Suction accumulator

What is the purpose of a suction accumulator?

A suction accumulator is used to prevent liquid refrigerant flooding back to the compressor.  Accumulators are commonly used on heat pumps, refrigeration for transport, refrigeration system to cold rooms, and any place that liquid refrigerant in the return may cause concern.

The accumulator is installed in the suction line before the compressor. Generally, it is a vertical recipient with an internal U-shaped tube. At the top, opposite of U-shaped tube, its outlet is located just below the top of the recipient. This allows the accumulator to become practically full before the level of liquid reaches the accumulator outlet.

A small diameter hole is drilled in the bottom of the U-shaped tube at its lowest point. This hole allows for recovery of any oil that may have accumulated, which will return to the compressor through this hole.

Sometimes heat source inside the recipient is necessary to evaporate the liquid refrigerant. This can be an electrical resistor or heating jacket in the body of the separator. Some accumulators have connections for a trap in the liquid line at the bottom of the accumulator which is cooled. This improves system performance by undercooling the liquid refrigerant and protects the compressor from liquid slugging, while superheating the suction gas.

High discharge temperature

What are the signs of high discharge temperature?

Signs of high discharge temperature are discolored valve plates, burned valve reeds, worn pistons, rings and cylinders, or twisted stator due to metal debris.

What causes high discharge temperature?

High discharge temperature is the result of temperatures in the compressor head and cylinders becoming so hot that the oil loses its ability to lubricate properly.  This causes the rings, pistons and cylinders to wear out, resulting in leakage, leaky valves, and metal debris in the oil.

What can be done to avoid high discharge temperatures?

(1) Correct abnormally low load conditions.

(2) Correct high discharge pressure conditions.

(3) Insulate the suction lines.

(4) Provide proper compressor cooling.

Global warming

What is global warming?

The US Environmental Protection Agency (EPA) defines global warming as “an increase in the surface temperature of the earth.”

Global warming occurred in the distant past as a result of natural influences, but the term is most often used today to refer to global warming expected to occur as a result of increased emissions of “greenhouse gases.” The release of refrigerants into the atmosphere is considered a major factor contributing to the increase in global warming. In general, scientists agree that the earth’s surface has warmed by about 1 degree Fahrenheit over the past 140 years. Although this does not seem like a big change, atmospheric scientists are concerned about this trend of general warming and the impact it has on many aspects of our lives, such as economic prosperity, agricultural production, and pollution.

What is “direct” and “indirect” global warming?

Direct global warming is the measure of global warming potential (GWP) that each greenhouse gas contributes to the warming process if it is released “directly” into the atmosphere

Indirect global warming considers the amount of contributing effect to global warming by the manufacture of greenhouse gases and their efficiency of operation.  In other words, it takes energy from power plants, which also emit greenhouse gases, to manufacture the gases and operate the equipment that the greenhouse gases are used in. An example of such equipment would be an air conditioner with a SEER of 10 versus one with a SEER of 13. The SEER-10 unit has a higher direct potential, since it does not work as efficiently.

What is the total equivalent warming impact (TEWI)?

TEWI is the sum of a greenhouse gas’s direct and indirect GWP.  This value takes into consideration both the direct factor of release of the gas into the atmosphere and the indirect factor of the manufacture and lifetime operation of the system in which the gas is used. This factor is important because some greenhouse gases can have a low direct impact on GWP, but require more energy to be manufactured or do not work as efficiently as other gases with higher direct GWP impact.

Solenoid Valves

How are solenoid valves rated?

Solenoid valves are rated in terms of Maximum Operating Pressure Differential (MOPD) against which the valve opens. For example, with the valve closed against an inlet pressure of 250 psi and an outlet pressure of 50 psi, the pressure differential across the valve is 250-50 or 200 psi.

The MOPD rating for the valve is the maximum pressure differential against which the valve will operate reliably. If the pressure differential is greater than the rated MOPD for the valve, the valve will not function properly.

The coil winding temperature and applied voltage have a significant influence on the MOPD rating. The MOPD is reduced as the temperature of the coil increases or the voltage decreases. For this reason, the MOPD rating is established by valve operation at 85% of rated voltage, rated after the coil has reached the maximum temperature, operating at the full rated voltage.

Why do some valves have a minimum OPD rating and what does that mean?

Minimum OPD is the minimum operating pressure differential.  All pilot-operated valves (such as our series 200 and 240) require a small amount of pressure differential to enable the piston or diaphragm to rise off the main seat.  Typically, 2 to 5 psig differential is needed to accomplish this.  If the pressure differential is less than the Minimum OPD, then the valve will not open when actuated or will fail to remain open.

If the valve is much larger than necessary for the application, it may suffer this influence since the pressure drop across the valve with low flow rates may be lower than the minimum OPD.

The solenoids for direct operation (such as our 50RB and 100RB) do not have a minimum OPD specification, since they do not rely on a pressure system for operation.

Pressure Controls

What are pressure controls for?

There are two main categories of pressure controls: high pressure and low pressure.  These controls may be individual or combined into one control.

The main function of low pressure control is to turn off the compressor when the suction pressure becomes too low. This is to protect the compressor from overheating and/or to prevent the product from freezing.

High pressure control is a safety control to protect the compressor from operating at excessive discharge pressures. High pressure control should be pre-set by the manufacturer and should never be set higher than the factory setting. Most have a stop to prevent them from being high in the field, but can be adjusted to a lower setting. The control setting is determined by the refrigerant fluid used in the system and its operating range, although the same compressor may be used.

While the high pressure control can be adjusted manually or automatically, low pressure controls are almost always automatic. Some controls can be converted from automatic to manual in the field, if desired.

There are other applications for pressure control in the cooling system. These include condenser fan cycling, oil pressure safety switches, and lock-out for heat recovery.

Measuring device

How does a thermostatic expansion valve work?

Many air conditioning systems incorporate a metering device in the Thermostatic Expansion Valve style, as standard equipment. It is extremely important that HVAC technicians understand the design and operation of these valves. If adequate service practices are not followed, this could result in serious damage to the system.

When charging the system, follow the manufacturer’s recommendations. If additional charge is required due to long lengths of the rows of the assemblies and the system includes a thermostatic expansion valve measuring device, the charging must be done with relation to the undercooling at a greater charge. The greatest chance of the thermostatic expansion valve’s losing control of the evaporator charge is during this time. If undercooling is present during greater charge, sufficient refrigerant is circulating throughout the system to control the charges of the evaporator.

To adjust the superheating of the evaporator coil, follow the manufacturer’s recommendations. If these are not available, the following recommendations may apply, depending on the design temperature of the system:

High Temperature 4°C – 7°C

Medium Temperature 3°C – 5ºC

Low Temperature 1.5°C – 3.5ºC

THERMOSTATIC EXPANSION VALVE

P1 = Bulb pressure (opening force)

P2 = Evaporator pressure (closing force)

P3 = Superheating spring pressure (closing force)

P4 = Liquid pressure (opening force)

Pressure balance Equation  Thermostatic Expansion Valve

P1 + P4= P2+ P3

Refrigerant distributors

What does a refrigerant distributor do?

Distributors are used on multi-circuited evaporator fins. By using multiple circuits in evaporators, the pressure drop through the evaporator is minimized. The purpose of the distributor is to provide equal feeding of the refrigerant to each individual circuit.  Because of this, it is important that each connecting tube from the distributors to the evaporator is the same size and length. Additionally, it is recommended that the distributors be installed in a vertical position to maintain equal flows under low load conditions.

There are two distributors commonly used: nozzle or Venturi. The nozzle type uses an orifice plate to generate the pressure drop that creates turbulence to provide equal feed of the circuits. Venturi type distributors use an internal Venturi design to provide equal flow to the circuits. Because the Venturi type does not depend on any turbulence to equalize the feed to the circuits, there is a very low pressure drop through it. In both cases, an externally equalized expansion valve should always be used with a distributor because of the pressure drop that the distributor generates.

Thermostatic Valve Equalizer

What’s the difference between a thermostatic expansion valve equalized internally or externally?

An internally equalized Thermostatic Expansion Valve uses the evaporator inlet pressure to create the pressure acting on the valve. An externally equalized valve uses the evaporator outlet pressure, thereby compensating for any pressure drop through the evaporator, for the same purpose.

If an internal equalization valve is used in a system with a large pressure drop through the evaporator, the pressure below the diaphragm will be greater, causing the valve to enter a more limited opening position, resulting in superheating higher than desired (lack of gas).

When should I use an external equalizer in the Thermostatic Expansion Valve?

  1. In any large system, generally more than 1-ton capacity.
  2. In any system that uses a distributor.

Note: For field replacement, you can always replace an internally equalized valve with an externally equalized valve, but you should never replace an externally equalized valve with an internally equalized valve.

If I need to replace an internally equalized valve and all available valves are of the externally equalized type, can I just “plug up” the equalizer connection?

No, the equalizer must be connected to the suction line at the bulb. Plugging up the equalizer connection will prevent the valve from operating properly.

Will an externally equalized Expansion Thermostatic Valve allow the system pressures to be equalized during the off cycles?

No, an externally equalized valve will not allow system high and low sides to equalize during the off cycle.  The only way to do this is through by using a Thermostatic Expansion Valve.

Where should the external equalizer be installed?

The external equalizer line should be installed on top of the suction line before any traps and located within 15 cm of the sensing bulb position.  If this is not possible, and a different location is required, it must first be confirmed that the pressure at the desired location is identical to the pressure at the bulb height

Q: What happens if the equalizer tube gets kinked?

If the equalizer line becomes kinked or bent, the pressure sensed at the underside of the diaphragm will no longer correspond to the evaporator outlet pressure and the valve will not be able to operate as intended.

I’ve seen some equalizer tubes “frosted”. Is this normal?

Frost on the equalizer line is an indication that the packing seal has failed, allowing higher pressure refrigerant to leak past and expand into the equalizer line.  Depending on the valve type, either the cage assembly or the entire valve should be replaced.

Heat exchanger

What is the advantage of using a liquid heat exchanger for suction?

A liquid heat exchanger for suction is beneficial in the following ways:

  1. It provides undercooling of the liquid refrigerant before it enters the expansion valve. This eliminates the possibility of formation of a gas spark in the liquid line and allows the expansion valve to operate with greater stability.
  1. The undercooling of the refrigerant increases system efficiency.
  2. The heat is transferred from the liquid to the suction increases the superheating of the suction gas, thereby reducing the possibility of liquids returning to the compressor. The return of liquids to the compressor is considered as one of the major causes of compressor failure, so any steps taken to minimize this will result in improved reliability of the compressor.

Flooding

What does ‘flooding’ mean?

Flooding (also known as ‘flood back’) is the term used to describe the condition when liquid refrigerant reaches the compressor.  This typically occurs when the amount of liquid fed to the evaporator is more than can be evaporated. There are a number of possible causes for flooding, including:

  • Expansion thermostatic valve oversized for the application
  • Expansion thermostatic valve poorly adjusted (overheating also 10C)
  • System overcharged with refrigerant
  • Insufficient air flow across the evaporator
  • Dirty evaporator
  • Evaporator fans inoperative
  • Bulb of the Thermostatic Expansion valve not properly adapted

Refrigerant flooding

What is refrigerant flooding?

Refrigerant flooding is a result of the return of liquid refrigerant to the compressor during the operating cycle. This oil is diluted with the refrigerant to the point where it cannot properly lubricate the surfaces of the load bearings.

What are the signs of refrigerant flooding in an air-cooled compressor?

Worn cylinders and pistons without evidence of overheating.

How does refrigerant flooding occur in an air-cooled compressor?

The liquid washed the oil off the pistons and cylinders during the suction stroke causing them to wear during the compression stroke.

What are the signs of refrigerant flood back in a refrigerant-cooled compressor?

The center and rear bearings are worn or seized; there is a dragging rotor and shorted stator, a progressively scored crankshaft, and worn or broken rods.

How does refrigerant flood back happen in a refrigerant-cooled compressor?

The liquid dilutes the oil in the crankcase and the refrigerant rich oil will be pumped to the rods and the bearings through the crankshaft.  As the refrigerant boils off, there will not be enough oil for sufficient lubrication at the bearings farthest from the oil pump.  The center and rear bearings may seize or may wear enough to allow the rotor to drop and drag on the stator causing it to short.

What can be done to prevent the refrigerant flooding?

(1) Maintain proper superheating of the evaporator and compressor.

(2) Correct abnormally low load conditions.

(3) Install accumulators to prevent uncontrolled flood back.

Cleaning the System

What kind of filter-dryer is recommended for a system after a compressor motor burn?

After a burn, both the cores and the liquids in the suction lines (if provided) should be replaced with special ‘burn blocks’. These cores have an “HH” in their naming convention model. The system is then operated with these cores positioned until the refrigerant and oil are clean and free of acids. Once this condition is met, the liquid core must be replaced with a type of standard core (UK48 or H48). The suction core should be replaced by an F-48 filter.

My system doesn’t have a removable suction core and there is not enough room to install one. What should I do?

In smaller independent systems, where a removable suction core cannot be installed, you must install a suction line hermetic filter dryer (ASK-HH). This should be left in the system until the refrigerant and the oil are cleaned, and then removed or replaced with a new one to avoid excessive pressure drop.

How do I know when suction filters should be replaced?

A humidity indicator in the sight glass or a differential pressure higher than the equipment manufacturer’s recommendation indicates the need for replacement.     If the equipment manufacturer’s recommendation is not available, the following maximum pressure drops are suggested:

MAXIMUM PRESSURE DROP RECOMMENDED (PSI) OF THE SUCTION LINE FILTER DRYER

The pressure drops indicated in the column titled TEMPORARY FACILITIES must be used only during the cleaning operation as an indication of when the core needs to be replaced. During normal operation, the pressure drop must not exceed the indicated in the column “PERMANENT INSTALLATION.” The operation with high pressure drops in suction filter dryer decreases the efficiency of the system and must be avoided.

Location of the valve bulb

Thermostatic Expansion Valve

What is the correct position for the bulb of the thermostatic expansion valve?

The location of the bulb of the thermostatic expansion valve in the suction line is essential for the proper performance of the thermostatic expansion valve. The following important points are:

  • Clean the suction line next to the evaporator outlet
  • The total length of the bulb must be in contact with the linear part of the suction line
  • The bulb must be placed upstream of the connection of the external equalizer
  • The bulb must be adapted at the 12 o’clock position on all suction lines up to 7/8″ diameter or smaller. In lines larger than 7/8″ diameter, the bulb must be placed at the 4 or 8 o’clock position. The bulb must never be placed at the 6:00 o’clock position.
  • Always externally insulate the whole bulb + the tube after the installation.
  • The bulb can be installed in a vertical suction line if necessary, but never place the bulb downstream of a siphon. The placement of the bulb before the siphon (upstream) is recommended.

Location of the valve bulb

Thermostatic Expansion Valve

What is the correct position for the bulb of the thermostatic expansion valve?

The location of the bulb of the thermostatic expansion valve in the suction line is essential for the proper performance of the thermostatic expansion valve. The following important points are:

  • Clean the suction line next to the evaporator outlet
  • The total length of the bulb must be in contact with the linear part of the suction line
  • The bulb must be placed upstream of the connection of the external equalizer
  • The bulb must be adapted at the 12 o’clock position on all suction lines up to 7/8″ diameter or smaller. In lines larger than 7/8″ diameter, the bulb must be placed at the 4 or 8 o’clock position. The bulb must never be placed at the 6:00 o’clock position.
  • Always externally insulate the whole bulb + the tube after the installation.
  • The bulb can be installed in a vertical suction line if necessary, but never place the bulb downstream of a siphon. The placement of the bulb before the siphon (upstream) is recommended.

Micron

What is a micron?

A micron is a metric measure and is defined as 1 millionth of a meter, or one thousandth of a millimeter.

Most people think that a perfect vacuum is equivalent to 30 inches of mercury (Hg). The last inch (29-30) of the vacuum is equal to 25,400 microns. The micron, then, is a more accurate method for measuring the deep vacuum.

Micron = 0.001 mm Hg c 0.000039 inches of Hg = 1 millitorr

Migration

What is refrigerant migration?

Migration is the term used to describe when the refrigerant moves to somewhere in the system where it should not be, for example, when liquid ‘migrates’ to the crankcase of the compressor. This phenomenon occurs because the refrigerant always migrate to the coolest part of the system. As an example, in a split air conditioning system with external compressor/condenser, the cooling liquid of the evaporator will migrate to the compressor during the winter months because the compressor is cooler than the internal temperature (evaporator). If this is not prevented, dirt and damage may occur to the compressor at startup in the spring.

How can I prevent migration?

There are two common methods used to prevent migration:

  • Use of a “gas collection system”.
  • Use of crankcase heaters to “vaporize” any cooling liquid.

Alternative oils and refrigerants

What is the right oil to use with the new refrigerants?

With the introduction of HFC refrigerants as alternatives to CFC and HCFC refrigerants, the question of the appropriate oil to use still comes up.

The generally preferred oil for use with HFC is a polyolester (POE), which is an additive package for refrigeration applications. The mineral oil (MO) is not recommended because the oil return is considered to be compromised.

Do I have to remove all MO from the system MO in rehabilitation?

If the rehabilitation is for a system with HFC refrigerant, the current recommendations are to remove the mineral oil until only 5% or less is in the system before changing to POE oil). The percentage can be measured by a refractometer.

What are the recommendations on the alkylbenzene oil?

Most temporary HCFC refrigerants can also use alkylbenzene oil (AB), if approved by the compressor manufacturer.  When in doubt about which oil to use with the refrigerant you are using, always consult the compressor manufacturer.

Loss of oil

What are the signs of loss of oil?

The oil loss signs are: all the rods and bearings are worn or beaten, unevenly worn crankshaft unevenly, rods broken from seizure, or little or no oil in the crankcase.

What causes the loss of oil?

The loss of oil is a result of the lack of oil in the crankcase for properly lubricate the loading surfaces. When there is not enough refrigerant mass flow in the system to return the oil as fast as it is pumped out, there will be uniform wear or scratching of all load surfaces.

What can be done to prevent the loss of oil?

(1) Check the operation of oil failure control, if applicable.

(2) Check the refrigerant charge of the system.

(3) Correct abnormally low load conditions or in short cycles.

(4) Check for pipe sizes incorrect and/or all the siphons.

(5) Check for improper defrosts.

Maximum operating pressure

What is meant by MOP (maximum operating pressure and/or motor overload protection) in an expansion valve?

The MOP refers to the maximum suction pressure that is allowed before the expansion valve tends to restrict and dose the flow increase. This is done at the point where the gas in the load of the power element of the expansion valve becomes superheated and can exert opening pressure only slightly higher with the increase of temperature.

The purpose of the MOP is to prevent the suction pressure to rise so high that the compressor motor cannot start due to an initial charge that is too high.

A MOP type valve tends to function as a pressure regulator of the crankcase valve (CPR), however, it will not control as accurately as the CPR valve. It is generally not recommended to use both a CPR type valve and a MOP type valve in the same system, as there is the possibility that they will “fight” with each other, as both try to control.

R-12 and Polyolester Oil

Is the mixture of R-22 and POE oil allowed?

There seems to be a misunderstanding about the use of R-12 and POE oil. Many end users believe there is no problem in using them together, but in fact, they may be creating some real problems: when the humidity is induced in a system using R-12 refrigerant and POE oil, the refrigerant becomes acid and may clog the capillaries and expansion holes.

Refrigerants

Is there a standard refrigerant?

Many air conditioning systems incorporate an HFC refrigerant as the standard. It is extremely important that the HVAC technician understands the properties of these refrigerants. If adequate service practices are not followed, it can result in serious damage to the system.

The air conditioning market is turning to the new and more environmentally friendly R-410A refrigerant, with the elimination of R-22 in 2010. The R-410A will represent a third of the market in 2006, together with the new regulation of high efficiency of 13 SEER.

As the R-410A is classified as an HFC, the only recommended oil is the polyolester oil (POE). The POE oil is highly hygroscopic and will absorb moisture at a fast rate. Tests have shown that the POE oil may be saturated with moisture in less than 15 minutes if exposed to an environment with a relative humidity of 90%.

Selecting the Thermostatic Expansion Valve

How do I select a thermostatic expansion valve for a particular application?

In order to select the thermostatic expansion valve it is necessary to combine the capacity (in tons of refrigeration) of the thermostatic expansion valve with the ability of the evaporator. The following procedure is recommended:

  • Check the refrigerant of the system
  • Determine the evaporator capacity under the operating conditions
  • Determine the temperature of the liquid refrigerant at the entrance of the thermostatic expansion valve
  • Calculate the pressure drop through the thermostatic expansion valve subtracting the suction pressure (low-pressure side) of the condensing pressure (high-pressure side). Subtract the pressure drop of the distributor, if any. The difference is the available pressure drop for the thermostatic expansion valve.
  • See the table of adequate expansion capacity in the catalog for the correct refrigerant to the evaporation temperature of the operation. Then find the nearest pressure drop column of the calculated one that gives the nearest capacity in tons (for the evaporator tonnage). Scroll left to select the specification of the valve nearest the capacity. You will have to recalculate the capacity using the Correction Factor Table for the actual temperature of the liquid if it is different from 38C, used as standard.

Oil separators

What is the purpose of oil separators and how do they work?

The oil separators are used in refrigeration systems where it is difficult for the oil to return from the evaporator.

These systems are typically built on the field such as in supermarkets, and ultra-low temperature system.

The oil separators are installed in the discharge line of compressors. They are generally a vertical container with the gas of the discharge connection in the upper part and an oil return port in the lower part. This return line may be piped directly to the suction line in units of a single compressor or in multiple compressors racks it can be piped to a tank called oil reservoir.  Some oil separators have a reservoir built at the bottom of the container with the top being the separator.

From the reservoir, the oil is then returned to the compressors with a mechanical or electronic control of oil level attached to the crankcase of the compressor.

The oil separators use various oil separation methods to remove the oil from the discharge of gas when leaving the compressor. These methods include speed reduction, shock, centrifugal action or coalescing elements. The oil separators vary in capacity and efficiency, depending on the mass flow being pumped through them and no oil separator is 100% efficient.

Gas collection systems

What is a system of “gas collection” and when should it be used?

A gas collection system consists of a normally closed solenoid valve installed in the liquid line and a low-pressure control that detects the suction pressure. The system operation is as follows:

  • A thermostat is connected to the solenoid valve of the liquid line. When there is need of cooling, the thermostat contacts close. This causes the solenoid coil to be energized, opening the valve. The refrigerant liquid flows into the evaporator and the suction pressure rises above the set point of the low-pressure control. The contacts on the low-pressure control close and the compressor starts.
  • When the thermostat is operational, its contacts open, causing the solenoid valve to close. This stops the flow of refrigerant to the evaporator. As the compressor continues to operate, the refrigerant is pumped out of the evaporator and the suction pressure drops. When the suction pressure reaches the threshold value in the low pressure control its contacts open, stopping the compressor. This removes all refrigerant from the low-pressure side of the system during the “off” cycle.

What is the advantage of the gas collection system?

The advantage of a gas collection system is that all of the refrigerant liquid is stored in the liquid tank and in the condenser when the compressor is not operating. This prevents the migration of liquid into the compressor crankcase during the off cycle and the consequent possibility of liquid when starting the compressor.

Subcooling

What is subcooling?

The subcooling is the condition in which the refrigerant liquid is colder than the lowest temperature (saturation temperature) required for preventing boil and therefore the change of the liquid to a gaseous phase.

The amount of subcooling in a given condition is the difference between its saturation temperature and the actual temperature of the refrigerant liquid.

Why is the subcooling desirable?

The subcooling is desirable for several reasons.

  1. It increases the efficiency of the system since the amount of heat to be removed by pound of refrigerant circulated is increased. In other words, less refrigerant is pumped through the system for maintaining the desired refrigerated temperature. This reduces the amount of time that the compressor must operate for maintaining the temperature. The amount of capacity increase obtained with each degree of subcooling varies according to the refrigerant being used.
  1. The subcooling is beneficial because it prevents the refrigerant liquid enter into the gaseous state before it gets to the evaporator. The pressure drops in the liquid piping and the vertical increases can reduce the refrigerant pressure to the point where it will boil or spark in the liquid line. This phase change causes the refrigerant to absorb the heat before it prepares the evaporator. The inadequate subcooling prevents the expansion valve to properly measure the refrigerant liquid entering the evaporator, resulting in poor system performance.

Overheating

What is “overheating”?

Overheating refers to the number of degrees above the saturation temperature (boiling point) of the steam at a certain pressure.

How do I measure overheating?

The superheat is determined by taking the reading from the gauge at the side of the low pressure and converting this pressure at the temperature using a PT chart and then subtracting this temperature from the actual temperature measured (using an accurate thermometer or thermocouple) at the same point in which the pressure was measured.

Why is it important to know about the system overheating?

The overheating indicates whether the amount of refrigerant flowing into the evaporator is suitable for the load. If the overheating is too high, then an insufficient amount of refrigerant is being fed, resulting in poor cooling and excessive energy consumption. If the overheating is too low, too much refrigerant is being fed, possibly resulting in flooding of the liquid into the compressor and causing damage to the compressor.

When should I check the overheating?

The overheating must be checked whenever any of the following occurs:

  • The system does not seem to be cooling properly
  • The compressor is replaced
  • The expansion thermostatic valve is replaced
  • The refrigerant is changed or added to the system

Note: The overheating must be checked with the system running at full load under steady-state condition.

How do I change the overheating?

Passing the rod setting on the thermostatic expansion valve changes the overheating.
Clockwise Direction – increases the overheating
Anti-clockwise direction – decreases the overheating

Note: To return to the approximate values of the original factory settings, turn the adjusting rod in the anti-clockwise direction until the spring is fully discharged (comes to the stop point or begins to ‘engage’). Then turn it on again in ½ “full of twists” shown in the chart.

Overheating of the compressor

How does overheating occur and how can it be solved?

The overheating has been a major cause of the compressor failure. The temperatures in the head and cylinder of the compressor become so hot that the oil is diluted and loses its ability to lubricate. This can cause wear of the rings, pistons and cylinders, resulting in dirt, leaking in the valves in filling in the oil.  It can also cause the stator to become grounded due to localized burning.
At cylinder temperatures above 150C, the oil dissociation will begin and at 180C, the oil will evaporate.   For measuring the cylinder temperature, position the thermometer @ within 15 cm from the compressor discharge line. For most applications, the temperature must be less than 100C. These values consider a temperature drop of 10-24 degrees from the cylinder to the measured point.

The correct setting of the high and low-pressure controls can help identify and remediate the system problems.

Overheating of the evaporator and overheating of the system

 

 

What is the difference between the overheating of the evaporator and the overheating of the system?

The overheating varies within the system, depending on where it is being measured. The overheating that the expansion valve is controlling is the overheating of the evaporator.  This is the measure at the evaporator outlet. The refrigerant gets overheating while traveling through the evaporator basically from the entry into the evaporator and reaching a maximum in the output, as the refrigerant passes through the evaporator absorbing heat.

The overheating of the system refers to the overheating of the gas entering the compressor suction. Some people mistake the overheating of the system with “the temperature of the returning gas”. It must be remembered that the overheating varies according to the saturated suction pressure of the refrigerant. The temperature of the returning gas is the temperature value measured by a thermometer or other temperature-sensing device. It does not vary due to pressure changes.

Which overheating must I watch at the compressor inlet?

The compressor manufacturers prefer to see at least about 10 degrees of overheating at the compressor inlet. This is to assure them that no refrigerant liquid enters the compressor.

Liquid tank for refrigeration

What are the types of tanks and when are they used?

A liquid tank is basically a storage tank of liquid refrigerant that is not in circulation. The small systems using capillary tubes can have very small loads and if the operation load is constant, the careful design of the evaporator and the condenser may allow the elimination of the liquid tank. If the condenser has sufficient volume to provide storage space, a separate liquid tank is not necessary, and this is a common practice in water refrigeration unit projects with condensers of tube and hull. However, in virtually all air refrigeration units equipped with an expansion valve, a separate liquid tank is required.

There are two basic projects for liquid tanks that may be of vertical or horizontal construction.

The most common liquid tank is the “flow-through” in which the refrigerant liquid enters the upper part and the outlet removes the liquid from the bottom in a separate connection.

The other project is a liquid tank in ‘wave’. This liquid tank has a single connection for transferring the refrigerant liquid.  In this project, the connection is in the liquid tank bottom with a “T” connection. One side of the “T” is connected to the liquid return line from the condenser. The other side of the “T” is connected to the source of liquid that feeds the evaporator.

The advantage of the wave liquid tank is that it tends to preserve any subcooling of the environment that is contained in the liquid returning to the condenser. The disadvantage is that during high ambient conditions, when there is little environmental subcooling available, there may be a tendency to have gas spark in the liquid supply. During high ambient conditions, with a ‘flow-through’ liquid tank, this may not be a big problem as the liquid refrigerant in the liquid tank can actually reach various degrees of subcooling as it travels from the input to the output.

Vacuum in refrigeration system

What is the purpose of vacuum in the refrigeration system?

Vacuum is a refrigeration system with two main purposes:
1. To remove condensable gases
2. To dehydrate (to remove the water vapor)

If noncondensibles such as air are not removed, the system will operate at condensing pressures higher than normal. This happens because the air is trapped at the top of the condenser, thereby reducing effectively the capacity of the condenser. The increase of the condensing pressure results in higher compression ratios and higher exhaust temperatures, both of which decrease the efficiency of the system and can lead to a reduced life.

The water vapor must be removed from the refrigeration system for several reasons. The water vapor can cause freezing in the expansion device (thermostatic expansion valve or capillary tube) causing loss of the refrigerating effect. The moisture, refrigerant and heat can also combine to form acids. These acids are mixed with the oil and the particles that wear the metal resulting in the formation of filings and deposit. These deposits tend to accumulate in the warmer areas, generally in the discharge valve plate and, if accumulated, can impair the proper sealing by the discharge valves.

Does the vacuum really pull liquid water out of the system?

No, the vacuum does not pull liquid water out of the system. When you evacuate a system, in fact, you are reducing the pressure sufficiently to allow the water to “boil” at room temperature. As the water boils, it changes into the gaseous state, and this steam is removed by the vacuum pump.

How much low vacuum do I need to properly evacuate my system?

Modern deep vacuum pumps must be used for this purpose. These pumps have the ability to evacuate up to 20 microns in field situations. The equipment manufacturer must be consulted to determine its recommended vacuum levels, however, if a vacuum of 250 microns can be achieved, this is generally considered suitable.

Care must be taken to ensure that the vacuum measured in the calibration is equal to the vacuum level in the system being evacuated. Use the largest possible hose to connect the evacuation equipment to the refrigeration system. It is also recommended to remove any Schrader cores before attaching the evacuation lines, in order to eliminate large pressure drops. When the system is evacuated, it is also recommended to isolate the system pump and to observe if the system holds its low vacuum. Some increase is acceptable (up to about 500 microns), but if the vacuum level of the system exceeds this value, a second or even a third evacuation may be required. If during the equalization the vacuum level of the system returns to the atmospheric level it is an indication that there is a leak.

When the vacuum pump is no longer capable of pulling a deep vacuum, this is generally an indication that the oil in the pump is contaminated and must be replaced. Make sure to use oil specifically produced for vacuum pump applications. This oil has a much lower vapor pressure than conventional oils. It is recommended to replace the oil of the vacuum pump at regular intervals usually after each use to ensure that a low level of vacuum can be obtained. The oil must be replaced while still warm enabling better drainage.

Thermostatic expansion valve and SEER

What is the increase in the assessment of SEER obtained by passing a flow adjuster (fixed measuring device) to an expansion valve since everything else remains the same?

To explain why a SEER rating of a system is improved with the use of a thermal expansion valve (thermostatic expansion valve) instead of a fixed orifice device, we must understand how the SEER is determined. The SEER (Seasonal Energy Efficiency Ratio) is a measure of how efficient an air conditioner or heat pump will operate for an entire refrigeration season rather than just a single operating condition.
For single-speed systems, the SEER is calculated as follows:

SEER = EERb*(1-Cd/2)
– Where:
EERb = energy efficiency index in 95/75F (DB / WB) outdoor temperature and room temperature 80/67F
Cd = coefficient of cyclic degradation determined by two tests of dried coil (one in steady state and the other in cycle). This factor quantifies the system’s efficiency at partial load (cycle).

In order to improve the SEER, it is important to keep the CD as low as possible. This can be achieved by minimizing the amount of refrigerant entering the evaporator during the “off” cycle. With a fixed orifice device, the high and low sides of the system equalize during the off cycle, resulting in a high Cd. In comparison, a thermostatic expansion valve free of leaks closes properly when the compressor is off, preventing the equalization and, thus minimizing the Cd. For this reason, the thermostatic expansion valves usually increase the rank of a HVAC system by approximately 0.5 SEER.

It must be noted that a system may be designed with a fixed orifice and a liquid line solenoid to achieve the same SEER rating of a thermostatic expansion valve. However, this system would not be as efficient throughout the entire operating range of the equipment as it would be with a thermostatic expansion valve. The thermostatic expansion valve regulates the flow of refrigerant to maximize the efficiency of the evaporator in all operating conditions, whereas a fixed orifice can be optimized only in a particular condition. For this reason, if energy efficiency is the goal, a system that contains a thermostatic expansion valve must be specified.

Double Flow Thermostatic Expansion Valve

It is possible to use a HFES (or another balanced port Thermostatic Expansion Valve) of double flow in a heat pump system?

The HFES series (or balanced port valve) will measure the refrigerant flow in either direction. The valve bulb, in that case, would have to be located in a common suction line, as a central pipe of a four-way valve.

Furthermore, for this system to operate properly, the system would have to be “close coupled”, meaning that the evaporator and the condenser would have to be physically located near each other, as in a package system.

For the “split” type systems, the long piping between the thermostatic expansion valve and the evaporators makes impractical to use such an approach.

For such systems, two expansion valves must be used: one in the inner coil and the other in the outer coil. The check valves must be installed around each thermostatic expansion valve to allow flow around the valve when operating in reverse.

Expansion Valves

What is the difference between the “automatic expansion valve” and the “thermostatic expansion valve”?

The automatic expansion valve has a first valve designed to prevent manual adjustment of fluid registry, then used as the expansion valve. The valve is designed to maintain constant pressure at the outlet of the expansion valve. By keeping the pressure constant, it also indirectly controls the temperature, but does not ensure overheating, which will protect the compressor.

As the evaporator capacity decreases, there is less evaporation of liquid. On the other side, because the valve maintains the pressure, it also maintains the value of this flow. In doing so, however, even excess of refrigerant that is still liquid is fed to the evaporator, which results in their return to the compressor with great mechanical loss. Alternatively, if there is increased load, there will be more vaporizing of liquid and if the valve maintains the flow, it will increase the overheating of gas and there will be little use of heat exchange surface. Unfortunately, this results in evaporator operation contrary to the cold production, at the time when its load is greater.

These problems have led to the replacement of the automatic expansion valve for the thermostatic expansion valve in most applications. The thermostatic expansion valve corresponds to the overheating at the outlet of the evaporator and as a result, it responds better to the actual load, resulting in a more efficient system.

Regulating valves of the crankcase pressure

What is the function of a pressure regulator of the crankcase?

The regulating valve of the crankcase pressure (CPR) regulates the downstream pressure to a maximum value. Generally, they are installed in the suction line before the compressor to limit the compressor inlet pressure. Limit the inlet pressure prevents the compressor “to stop” during startup if the compressor is overloaded.

The CPRs are also used in other applications such as flood systems of two valves to maintain a minimum pressure of the head. In this application, the CPR has a higher pressure range and is used to pressurize the liquid tank for maintaining the pressure of the liquid.

 

 

How do I set the CPR?

To limit the compressor inlet pressure, get the compressor manufacturer’s specifications for the maximum suction pressure allowed for the refrigerant and the temperature. Using a low-pressure gauge set the valve several pounds below the maximum pressure. An alternative method is to use an ammeter to open the valve to set a benchmark that does not exceed the maximum amperage rating of the compressor.

How do I set a flood valve application?

The valve must be set to maintain a minimum liquid pressure to prevent spark in the gas in the liquid line and maintain the proper feeding of the expansion valves. This setting must be approximately 10 psig lower than the flooding prevention valve.

Service valves

How to work with a service valve?

The typical service valve consists of four main parts:
• Line Connection • Gauge Connection
• Valve Rod • Connection to the compressor

Normally, the service valve has a common connection that is always open. When the valve is positioned behind, (the entire rod is outside) the gauge connection and the valve are opened, allowing the flow of refrigerant through the system. If the valve is positioned in front (fully internal rod), the gauge connection is open to the compressor connection and the refrigerant line connection (suction or discharge) is closed. To read the pressure while the valve is open, the valve must be in the rear position and then it must be rotated once or twice to slightly open all three connections: the gauge, line and compressor connection. This allows the compressor and the refrigerant line to be open and the vapor pressure flows. In the gauge connection, the system pressure can be checked and the refrigerant can be loaded or removed.

When welding a service valve:
Make sure that the valve rod is positioned in the middle before welding. The heat of the welding of a valve rod positioned completely in front of or behind can melt the valve rod seat (inside the valve) in the welding area inside the valve body.

The technique called “wet rag” may help. Soak a rag in cold water and wrap it around the service valve before welding. Prevent the water from reaching the interior of the valve.

When opening a service valve:
Make sure the valve is firmly attached (in bench vise or screwed or fixed by Rotalock connection) before opening the sealing cap or the valve rod. Make sure the valve has package fastener. The package fastener helps to ensure a sealing free of leakage. It is typically made of brass and is at the base of the valve rod (see the illustration on the other side). It must be loosened ¼ to a full circle before opening the valve. Be sure to tighten the nut when you have finished handling the valve rod.

Use the right tools! You will only open a service valve with the properly keys to the service valve. Do not open a service valve with a wrench. You can round the edges of the valve rod and the valve will be useless.

Mechanical verification of the thermostatic expansion valve

How can I determine if a thermostatic expansion valve still has the appropriate load on its bulb?

A valve with a low load (or no load) in the bulb will tend to reduce the evaporator supply. This occurs because the pressure above the diaphragm (opening force) is reduced.
To check this the following procedure is recommended:

  1. In a valve with equalizer, turn the adjusting nut to the full counterclockwise position. Check if the overheating is still too high before proceeding to the next step.
  2. Remove the bulb and hold it in your hand for a few minutes to warm it.
    Note the suction pressure. If the valve is loading, you must see an increase in the suction pressure.
  3. If there is no change in the suction pressure, it is reasonable to conclude that the valve lost its charge and must be replaced.

Note: Some types of valves have removable control elements, which may be replaced rather than replacing the entire valve.

If the thermostatic system can be removed, the bulb can be checked by trying to push the diaphragm with the thumb. You must NOT be able to depress the diaphragm manually. If you are able to it, the valve lost its charge.