what is liquid subcooling

Full Answer

What is subcooling in a condenser?

Subcooling is defined as the difference between the measured liquid temperature and the liquid saturation temperature (at a given pressure), or any sensible heat taken away from the 100%. Liquid subcooling can be defined as the saturated liquid point in the condenser.

What is sub-cooled liquid refrigerant?

A sub-cooled liquid refrigerant is at a temperature below (colder-than) the temperature at which the refrigerant would evaporate ("boil" or change from a liquid refrigerant to a gas - also called the refrigerant saturation temperature ).

What is the purpose of subcooling the metering liquid?

Our objective is to provide subcooled liquid to the metering device at the lowest temperature possible while maintaining the minimum pressure drop required across the metering device.

What is subcooling temperature measured in?

- Danfoss (2005) Subcooling is measured in Kelvin (K) or °C. Subcooling temperatures are measured on the refrigerant line just ahead of (supplying refrigerant into) the TEV.

What is the liquid sub-cooling?

subcooling refers to a liquid whose temperature is below its saturation point . As a general rule, all refrigeration systems have a subcooling stage, which can take place both inside and outside the heat exchangers.

What is subcooling and why is it important?

In the refrigeration cycle, subcooling is an important process that ensures liquid refrigerant enters the expansion device.

How is liquid subcooling calculated?

Calculate the subcooling as follows: subcooling = CT – T. For a refrigerant blend use the saturated liquid (bubble) temperature as the condensing temperature. Subcooling will not usually occur in the condenser unless liquid backs up in the condenser.

What is the meaning of Subcool?

(sʌbˈkuːl ) verb (transitive) physics. to cool (liquid) to a temperature lower than the temperature it was when it condensed into steam.

What happens if subcooling is too high?

If the subcooling is too high, the system will be overcharged, reducing performance, efficiency, and ultimately damaging compressor valves and start components.

What is good subcooling?

Subcooling on systems that use a thermostatic expansion valve (TXV) should be approximately 10°F to 18°F. Higher subcooling indicates excess refrigerant backing up in the condenser.

What causes high subcooling?

High Subcooling is an indication that more than the designed amount of refrigerant is “backing up” or “packed” into the condenser. This can be caused by overcharge, restriction (such as a contaminated line drier or kinked liquid line), or an undersized or failing closed metering device.

Where do you measure for subcooling?

Measure the liquid line temperature of the device. The liquid line is the pipe that carries the refrigerant from the unit's condensing coil to the expansion valve. To measure the temperature, strap the thermometer's probe on the liquid line approximately six inches from the expansion valve and record the results.

How do you increase subcooling?

The temperature that you read with the thermometer should be lower than the saturated condensing temperature. The difference between the measured liquid line temperature and the saturated condensing temperature is the liquid subcooling. Add refrigerant to increase subcooling. Recover refrigerant to reduce subcooling.

What is sub cooling in HVAC?

Superheat and Subcooling are technical readings in an HVAC that measure the Freon (refrigerant) reading. Superheat measures the Freon boiling point in gas form while subcooling measures the Freon in liquid form below the evaporation level.

What is sub cooling in HVAC?

Simply put, subcooling is the lowering in temperature of the liquid refrigerant in the condenser coil. On a split system air conditioner, the condenser coil is in the outdoor unit.

What does low subcooling mean?

Low Subcooling is an indication that not enough refrigerant is contained or “packed” in the condenser. This can be due to undercharge, poor compression, or a metering device oversized or failing open (overfeeding).

How do you do subcooling?

The Liquid Line Temperature is also measured at the outlet of the condensing unit but now is measured on the small liquid line (B). For this example the Liquid line Temp is 95 degrees. Subtract the Liquid line Temperature from the Liquid Saturation Temperature and you get a Subcooling of 15.

What is subcooling in chemistry?

Subcooling is a measurement of temperature DECREASE of a liquid below its saturation (mixed liquid/vapor) temperature at a given pressure.

What does zero subcooling mean?

Zero Subcooling means that the refrigerant in the liquid line is a mix of liquid and vapor; this is not an acceptable condition EXCEPT in cases where the system is designed to inject discharge gas into the liquid line on purpose to increase liquid pressure (headmaster).

Why is subcooling reading in range?

This is often because the previous tech simply “set the charge” by subcooling without fully testing all aspects of the equipment, such as airflow.

What is subcooling on a TXV?

Many techs will say that subcooling is how you “set a charge” on a TXV/TEV/EEV metering device system.

What does it mean when a refrigerant is high subcooling?

High Subcooling is an indication that more than the designed amount of refrigerant is “backing up” or “packed” into the condenser. This can be caused by overcharge, restriction (such as a contaminated line drier or kinked liquid line), or an undersized or failing closed metering device.

How much subcooling is needed for a condenser?

Generally speaking, 10-12° of subcooling at the outlet of the condenser coil is most common. However, you must look for the proper design subcooling for the particular system you are working on. Some systems will require subcooling readings of up to 16° for maximum efficiency and capacity.

Is 202° water a liquid?

If you measure that same water, and it is at 202°, you can be sure that it is fully liquid and that it is no longer in the process of either boiling (changing from liquid to vapor) or condensing (changing from vapor to liquid). Because the water is at 202° instead of 212°, we know it is liquid, and we can also say it is subcooled by 10°.

What is subcooling liquid?

Not to be confused with supercooling. The term subcooling (also called undercooling) refers to a liquid existing at a temperature below its normal boiling point. For example, water boils at 373 K; at room temperature (293 K) the water is termed "subcooled". A subcooled liquid is the convenient state in which, say, ...

Why is subcooling used?

Subcooling is normally used so that when the refrigerant reaches the thermostatic expansion valve, all of it is in its liquid form, thus allowing the valve to work properly. If gas reaches the expansion valve a series of unwanted phenomena may occur.

Why is subcooling more efficient?

As compressors that are subcooling work on this easier conditions , higher pressure makes their refrigerant cycles more efficient , and the heat withdrawn by this means, cheaper than the one withdrawn by the main system, in terms of energy.

How does refrigerant change?

In a common refrigeration system, the refrigerant undergoes phase changes from gas to liquid and from liquid back to gas. This enables to consider and discuss superheating and subcooling phenomena, mainly because gas must be cooled to become liquid and liquid must be heated back to become gas. As there are little possibilities of completing this for the totality of the flowing refrigerant without undercooling or overheating, in conventional vapor-compression refrigeration both processes are unavoidable and always appear.

What is the energy equivalence between subcooling and superheating?

Normally, the fluid that is being subcooled is hotter than the refrigerant that is being superheated, allowing an energy flux in the needed direction.

How does subcooling differ from total subcooling?

The total subcooling is the complete temperature drop the refrigerant undergoes from its actual condensing temperature, to the concrete temperature it has when reaching the expansion valve: this is the effective subcooling.

What is subcooling in a condenser?

The most commonly known subcooling is the condenser subcooling, which is usually known as the total temperature drop that takes place inside the condenser, immediately after the fluid has totally condensed, until it leaves the condensing unit.

Effects of Subcooling on coefficient of performance (COP)

Due to subcooling, the refrigerating effect increases. Therefore, COP of system increases. Also, the required mass flow rate of refrigerant (m) per tonne of refrigeration reduces and it is less as compared to simple saturated cycle. Therefore, less amount of refrigerant is required for the refrigeration system of same capacity.

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What is subcooling refrigerant?

A sub-cooled liquid refrigerant is at a temperature below (colder-than) the temperature at which the refrigerant would evaporate ("boil" or change from a liquid refrigerant to a gas - also called the refrigerant saturation temperature ). Subcooling is defined as the difference between condensing ...

Why is subcooling necessary?

Subcooling of the refrigerant is necessary to avoid vapour bubbles in the refrigerant ahead of the expansion valve. Vapour bubbles in the refrigerant reduce capacity in the expansion valve and thereby reduce liquid supply to the evaporator. ubcooling of 4-5K is adequate in most cases. - Danfoss (2005)

What does higher subcooling temperature mean?

Higher subcooling temperatures of a liquid refrigerant mean a more efficient HVACR system operation beause more heat is being removed per unit volume (or unit weight) of refrigerant circulating in the system .

Where is subcooling temperature measured?

Subcooling temperatures are measured on the refrigerant line just ahead of (supplying refrigerant into) the TEV.

What does Danfoss (2005) TEV fitters notes provide details about?

Danfoss (2005) TEV fitters notes provides details about subcooling on HVACR equipment, from which we quote this excerpt:

What is subcooling in refrigeration?

Subcooling is a temperature decrease below the condensing temperature of the refrigerant that occurs once the refrigerant is 100% liquid. Our objective is to provide subcooled liquid to the metering device at the lowest temperature possible while maintaining the minimum pressure drop required across the metering device.

Why is subcooling the answer?

The “subcooling is the answer” mistake occurs when a technician overcharges a system to get a higher subcooling number at the expense of higher head pressure rather than lower liquid temperature. The condenser's job is to reject heat from the refrigerant to the condensing medium (generally air); the liquid temperature cannot drop below ...

What is the primary role of setting an appropriate level of subcooling?

The primary role of setting an appropriate level of subcooling is to ensure that we deliver a full line of liquid refrigerant to the metering device. We want to do this at: A pressure differential required by the metering device At a temperature and pressure no higher than required for…

What happens when refrigerant is added to a system?

So, as refrigerant is added to a system, the quantity of liquid contained in the condensing coil increases, resulting in a higher subcooling number but ALSO resulting in higher head pressure, conden sing temperature, and compression ratio .

Does subcooling increase liquid saturation?

However, much of the increased subcooling will come from an increase in conden sing (liquid saturation) temperature rather than actually cooler liquid. Take a look at the system readings above. The outdoor temperature was 80°, and the liquid ...

Is subcooling better than liquid line temperature?

Subcooling vs. Liquid Line Temperature. There is a common belief in the trade that the higher the subcooling, the better the system efficiency because lower liquid line temperature means less flash gas.

Does a lower liquid line temperature reduce the amount of flash gas?

While it is true that a lower liquid line temperature entering the metering device does reduce the amount of “flash gas” converted directly from liquid to vapor as it leaves the metering device, this is not an efficiency gain when the subcooling is artificially increased due to overcharging or other methods of increasing head pressure.

What is liquid subcooling?

Liquid subcooling can be defined as the saturated liquid point in the condenser. Liquid subcooling may occur from the start of the 100% saturated liquid point in ...

How to measure condenser subcooling?

It can be measured by subtracting the actual condenser liquid “out temperature” from the saturation temperature measured at the condenser outlet. The formula is:

What happens when a liquid stays in a receiver?

The vapor will recondense to a liquid in the receiver. Both the liquid and the vapor will reach a new saturation temperature. As the vapor recondenses, it rejects heat, which will be absorbed by the subcooled liquid. Subcooling can be lost in this situation if the liquid stays in the receiver too long.

Does refrigeration need liquid subcooling?

In today’s competitive service market, every conscientious service technician should understand why a refrigeration or air conditioning system must have the proper amount of liquid subcooling. The amount of liquid subcooling not only affects system capacity, but also the effectiveness and capacity of electronic, automatic, ...

Does liquid subcooling take place after the condenser?

This means that total subcooling includes condenser subcooling and any subcooling that takes place after the condenser. Liquid can lose heat and subcool in the receiver, filter drier, and liquid line before it gets to the metering device. However, if there is any refrigerant vapor in the receiver, the subcooled liquid will not subcool further.

Overview

The term subcooling (also called undercooling) refers to a liquid existing at a temperature below its normal boiling point. For example, water boils at 373 K; at room temperature (293 K) the water is termed "subcooled". A subcooled liquid is the convenient state in which, say, refrigerants may undergo the remaining stages of a refrigeration cycle. Normally, a refrigeration system has a subcooling stage, allowing technicians to be certain that the quality, in which the refrigerant reac…

Applications

Natural and artificial subcooling

Economizer and energetic efficiency

Transcritical carbon dioxide systems

See also