Saturated Suction Temperature is one of the most important parameters to consider while designing a refrigeration system.
Determination of saturated suction temperature plays a key role in deciding the other key parameters of a refrigeration system such as the operating pressures, total power requirement, refrigeration capacity, mass flow rate of refrigerant and the lifetime operating cost.
By definition, saturated suction temperature is the temperature at which the vapor pressure of a substance in liquid form equals the pressure of the system allowing the substance to change its state from liquid to vapor. This transformation from liquid to vapor state is carried out at constant temperature and is called the saturation temperature. With increase in system pressure, the amount of heat required to generate enough vapor so that the vapor pressure exerted by the liquid equals the system pressure increases. That is why, with increase in pressure, the corresponding saturation temperature increases. In a refrigeration system, this temperature can be termed as the evaporating temperature.
The refrigerant employed in a refrigeration system passes through a series of thermodynamically controlled systems governed by the laws of nature. The economics at play here are governed by how much refrigerant is to be pumped and at what rate. This decides the compressor capacity, power rating and amount of energy it would consume in its life cycle.
What is Suction Temperature?
Suction temperature is nothing but evaporator outlet temperature in a refrigeration system.
The suction temperature thus is the temperature at which the refrigerant is available at 100% vapor state before its entry into the suction of the compressor in a refrigeration system. Ideally, this temperature should be slightly above the saturation temperature at the available system pressure.
If the suction temperature is kept high, it means the saturation temperatures are higher, which also means the system is operating at high pressures. If the pressures are higher, it indicates the density of the gas being circulated is higher. Higher density indicates low specific volume or higher mass of the refrigerant. Now, if the mass of the refrigerant being circulated is high to obtain same refrigeration effect, it would call for higher work done by compressor leading to higher compressor rating and power consumption.
This can be elaborated by the manufacturers table for a R290 reciprocating compressors of German manufacturer SECOP below.
Comparison of Performance of R290 compressor at two different Condensing temp conditions, Image credit: linkedin.com
As can be seen above in both the cases, as the evaporator temperature is increased from say -35°C to 7.2°C, the corresponding power consumption has increased. The corresponding power requirement for compressor has also increased indicating higher capital cost. Before we dwell into the matter further we must know, what is this evaporator temperature?
Saturated Suction Temperature Formula
The saturated suction temperature for a refrigerant can be calculated by use of the clausius clapeyron equation thermodynamics
ln (P2/P1) = ΔHvap /R(1/T1–1/1T2)
ΔHvap is the enthalpy of sublimation of liquid or vaporization of liquid
P2 is the saturated or equilibrium vapor pressure of the solid or liquid at temperature T2.
P1 is the saturated or equilibrium vapor pressure of the solid or liquid at temperature T1.
R is the universal gas constant.
The above equation can be used for calculating the saturated pressure P2 of the refrigerant for the measured temperature T2. This would however be possible, if the saturated temperature and Pressure conditions are known for another set T1 and P1 respectively along with the latent heat of vaporization value. Once the saturation pressure is calculated, the Pressure Temperature chart can be used to find the saturated temperature.
Another method to calculate the saturation temperature is to measure the pressure at the location in question by a tool such as a pressure gauge and then use the Pressure temperature equilibrium chart to find out the saturation temperature.
Low Saturated Suction Temperature
Saturated suction temperature is called low when the degree of superheat is low.
If the complete evaporation of the refrigerant occurs very close to the evaporator’s outlet, the refrigerant may not have sufficient degree of superheat in ensure no liquid is carried over to suction of the compressor. In this case, suction temperature is considered low.
For refrigeration system to work correctly there should be a certain degree of superheat present in the refrigerant as it enters the compressor suction. This ensures that all the liquid refrigerant has evaporated before it enters the compressor. In most refrigeration systems the degree of superheat of refrigerant measured at evaporator outlet should be 10 °F and at compressor suction the value is 20-25°F.
What Causes Low Saturated Suction Temperature?
Low suction temperature indicates ineffective heating in the evaporator.
Low suction temperature results from inadequate heating of refrigerant in the evaporator, which may be due to problem in TEV, quantity of refrigerant inadequate air flow, higher sub-cooling in condenser etc.
How to calculate Saturated Suction Temperature?
Saturated suction temperature is effectively the boiling point temperature of the refrigerant at the suction pressure.
The saturated suction temperature or boiling point temperature depends upon the pressure of the refrigerant measured at the suction of the compressor.
The pressure at the suction can be measured by means of a tool such as a pressure gauge and the corresponding saturated temperature can be measured by utilizing the standard PT charts for refrigerant in question.
High Saturated Suction Temperature
The saturated suction temperature is termed high if it has high degree of superheat
High saturated suction temperature results if the total evaporation of the refrigerant occurs towards the entrance of the evaporator coils. This allows the refrigerant to get overheated and attain higher degree of superheat than desired.
Higher degree of superheat is disadvantageous for refrigeration cycle as high suction temperature also results in higher discharge temperature. A higher discharge temperature requires higher condensing duty. Moreover, operating the compressor at higher temperatures might thermally degrade the refrigerant as well as lubricating oils leading to formation of sludge within the system. This also causes higher wear and tear of the compressor parts.
Saturated Suction Temperature vs Suction Temperature
Suction temperature of the refrigerant is higher than the saturated suction temperature.
For refrigeration system to work effectively, the suction temperature must be few degrees higher than its saturated suction temperature.
Suction temperature is a measured temperature of refrigerant at the suction of compressor, whereas saturated suction temperature is the boiling point of the refrigerant at the given suction pressure. Obtained from the PT charts for different refrigerants, the saturated temperature of a refrigerant increases with increase in pressure.
The actual suction temperature of the compressor in a refrigeration system is kept higher than its corresponding saturated temperature to enable complete vaporization of liquid before the refrigerant enters the compressor. The degree of superheat desired in the refrigerant at the suction of compressor is around 20-25°F. A higher degree of superheat than this value is disadvantageous as it adversely effects the compressor performance and it increases wear and tear.
Saturated Suction Temperature of r134a
The saturated suction temperature chart for r134a refrigerant is shown below:
r134a Pressure-Temperature chart, Image credit: igasusa.com
Saturated Suction Temperature of r404a
r404a Pressure-Temperature Chart Image Credit: igasusa.com
What is Saturation Temperature of r410a
r410a Pressure-Temperature Chart, Image Credit: www.igasusa.com