerant must be recompressed, using between 5 to 15% of total compressor power, depending on compression ratio.
The Thermosyphon Oil Cooling diagram illustrates another method to cool compressor oil. A heat exchanger beside the sump of the compressor cools the oil using refrigerant driven by buoyancy forces. A small receiver feeds the exchanger with liquid refrigerant that evaporates. The gas rises to a condenser. There is not additional power used for this type of cooling because the refrigerant is not re-injected into the compressor and does not have to be reconnected.
Existing condenser fan energy will not increase significantly because of the reduction in compressor power. A new thermosyphon condenser will increase fan power, but will improve performance of the existing compressor by reducing load and condensing temperature.
LOOK FOR: A liquid refrigerant line and expansion valve in the side of the compressor.
- 4) VSD on Condenser Fans
Heat transfer in a condenser can be controlled by adjusting fan speed. Most condenser fan motors have one speed, so heat transfer is controlled with on-off control (cycling). When the fan is on, it draws full power.
There are two common methods that control motor speed: Variable Speed Drives (VSD) and two-speed motors. Fans won't consume full power at partial capacity, saving power and energy.
Methods to control motor sped include solid state drives that can be applied to AC motors to reduce the motor speed.
When condensing pressure rises above the pressure settings, fans increase speed or cycle to increase the condenser heat transfer. If there are multiple single speed-fans, they turn on in separate until all are on, or condensing pressure settles back to the set point.
When multiple fans are used in sequence on a single condenser, if is important to operate them all together after VSD's are installed. There will be a larger savings than if the fans operate in sequence wit VSDs.
LOOK FOR: Singer speed condensing fans. Otherwise identify the type of drive used.
- 5) VSD on evaporator Fans
As with condensers, single-speed evaporator fans either run continuously or cycle on and off depending on refrigerant load. When the refrigeration load is high, fans operate at full power. When the refrigeration laod decreases, the fans can turn off to save energy. refrigeration load will decrease once the cooled space and medium reach the temperature setpoint. Excess energy is wasted and airflow is only required to keep the air from stratifying in the cooled space. As in the condenser, a VSD and controls will reduce part-load fan power.
LOOK FOR: Evaporator fans that operate continuously (best opportunity) or cycle to control space temperature.
- 6) Avoid Excess Refrigeration Loads
In coolers or freezers, there are other refrigeration loads beyond the product: exterior heat gain, excess or inefficient lighting, and inefficient doors and seals. In a space with poor insulation, a large portion of the refrigeration load may be external heat gain from warm air and the sun. Poorly insulated walls, ceilings and doors, doors that spend a lot of time open, and poorly functioning air curtains or automatic doors also allow heat gain.
Energy added to the room by lights increase refrigeration load. Use low-wattage, efficient added to the room by the lights increase refrigeration load. Use low-wattage, efficient lighting to save energy at the compressor as well as in the lighting system. Some lamps can't be used in cold temperatures , so take care in recommending alternate lighting.
LOOK FOR: Temperatures in the freezer higher than desired. Cold exterior surfaces, warm interior walls or slow doors are signs of efficiencies. Incandescent lamps and high lighting levels in the cooled spaces.
- 7) Install Economizer
An economizer is used on screw compressor to obtain slightly lower refrigerant temperatures than possible with a simple cycle. Reciprocating compressors cannot be economized, because there is no way to inject refrigerant gas at an intermediate pressure.
LOOK FOR: Simple refrigeration systems used to achieve low temperatures.