Here is the summarized by our Lando’s expert. How to calculate your water chiller‘s refrigerant charge guide during our actual design or commissioning? we have many ways to judge whether the refrigerant charge is appropriate or not, the following are some common methods
1. Touching the inhaler and exhaust pipe to sense the heat and cold of the copper pipe.
2. Observe the bubbles in the retinoscope.
3. Measuring high and low-pressure pressure
4.Measuring the current of the compressor
5.Calculate the superheat
6.Calculate the subcooling degree
7. Measuring the temperature difference between the air in and out of the condensation coil and evaporation coil.
8. Observe condensation on the inhaler.
9. Weighing and filling
Of course, all of the above methods have certain drawbacks, let’s pick a few to explain to you where the drawbacks are.
For example, using the method of calculating superheat to determine the amount of filling, where is the downside?
Water Chiller with the capillary system
a. Hot days
b. Cold weather
c. Latent heat load
Water Chiller with Expansion valve systems
a. Excessive condenser liquid storage.
(01), by measuring the evaporator on the suction pipe suction temperature and pressure values to calculate the degree of superheat, the pressure value is converted to temperature value minus the difference between the suction temperature value, that is, superheat.
(02) For capillary systems, measuring the correct capillary flow rate depends on pressure and resistance. If the system is flushed on a cold day to ensure the correct superheat, the system will be overfilled with refrigerant on a hot day. For capillary tube systems with maximum heat load conditions, more refrigerant will be added, and since the maximum heat load is imaginary, it will reduce the condenser area and raise the high pressure.
(03) For expansion valve systems, it will be easy to overflush, and since the expansion valve will automatically open and close, too much or not enough refrigerant flow through the coil will maintain the same overheat, which, if recognized, will cause more refrigerant to accumulate in the condenser, causing the high voltage to rise, the current to be partial, and the compressor to wear out.
What are the drawbacks of measuring the compressor current to determine the refrigerant charge?
1) Cold weather flush Freon
2) Hot day flush Freon
3) The cleanliness and dirtiness of the coils
4) Dirty filters.
5) Wear and tear of motor bearings
6) The mains voltage is too low
7) The mains voltage is too high
The current tested varies under different conditions, for example, the current is influenced by the voltage value of the mains supply, and deviations in the voltage value cause deviations in the current.
Current is also influenced by the lubrication of the motor bearings, hot days and cold days.
So is there any way that we can, at the time of design, accurately determine the refrigerant adequacy?
First of all, we must know that most of the refrigerant in the refrigeration system is in the two vessels and the liquid tube, in which we also need to know the state of the refrigerant in the two vessels, the following is the calculation method of the volume ratio of the state of the refrigerant in the two vessels.
As we all know, in a condenser, depending on the state of the refrigerant, there are three sections: the superheating section, the condensing section, and the subcooling section. The relative positions of the three sections are determined by the heat transfer conditions between the condenser and the surrounding environment.
As shown in the figure above.
The superheated section is the straight-line section where the refrigerant enters the condenser and condenses until it reaches the saturated vapor critical point; the intersection of the exhaust to saturated vapor curves on the diagram.
Condensation: Straight line of refrigerant condensing from saturated vapor to saturated liquid; the position between the two saturation lines on the diagram.
Sub-cooling section: the section where the refrigerant condenses continuously from the saturated liquid to the subcooling position.
The enthalpy values of the three sections are shown in the table below.
The condensing temperature in this system is 45°C, and we assume the following state parameters on the airside, with an inlet temperature of 25°C and an outlet temperature of 30°C; the logarithmic average temperature difference for heat transfer is calculated as follows.
We differentiate the condenser into many smaller pieces, the principle of differentiation being that each piece has an equal enthalpy increase; according to the formula.
Refrigerant-side heat transfer: Q1=m*Δh (heat transfer = mass flow rate * enthalpy increase)
Air-side heat transfer: Q2 = K*A*ΔTm (heat transfer coefficient * heat transfer area * heat transfer temperature difference)
According to the conservation of energy we have.
So there is.
Perform the following transformations
A= m*Δh/ K*ΔTm
First, we can assume that the heat transfer coefficients of the entire heat exchanger should be equal (the reality may be slightly off and does not affect the calculations) since the cooling flow and enthalpy increase are equal.
Therefore, the subdivided heat transfer area is inversely related to ΔTm.
Based on the above elaboration, we have the following calculations.
According to the above method, we also calculate the heat transfer state of the evaporator.
The state of the refrigerant in the evaporator is the evaporation and superheat sections, as shown in the following diagram.
The volumetric ratio of the two sections is calculated as follows.
Many students are asking, what is the use of this volume ratio? Later we will use this volume ratio to directly calculate the refrigerant charge of the whole system.
The last lesson, we explained the use of EXCEL to calculate the volume ratio of the refrigerant state in the heat exchanger, through this volume ratio, we can accurately calculate the condenser and evaporator refrigerant charge, due to refrigeration systems in 80% – 90% of the refrigerant is in the two devices, so we can calculate the entire system refrigerant charge, then how to use EXCEl to calculate it?
Let’s start by looking at the condenser, we just need to know the volume of the condenser and multiply that by the average density to get the mass of refrigerant in the condenser.
The methodology is as follows.
Based on the introduction in the previous lesson, we have calculated the volumetric ratios of the three states (superheat, condensation, and supercooling) in the condenser; we have assumed that the coefficient of heat transfer for each stage is equal to the difference in temperature of heat transfer.
Based on the following formula.
A= m*Δh/ K*ΔTm
then each enthalpy increase is applied to the same heat transfer area.
The enthalpy difference is equally divided into 10 points and the volume ratio is also equally divided into 10 points, and the volume ratio and enthalpy values at each point are shown in the table: the corresponding densities are calculated from the pressure and enthalpy values at each point.
The above table shows the data for the superheated section.
The results for the other two paragraphs are as follows.
With this curved data, we can now plot the density-to-volume ratio.
With this curve, we can directly calculate the area under the curve, which is the mass of the evaporator inside the condenser.
It can be divided into three parts to calculate separately, superheat section, condensation section, and supercooling section, which will be more accurate; you can try it yourself, we are going to calculate the area of the whole curve directly by using the trend line method (the error is a little bit bigger than calculating the area of three curves).
Seek the area of the curve for general mathematics students is a little difficult, we need to function according to the trend line, to find out the original function, and then according to the original function to seek the area of the curve, with Excel is not good to achieve; here is proposed that the general mathematics students return to the original formula.
We use the average density of each segment multiplied by the volume ratio to get the amount of refrigerant.
The following table shows the calculation.
The following results have been calculated.
M (condenser) = 413.55V (condenser volume)
Similarly, we plot the density-to-volume ratio curve for the evaporator, with the following results.
Plotting the density-to-volume ratio curve yields the following.
Using the condenser idea above, Renren has calculated the following results.
M (evaporator) = 54.72 V (condenser volume)
M (refrigerant charge) = 1.15* (413.55V (condenser volume) + 54.72V (evaporator volume)).
Where: 1.15 is the empirical coefficient, you colleagues can use the same idea, respectively, to find the volume of the compressor, exhaust pipe, liquid pipe, and suction pipe, and then multiply it by the average density of the idea to solve, we do not do a detailed explanation here.
If there is a reservoir present in the system, this factor needs to be increased.
We do not explain the use of curves here to solve the process of area, this is a very difficult for the general student, the need to use calculus to find the original equation, we use EXCEL to do the principle of simulation is that everyone can grasp, so here to explain the average density of the method to obtain the results, practice shows that the deviation will not be very large.
We present this with a model of an evaporator.
The trend curve equation for the evaporator is.
y = -138.27×3 + 311.14×2 – 260.59x + 115.37
Find the original function to be.
We follow the original function to find the cumulative area of each point in the following table.
Calculated to give a final calculated area of 54.2175.
Compare the difference between.
The deviation value is only within 1%.
The more points you have, the smaller the bias will be.
So it is feasible to suggest here that a direct averaging is feasible for colleagues who are not good at math.
For this above inform you how to How to Calculate Your Water Chiller’s Refrigerant Charge Guide. But for different refrigerants with different water chiller systems, the result is not the same if the calculation method is the same. The ideas provided here are for the reference. you can combine theoretical calculation with actual, and make corrections, again and again, I believe that after several times, your refrigerant charge can be calculated very accurately.
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