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A large percentage of compressor failures in low-temperature vapor-compression refrigeration system applications originate from overheating. Typical compressor capacity rating data is based on an industry standard of a 65°F return vapor temperature. While a 65° return vapor temperature may be satisfactory for high-temperature applications and, perhaps, some medium-temperature applications, unsatisfactorily high-discharge temperatures are virtually unavoidable on low-temperature applications. This is due, in large part, to the thermodynamic properties of the refrigerant.
Refrigeration oils have been highly refined in an effort to elevate the temperature at which chemical decomposition occurs, and experience has shown mineral oil decomposition begins when compressor internal temperatures reach 350°. Compressor manufacturers typically indicate that discharge gas temperatures are 50° to 75° higher at the discharge valve inside the compressor than out on the discharge line, 6-8 inches from the service valve. Therefore, dangerously high temperatures already exist inside the compressor when discharge line temperatures are well below 350°. This lubricant decomposition is accelerated by residual contaminants that are present in a typical system. Studies have shown the rate of chemical reaction doubles with every 18° temperature increase. For example, a chemical reaction that takes 10 years to complete at 100° will only take five years to complete at 118°. At 136°, it would be completed in two-and-a-half years.
However, mineral oils are vulnerable to losing the lubrication film necessary to prevent metal-to-metal contact between bearings and journals, or piston rings and cylinders, prior to the temperature at which decomposition begins. With mineral oil, this occurs approximately between 310° and 330°. When these compressors’ internal temperatures are reached, the probability of extreme piston and ring wear is imminent.
Hydrocarbons (HCs) and other natural refrigerants were employed in early vapor-compression refrigeration application designs. Many of these were highly flammable, toxic, or both. Developments in the late 1920s and into the early 1930s delivered a small group of organic compounds — the chlorofluoocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) for use as refrigerants — and CFCs and HCFCs proliferated into all types of refrigeration and comfort cooling applications. This continued unimpeded for nearly 50 years. These refrigerants typically were selected for various applications in large part based on their favorable thermodynamic properties relative to the system type in which they were to be applied. That is, CFC-12 was predominantly used for medium-temperature refrigeration, CFC-502 was predominantly used for low-temperature refrigeration, and HCFC-22 was principally used for comfort cooling.
However, in the late 1970s, environmental concerns with CFCs and HCFCs arose owing to scientific theories linking these organic compounds to concerns of global ozone depletion. Chorine-containing CFCs were quickly targeted for elimination from use as refrigerants. HCFCs, which contained a smaller amount of chlorine, also were earmarked for eventual elimination.
To speed the move away from CFCs, HCFC-22 initially was more widely specified for new and retrofit use in both medium- and low-temperature refrigeration systems. Additionally, a number of zeotropic blend refrigerants containing predominantly HCFC-22 appeared on the scene and were deployed as interim replacements for CFC compounds.
Prior to this, HCFC-22 had already been in wide use in systems with higher saturated suction temperatures for comfort cooling. However, the extended use of HCFC-22 into medium- and low-temperature refrigeration applications was not without its challenges.
Because of the thermodynamic properties of HCFC-22 — which result in decreased vapor density and increased compression ratios — lower saturated suction temperatures clearly would result in unsatisfactorily high discharge temperatures and the associated oil decomposition and lubrication problems without further intervention.
Consequently, as HCFC-22 was considered to replace CFC refrigerants in medium- and low-temperature refrigeration applications, a need was quickly recognized to address these inherent high-discharge temperatures to ensure a reasonable life expectancy of compressors.
Compound compression techniques were implemented to combat high compression ratios, and liquid injection methods, including the use of desuperheating thermostatic expansion valves (TEVs) to address high discharge temperatures, were adopted for new equipment installations.
One cost-effective method of preventing destructive superheat at the compressor for these HCFC-22 medium- and low-temperature systems was the introduction of the temperature responsive expansion valve (TREV). Using liquid from the high side of the system, the TREV injects saturated refrigerant into the suction line ahead of the compressor in direct response to the temperature of the discharge line.
This valve does not have an equalizer line and is not influenced by system pressure as a desuperheating TEV is. A sensing bulb is placed on the compressor discharge line 6 inches from the compressor discharge outlet connection. The valve responds directly to a rise in discharge gas temperature. Temperature settings are available from 115° through 275°. A bellows assembly similar to that employed in a direct-acting pressure regulating valve is used. The valve power element is charged with a hydraulic fluid that expands as its temperature increases. Multiple capacity sizes are offered for HCFC-22 to a nominal capacity maximum of 5 ton (see Table 1 below).
A liquid supply line to the TREV is connected to a high side location that will provide vapor-free liquid to the valve inlet. The TREV is designed to feed saturated refrigerant directly into the compressor suction line 12-18 inches before the compressor. Other locations are possible, and the suitability of these locations should be determined by proper testing and evaluation. Good thermal contact between the sensing bulb and the discharge line is essential, and insulating the bulb with a high-temperature insulation, such as fiberglass or its equivalent, is recommended.
From the onset of the more extensive use of HCFC-22 to replace CFCs in medium- and low-temperature systems, it was recognized that eventually it, too, would be eliminated from use. Chemical companies conducted ongoing research and development to offer suitable long-term substitute refrigerant compounds, and hydrofluorocarbon (HFC) refrigerants with zero ozone-depletion potential were developed. HFC-404A and -507 quickly became the commonly specified refrigerants for medium- and low-temperature refrigeration systems.
HFC-404A and -507 were considered “cooler” refrigerants by comparison to HCFC-22 regarding discharge temperature. However, even with these HFC refrigerants, destructive superheat caused by higher condensing temperatures has been experienced periodically in some geographical areas of the U.S. during operation in peak summer conditions, specifically in low-temperature applications. Liquid injection has continued to be employed but with less frequency.
As compressor designs evolved, an inherent reduction in compressor temperatures was achieved on specific applications. These design improvements fall into several areas, such as reduced contact between the motor and compressor housing, improved motor designs, reduced volumetric clearances due to improved machining tolerances, and minimization of heat transfer between the high- and low-pressure areas of the compressor.
Meanwhile, the international scientific community initiated concern over global warming. Concerns about greenhouse gases enter the worldwide consciousness, and chemical compounds were evaluated for their global warming potential (GWP). This led to the introduction of HFC refrigerants with comparable thermodynamic properties but lower GWPs than HFC-404A and -507 for medium- and low-temperature applications. HFC blend refrigerants designated as HFC-407A and -407F were developed. Compared to HFC-404A, (GWP = 3,922), HFC-407A (GWP = 2,107) offers a reduction of just less than 50 percent and HFC-407F (GWP = 1,824) provides a reduction of just more than 50 percent in GWP. Of these two refrigerants, HFC-407F exceeds in capacity and efficiency. However, discharge temperatures for HFC-407F are greater by comparison than that of HFC-404A and -507.
Once again, a means of reducing discharge temperatures to maintain quality of lubricant and extended compressor life was desired. Liquid injection using the TREV, while having been developed originally to address high discharge temperatures associated with HCFC-22 use on low-temperature applications, has proven to be a suitable device for application with HFC-407F as well. Because it is a temperature-responsive valve, which is not impacted by system pressures, this liquid injection valve may be satisfactorily applied in HFC-407F applications to eliminate potential lubricant decomposition due to high-discharge temperature operation. And, it can be field-applied as needed to existing systems as they are converted from a previously specified refrigerant to HFC-407F.
The chemical companies continue to pursue safe, low-GWP refrigerants while maintaining suitable performance characteristics to replace those that have gone before. The most recently introduced low-GWP refrigerant alternatives are hydrofluoroolefin (HFO) compounds. Pure HFO refrigerants have GWP numbers below 1, meaning they are of less concern regarding global warming than even carbon dioxide. They are safety-classified as A2L by ASHRAE (nontoxic but mildly flammable); however, it was discovered that by blending HFOs with HFCs, the mild flammability is mitigated, resulting in an ASHRAE classification of A1 (nontoxic and nonflammable) although with an increase in the GWP number.
These HFO/HFC blend refrigerants are entering the marketplace. They are considered as chemically safe, similarly performing refrigerant alternatives with a markedly lower GWP than HFC-404A and -507. HFO/HFC-448A and HFO/HFC-449A are proving to be very suitable alternatives to continue the transition away from HCFC-22. Additionally, they are also considered useful substitutes for the higher-GWP HFC-404A and -507.
In the evaluation of HFO/HFC-448A and HFO/HFC-449A, it has been recognized that, once again, dangerously high discharge temperatures and the associated probability of lubricant decomposition may be unavoidable, particularly on low-temperature systems. Thankfully, the TREV continues to be available as an effective component that may be deployed to allow systems to operate at acceptable compressor discharge temperatures with these new blended refrigerant compounds.
The TREV, as a true temperature responsive valve that is not influenced by system pressures, continues to be an effective problem-solving tool for new alternative refrigerants that may experience potentially destructive high compressor discharge temperatures. It will also more than likely continue in this role as the refrigerant landscape continues to unfold.
HVACR Tech Tip Article contributed by Gene Ziegler, technical training manager, Sporlan Division of Parker Hannifin
Additional resources on HVACR Tech Tips:
HVACR Tech Tip: Guide to Servicing Blended Refrigerants
HVACR Tech Tip: When Should a Catch-All Filter-Drier be Changed?
HVACR Tech Tip: Obtaining Oil Samples in a Refrigeration or Air Conditioning System
15 May 2018
Monitoring the acid level in a refrigeration or air conditioning system is of the utmost importance. Contaminants such as water and air are important factors in the formation of acids in the system. Also, high discharge temperatures caused by dirty condensers, high compression ratio, high superheat of suction gases returned to the compressor, fan failure, and other factors can cause oil to exceed its thermal stability and create oil breakdown.
In addition to high discharge temperatures, there are certain catalytic materials that contribute to the oil-refrigerant mixture breakdown. The most noted of these in a refrigeration system is iron. In adverse conditions, materials like iron provide a location (surface) for acid generating reactions to occur.
Sufficient acid within a refrigeration system is detrimental to system materials and may cause a compressor to fail. Therefore, monitoring acid levels within a system alerts the user of potential problems caused by acidic conditions.
Various methods exist for obtaining an oil sample from a refrigeration or air conditioning system. Since less than one ounce is required for the Sporlan TA-1 Acid Test Kit, any of the following methods are satisfactory. Or, if a detailed laboratory analysis of the lubricant is desired, the Virginia OA-1 Oil Analysis Kit is recommended.
Refrigeration rack systems typically have compressors with oil drain ports. Oil analysis can be accomplished by draining an oil sample into the oil vial of the acid test kit. Often, a technician encounters a refrigeration system where oil sampling from the compressor cannot be accomplished easily.
Usually, the compressor would have to be disassembled and a small amount of oil drained from the suction port of the compressor. Since this is a labor intensive procedure, systems of this type are rarely checked for acid concentration until it is too late.
For refrigeration rack systems, the technician may want to use the following suggestions to make oil sampling more accessible.
Method one is ideal for a technician who is installing a new split system, or installation of a new compressor after a compressor burnout, and would like to obtain an oil sample after some runtime.
Method two is for a technician who needs an oil sample from an operating system.
The method shown in Figure 1 involves installing a small oil trap in the suction line of the refrigeration system. The oil trap is designed to have an oil sample tube long enough to hold approximately 1 ounce of oil and an adaptor installed in the suction line, ahead of the shell. The adaptor should be a Schrader fitting with a cap.
Note: Be sure to purge the remaining amount of oil in the trap so the next oil sample is representative of the oil circulating in the system.
This method is ideal for a technician who needs to obtain an oil sample without opening the system. This method is applicable for systems with service valves.
Construct the trap as shown in Figure 2 and connect it to the service valves with flexible refrigeration hoses. The trap should be connected to each access port and held upright to ensure the oil sample tube is collecting oil. The side connection is the inlet fitting and is connected to the discharge service valve. The suction service valve is connected to the top or outlet connection. When the oil level is visible in the sightglass, isolate and relieve pressure from the trap, then drain the oil sample from the device.
Note: This trap should be constructed in advance so it is ready to be used when the need arises. Also, the device can be reused. Flush the trap thoroughly with a readily available flushing solvent, drain, and evaporate solvent residues by evacuating the trap with a vacuum pump. Following these steps will ensure the trap is ready for the next sampling.
These two methods will help you to provide accurate monitoring of the acid level for your refrigeration or air conditioning system diagnosis.
For more information on the Sporlan Test-All Oil Acid Kit test see page 42 of Bulletin 40-10. For more information on the Virginia Oil Analysis Kit see Form P-452.
Article contributed by Chris Reeves, product manager, Contaminant Control Products, Sporlan Division of Parker Hannifin
For more articles on climate control:
Use of Suction Line Filter-Driers for HVAC Clean-up After Burnout
Clean-up Procedure for Refrigeration and Air Conditioning Systems
Compressor Overheating Is the Number-One Refrigeration Problem
10 Apr 2018
Here is information on Psychrometrics that should help remind the HVACR technician what this subject is all about... with perhaps a few tidbits the tech may not have known! Psychrometrics: the study of the physical and thermal properties of dry air and water vapor mixtures.
Degree of saturation (μ): See relative humidity.
Dew point temperature (tdp): Temperature at which water vapor starts to condense in the air.
Dry air: Air devoid of water vapor and pollutants. Dry air has relative humidity of zero.
Dry bulb temperature (tdb): Actual temperature of the air, as observed using a thermometer or temperature sensor.
Enthalpy (h): Total useful energy content in the air. It is the sum of the enthalpies of the dry air and water vapor.
Humidity ratio (W): The ratio of the water vapor in the air to the dry air. This value is often multiplied by 7000 grains/ lb and expressed simply as humidity in grains of moisture.
Relative humidity (RH): The ratio of the mole fraction of water vapor to the mole fraction of water vapor with saturated air. If you don’t like the term “mole fraction”, it is also the ratio of the partial pressure of the water vapor to the partial pressure of water vapor with saturated air. If you don’t like the term “partial pressure”, it simply refers to the fact that both water vapor and dry air exert a component pressure that sums up to the total air pressure. If you want to think of relative humidity as the ratio of water vapor in the air compared to the water vapor in saturated air, that’s ok, but it is not technically correct. This value is actually the degree of saturation, which happens to be close to the value of relative humidity.
Saturated air: Air having a relative humidity (and degree of saturation) of 100 percent. At this condition, air is also at its dew point temperature. See the “Concerning Dry Air and Water Vapor Mixtures” section below.
Specific heat ratio (SHR): The ratio of the sensible heat load to the total heat load. Matched air conditioning systems typically have SHRs in the 68% to 80% range. Systems having a low SHR will remove more moisture from the air than systems having a high SHR. SHR can also be used to determine the required supply air temperature to maintain a room at desired conditions.
Specific volume (v): The volume occupied by a unit mass of dry air.
Psychrometer: A device used to measure relative humidity. It consisting of two thermometers, one that measures wet bulb temperature, and the other dry bulb temperature.
Psychrometric state: The state of an air sample. It is represented as a point on a psychrometric chart.
Standard air (for fan ratings): Air having a density of 0.075 lb/ft3 at 70°F and 14.696 psia (29.921 in. Hg) barometric pressure. Used to rate fans in standard cubic feet per minute (SCFM).
Standard atmosphere: Reference for estimating properties at various altitudes. It is air at 59°F and 14.696 psia (29.921 in. Hg) barometric pressure.
Wet bulb temperature (twb): Temperature of a wetted wick thermometer exposed to high velocity air. It is normally used with dry bulb temperature to determine relative humidity.
It is a misconception that water vapor is somehow held, absorbed, or dissolved in the air. Water vapor is only a resident in the air, somewhat like dust. Air acts as a “transporter” of water vapor. But unlike dust, atmospheric water constantly changes state, and it is a major regulator of air temperature.
The term “saturated air” is a bit of a misnomer as it suggests water vapor is absorbed or dissolved in the air. In this context, “saturation” simply refers to the state of water vapor, and that water vapor and dry air behave largely independent of each other.
Adiabatic mixing: Mixing of two or more air streams while maintaining constant enthalpy (no heat loss or gain).
Cooling and dehumidifying: Reducing both the dry bulb temperature and humidity ratio of the air.
Evaporative cooling: Reducing the dry bulb temperature and increasing humidity ratio of the air while maintaining constant enthalpy (no heat loss or gain).
Heating and humidifying: Increasing both the dry bulb temperature and humidity ratio of the air.
Sensible cooling: Removing heat from the air without changing its humidity ratio.
Sensible heating: Adding heat to the air without changing its humidity ratio.
We hope this blog helps you in your HVACR career and you learned some valuable information along the way. If you need other helpful documents on HVACR related topics please visit our additional blogs below.
HVACR Tech Tip article contributed by John Withouse, senior engineer - refrigeration, Sporlan Division of Parker Hannifin.
Related content for you:
HVACR Tech Tip: Can You Have Subcooled Refrigerant in the Receiver?
Using P-T Analysis as a Service Tool for Refrigeration Systems
HVACR Tech Tip: Where Should the TEV External Equalizer Be Installed?
13 Mar 2018
Superheat is the temperature of refrigerant gas above its saturated vapor (dewpoint) temperature. Superheat as it relates to thermostatic expansion valves, can be broken down into three Superheat categories:
Static Superheat – The amount of superheat necessary to overcome the superheat spring force biased in a closed position. Any additional superheat (force) would open the valve.
Opening Superheat – The amount of superheat necessary to open the valve to its rated capacity.
Operating Superheat – The superheat at which the valve operates at normal running conditions or normal capacity. The operating superheat is the sum of the static and opening superheat. The figure below illustrates the three superheat categories. The reserve capacity, as shown in the graph, is important since it provides the ability to compensate for occasional substantial increases in evaporator load, intermittent flash gas, reduction in high side pressure due to low ambient conditions, shortage of refrigerant, etc.
For more information, download TEV &AEV Theory and Application - Catalog E-1a.
Determine the suction pressure at evaporator outlet with gauge. On close coupled installations, suction pressure may be read at the compressor suction connection.
Use the Pressure-Temperature Chart to determine saturation temperature at observed suction pressure. For example, with a R-22 system: 54.9 psig = 30°F.
Measure the temperature of suction gas at the expansion valve’s remote bulb location. For example: 40°F.
Subtract saturation temperature of 30°F (step 2) from suction gas temperature of 40°F (step 3). The difference, 10°F, is the superheat of the suction gas.
Parker “sets” the thermostatic expansion valve superheat at the static condition described above. Turning the adjusting screw clockwise will increase the static superheat. Conversely, turning the adjusting screw counterclockwise will decrease the superheat. Parker valves can also be adjusted at the operating point, indicated above. When a system is operating, any adjustments made will change the operating superheat. The static superheat range of adjustment is 3°F to 18°F. One full turn clockwise will typically increase superheat 2°F to 4°F. Note: Refer to the valve’s installation bulletin for specific directions on superheat adjustment.
For more information, download TEV &AEV Theory and Application - Catalog E-1a.
For more details on Thermostatic Expansion Valves - Theory of Operation, Application, and Selection - Bulletin 10-9.
Article contributed by Glen Steinkoenig, product manager, Thermostatic Expansion Valves, Sporlan Division of Parker Hannifin.
Related, helpful climate control content for you:
HVACR Tech Tip: 12 Solutions for Fixing Common TEV Problems
HVACR Tech Tip: Understanding and Preventing Superheat Hunting in TEVs
HVACR Tech Tip: 12 Solutions for Fixing Common TEV Problems
13 Feb 2018
In 2017, many changes impacted the HVACR, HVAC climate control industry. New technologies emerged that not only changed the way we do business – most for the better! But also brought a shift in the way we do our jobs, our way of thinking. Flame-free refrigerant fittings? Changes in refrigerant choices for retrofits? Troubleshooting a refrigeration system in the field? The smartest technicians understand the importance of not only staying informed but having a go-to resource for reference while on the job. Our Climate Control engineers team are dedicated to bringing you the knowledge you need when you need it through this blog; so you can do your job smarter, more efficiently, more profitably.
Answers to your questions and solutions to your challenges can be found in the top 5 most read blogs in 2017 below which addressed -
As HVACR technicians, you need some ideas in your back pocket for basic troubleshooting in a refrigeration system. How about a simple chart that helps you diagnose a system with 3 data points for starters? Using this chart is simple and can greatly speed up the troubleshooting of a system while in the field.
A common problem facing refrigeration and air conditioning service technicians and contractors is superheat hunting by thermostatic expansion valves (TEVs). Here is a better understanding of a commonly overlooked cause of superheat hunting and how the problem might be corrected.
Refrigerant choices for refrigeration systems are undergoing significant change, including choices for retrofits and new systems. This article is Part 1 of a 3 part series addressing such retrofits and deals with the basics of refrigerant blends and temperature glide.
The thermostatic expansion valve (TEV) provides an excellent solution to regulating refrigerant flow into a direct expansion type evaporator. The TEV controls the flow of liquid refrigerant entering the direct expansion (DX) evaporator by maintaining a constant superheat of the refrigerant vapor at the outlet of the evaporator. To understand the principles of TEV operation, a review of its major components is necessary.
Six questions and answers that will help you learn the key points of what you need to know about flooded head pressure control.
Watch the video and learn more about our Climate Control technologies:
11 Jan 2018
The HVACR service technician uses an array of tools and test instruments to diagnose problems and evaluate system performance. One tool that is readily available, inexpensive and yet rarely used to its fullest advantage is the pressure-temperature card or P-T Chart.
Proper analysis of the pressure to temperature relationship can help service technicians diagnose a refrigeration system issue quickly. The ChillMaster P-T Chart allows quick pressure to temperature conversion by providing essential refrigerant data to mobile devices. Download the app today in IOS or Android version
Contractors and technicians will enjoy the P-T Chart’s user-friendly design and precise calculations based on National Institute of Standards and Technology (NIST) Refrigerant Properties. Other features include the ability to customize screens for certain refrigerants and preferred units of measurement.
More than 70 refrigerants (traditional and natural) are featured in the P-T Chart. Extras included with the app - a training article “Using the P-T Card as a Service Tool.”
The app is available for both iOS and Android platforms. Click on your chosen platform to download the ChillMaster P-T Chart app for free.
Other helpful resources:
How to Use the Smart Service Tool Kit for HVACR Diagnosis
Wireless Transmission of Performance Data Extends Equipment Life
How the “Information Anywhere” Revolution Helps Boost Production
Use of Suction Line Filter-Driers for HVAC Clean-up After Burnout
15 Nov 2017