Cooling cycle and temperature pressure flow measurements.
The refrigeration cycle consists of six basic processes.
1. Compressing superheated steam in the compressor
2. Condensation
3. Subcooling
4. Expansion
5. Evaporation
6. Superheat
Among these processes, superheating and supercooling values are considered to be the most important indicators of whether a cooling system fulfills its expectations. Failure to measure properly during fault diagnosis and energy studies may lead to incorrect evaluations. Since measuring only temperature and pressure is not sufficient to determine cooling system performance, it is necessary to measure the mass flow rate of the refrigerant.
The pressures that need to be measured are suction and discharge lines, oil pressure in pump-lubricated compressors, and static pressure differences should be measured to determine frost in the evaporator and contamination in condensers. Temperature measurements should be made from the suction, discharge, liquid and expansion lines of the cooling cycle, but the sensors must be contacted with the refrigerant via T connections, otherwise measurement errors of 4–6 C will occur.
Again, it is very important to monitor the lubricating oil temperature in screw and centrifugal compressors. Apart from this, measuring air inlet and outlet temperatures in air-cooled condensers and evaporators is important in terms of system performance. Since air-cooled condensers generally operate with a temperature difference of 14 K, the difference between the condensation temperature and the ambient temperature gives us information about the suitability of the design and the appropriateness of the amount of refrigerant.
Again, in air cooler (unit) evaporators in cold rooms, the temperature difference between the ambient temperature and the evaporation temperature is designed to be between 5–7 K. If the temperature difference is greater than these values, it is determined that the evaporator is chosen too small or the refrigerant is insufficient. Again, measuring the water/brine inlet and outlet temperatures in water-cooled or water/brine cooling systems is important in terms of performance and safe operation.
As mentioned before, thanks to flow measurements in cooling systems, we can precisely measure evaporator, condenser and compressor capacities. In addition, it is possible to approximately determine the thermal capacities of the system elements with the help of water and air flow rates.
What is Compressor Outlet Temperature ?
Temperature measurements in the cooling system are the most important measurements to evaluate the cooler in terms of performance. Although the targeted cooling takes place in the evaporator, that is, the cooler section, the compressor outlet temperature shows whether the gas circulation is as desired. The compressor is the element that compresses the gas in the cooling cycle. When heat is not removed, shrinking (compressing) the volume of a gas causes the temperature to rise. The temperature must exceed the ambient temperature so that the heat can be released and liquefaction can begin. Regardless of the gas type, the temperature of the condenser should not be below 35 °C. Considering that the cooling system must operate in environments up to 35 °C, especially in summer months, a condenser temperature of 35-40 °C can be considered a limit value.
Most of the refrigerants are ideal gases. For a gas to be ideal, it means that its pressure and temperature will change proportionally. In other words, the temperature of a gas whose pressure value is known or the pressure of a gas whose temperature value is known can be found. In refrigerants known as mixed gases (R-4xx), this temperature-pressure harmony may cause differences of several degrees, which is called temperature drift. For example, if we measure an output pressure of approximately 16 bar (230 psi) in a system using R22 gas, we can say that the gas has a temperature of 50 °C.
While measuring temperature, measuring pressure will ensure verification of the temperature value we measure. For example, if we measure 16 bar pressure and 20 °C temperature at the compressor outlet, our temperature measurement is incorrect. Technological products are manufactured in a way that allows us to measure temperature and pressure simultaneously. In addition to the digital manifold gas pressure, it provides 2 NTC type temperature sensors with which we can measure temperature from anywhere we want.
In the digital gas manifold, you can measure the compressor outlet pressure and check the temperature value by attaching the sensors to the outlet pipe. If you do not have a digital manifold, you can read the temperature value by attaching the sensor of the digital thermostat to the outlet pipe with an insulating tape.
After a while after the system starts operating, we start reading and interpreting the measurement values. Although it varies depending on the size of the system, waiting 15-20 minutes is sufficient for the system to find its thermal equilibrium.
If your compressor outlet temperature (for R-22 gas) is not around 50 °C, it can be concluded that the gas is missing or the compressor is not doing its compression job. If a capillary tube is used, it may occur that it is used with a large diameter or short and therefore cannot be compressed sufficiently. If you measure the compressor outlet temperature at 75 °C or above, it may be due to compressor jamming (clogging, expansion valve passing too little refrigerant, etc.) or excess gas. It is only possible to draw a conclusion for performance after checks on all points.
What is Condenser Outlet Temperature?
subcooling
The condenser is the element of the cooling system that releases heat to the outside. The heat load of the condenser is higher than the evaporator because, in addition to the heat absorbed by the evaporator, the compressor must also dispose of the heat it releases as a work-consuming element and puts into the gas. The condenser outlet can provide information about whether the heat rejection process is carried out correctly.
Considering the condenser outlet as the place where the gas liquefies by releasing heat, it should be at a lower temperature than the inlet. The transition of gaseous refrigerant to liquid occurs at constant pressure, that is, the pressure is constant from the compressor outlet to the expansion valve outlet. It is the removal of the latent heat of the gas that creates the temperature difference and the heat loaded by the compressor. Entropy shows the rate of useless energy in a substance.
Entropy shows the proportion of useless energy in a substance. The potential energy of matter with high entropy has decreased (released) and its disorder has increased. The picture shows the temperature (T)-entropy (S) diagram. The compression effect of the compressor causes an increase in temperature.The condenser first cools the gas slightly and then liquefies it at a constant temperature.
Temperature-Entropy diagram
The temperature drop of the condenser before liquefaction depends on the gas used, but is around 15-20 °C. Considering that the exit temperature of the gas from the compressor is 55 °C, the temperature after the condenser should be around 35-40 °C.
A higher temperature is a sign that the heat dissipation function is not fulfilled. At the same time, high evaporator inlet temperature reduces efficiency. A method called subcooling is used to change the temperature balance to increase efficiency. The aim is to further reduce the condenser outlet temperature and prevent hot gas from flowing to the evaporator.
For subcooling, the condenser outlet is forced to exchange heat with the cold gas at the evaporator outlet with the help of a heat exchanger. In domestic and small commercial refrigerators, the capillary tube is wrapped around the evaporator return pipe and acts as a heat exchanger, cooling the liquid refrigerant a little more by giving heat to the evaporator return.
Thus, the refrigerant, which cannot become liquid, comes to the evaporator as a completely liquid and lowers its temperature a little more.
When making measurements, measuring before or after subcooling will help us understand the difference between temperatures and the benefit of subcooling.
What is subcooling and how is it measured?
Subcooling and Its Measurement In order to find the liquid line subcooling value, it is necessary to find the condensation pressure and two temperatures. These are the condensing temperature at that pressure and the liquid line temperatures at the condenser outlet. We start by measuring the surface temperature of the pipe at the condenser outlet. The condensation temperature can be obtained from the pressure-temperature table of the relevant refrigerant.
For newrefrigerantmixtures with high shear temperatures (e.g. R-407C), bubble point (BP) should be taken as basis. To make measurements, the system must be allowed to enter the regime and it must be ensured that sufficient air flow is provided over the condenser. After measuring the liquid line temperature, it is necessary to measure the pressure in the liquid line service valve or, in the absence of such a valve, in the compressor discharge line and find the equivalent temperature using a pressure-temperature table.
The difference between both temperatures will give the subcooling value. For measurements of subcooling / superheating values, manifolds that can also measure temperature are now mostly used. The fact that pressure and temperature measurements can be made on the pipe surface at the same time allows superheating and supercooling values to be displayed simultaneously. This allows for a more accurate measurement.
Subcooling provides an increase in cooling capacity without any change in energy consumption in the compressor. The refrigerant, which cannot be turned into liquid in the condenser for any reason, is turned into liquid by passing through the system or circuits designed for subcooling and sent to the expansion valve as liquid.
Subcooling=(Temperature value corresponding to high pressure value) - (Liquid line temperature)
What are Evaporator Inlet and Outlet Temperatures?
super heat
The evaporator inlet is the point where the trapped liquid refrigerant enters the low-pressure suction line, where the lowest temperature and rapid evaporation occurs. The rapid evaporation seen at the entrance to the evaporator from the capillary tube can be seen as snow outside the tube. Frosting is also an indication that liquid refrigerant is present. The outlet temperature instead of the inlet temperature can be an indicator to evaluate the evaporator efficiency. Controlling the outlet temperature can provide an easier evaluation of the efficiency, as frost and low temperature will be seen at the inlet even if the gas in the system is excess or deficient.
Small and medium-sized companies operating in the cooling system manufacturing sector only consider the evaporator outlet temperature to determine the amount of gas and evaluate the system efficiency. Instead of measuring the evaporator temperature, they test the adequacy of the gas by checking the amount of frost (looking at the frost of the evaporator outlet). Although only the amount of snowfall usually gives accurate results, taking into account seasonal conditions, the characteristics of the environment and the product, making technical measurements will give more accurate results.
Whether the evaporator temperature is sufficient or not depends on the desired cold ambient temperature values. An air-cooled evaporator must send convection air into the cooler at a lower temperature than the ambient temperature. In practice, a cooler evaporator that will achieve an ambient temperature of 5 °C or above must have a surface temperature that is at least 7-10 °C lower than the ambient temperature, that is, between -2 and -5 °C. At temperatures below zero, this difference may vary depending on the type of refrigerant and defrost frequency.
Another issue that should be taken into consideration in measurements is the amount of superheat. As mentioned in the previous topic, subcooling (the term superheat is used for the evaporator) has an effect that cools the condenser outlet and heats the evaporator outlet. Superheat is the process of heating the refrigerant a little more after it has completely evaporated. Its most important benefits are that it prevents liquid from entering the compressor and increases system efficiency.
The amount of superheat is the difference between the temperature at the compressor suction line inlet and the temperature amount corresponding to the pressure value measured at the same point.
For example :
In a system using R22 gas, if the compressor suction line pressure is 3.5 bar and the temperature is measured as -3 °C at the same point (compressor suction line), first the temperature value corresponding to 3.5 bar pressure is read from the pressure - temperature (PT) table of the gas. The temperature of R22 gas at 3.5 bar pressure is read as -10 °C.
The difference between two temperature values (7 °C) is the amount of superheat. If the superheat amount is low, it may cause liquid refrigerant to flow to the compressor. High superheat (>7 °C) indicates that the evaporator is working inefficiently, there is a lack of gas, or the expansion valve is not passing enough gas.
As mentioned before, digital thermostats can be used for measurements. NTC sensor connections can be made using tape or shortcuts.
