Refrigerators are an integral part of everyday life to the point where it is almost impossible to image our day without them. As in our everyday life, refrigeration units are also widely used for industrial purposes, not only as stationary units but also for transporting cold goods over long distances. In this blog, we will focus on the simulation and modeling of such an industrial refrigeration unit.
Like any stationary refrigeration unit, a unit used for cooled transportation includes an intermediate heat exchanger, a pump, an evaporator, a compressor, a condenser, and a throttle. The most common refrigeration scheme uses three heat fluids in the industrial refrigeration cycle. There is Water, which is used for heat removal from Refrigerant- R134A and Propylene glycol 55%. These other fluids are used as intermediate fluids between the refrigerator chamber and refrigerant loop. The working principle of all fridge systems are based on the phase transition process that occurs during the refrigerator cycle shown in Figure 1. The propylene glycol is pumped into the evaporator from the heat exchanger, in which it cools and transfers heat to the refrigerant. In the evaporator, the refrigerant boils and gasifies during the heat transfer process and takes heat from the refrigerator. The gaseous refrigerant enters the condenser due to the compressor working, where its phase transition occurs to the liquid state and cycle repeats.
Simulating the Process
Keeping cold product at the proper temperature during transportation is of vital importance to many industries. The better the cooling system operates, the longer products can be saved safely without losing quality. It is also important to keep the refrigeration unit as small as functionally possible. The use of advanced methods for estimating the parameters of refrigeration equipment allows us to more accurately designing installations which keeps products at the proper temperature. The proper design of industrial refrigeration has several stages:
- – Thermal scheme cycle modeling. In this stage, pressure levels, flow rate, and heat capacity are considered and evaluated.
- – Preliminary calculation of scheme elements. This stage assumes HEX preliminary design calculation using the averaged parameters such as heat transfer coefficients averaged over all heat exchangers, temperatures, pressures, etc.
- – Refined thermal and hydraulic calculation of complete scheme. In this stage, the accurate discretized calculations of all HEX and heat gains in the transport networks are -compared to the newly obtained results. Geometric parameters are also clarified, if necessary.
- – Off-design analysis of developed schemes (including transient operation). This stage the transient analysis parameters are estimated for the extensive hydraulic networks with an intermediate coolant.
For this example shown in Figure 1, we estimated the refrigerator values in 0D thermodynamics software AxCYCLE™. HEX’s design was created by average parameters. Inputting this information, the following values of HEXs were obtained:
- – Due to technical and economic considerations, the shell layout with 12 passages and the tube heat exchangers with 3 tubes were chosen;
- – Refrigerant flows go in shell side for both evaporator and condenser;
- – Tubes are ribbed from the outside;
- – Condenser total length is 6 m;
- – Evaporator total length is 4 m;
- – The condenser contains parts that act as Refrigerant desuperheater and parts that act as a condenser.
Accurate discretized calculation of industrial refrigerator was provided by thermal-fluid network software AxSTREAM NET™ as shown in Figure 2. In the application, there are different modeling levels. The accurate approach provided by using the interval method of scheme simulating that allows users to automatically determine the condensation and evaporation process in the facilities and provides accurate heat transfer modeling that is critical for a successful design.
Detailed simulation of the refrigerator is necessary for obtaining the process of phase transition in a condenser. As you can see in Figure 3, the beginning of the phase transition is marked by a sharp change in the heat transfer coefficient, a critical data point easily shown in the first two sections in AxSTREAM NET™. This is where the condensation process starts. Tracking this critical processes helps to avoid errors in the design of any refrigeration units. This is especially true for units that transport cooled items over long distance.
It should be noted that the thermal-fluid network software such as AxSTREAM NET™ provides accurate values not only in the estimation of hydraulic resistance in the network, but also in the thermal processes simulating. This approach gives the assessment of refrigerant and propylene glycol for proper pump and compressor selections. Characteristics of the pump and compressor are chosen automatically depending on the main parameters of hydraulic analysis such as flow rates and pressure distribution. This approach provides an accurate calculations of off-design regimes.
Comparison of Averaged-Based Calculation vs AxSTREAM NET™ Simulation
Analysis of refrigerator performance shows that heat transfer coefficients strongly depend on wall temperature during refrigerant condensation and evaporation. So these processes need to be estimated with special care. Nowadays, using an averaged-based calculation simulation does not give adequate calculation precision. As shown in Figure 4, the averaged calculation model provides multiple inaccuracies compared to the discretized scheme calculated with AxSTREAM NET™.
When the updated calculations from AxSTREAM NET™ are considered for the redesign of heat exchangers, more accurate geometric parameters are obtained:
- – Condenser pipe length is 5 m (compared to 0.6 m previously, a 24.1% difference);
- – Evaporator is 5 m (vs. 1.4 m, a 2.8% difference);
- – Air-Propylene Glycol HEX is 5 m (vs. 7 m , a 3.7 % difference)
As shown in our analysis, the use of AxSTREAM NET™ provides an accurate estimation of HEX surfaces heat loads, which makes it possible to design heat exchangers with the smaller surface margin. The AxSTREAM NET™ scheme created can then be used for off-design modeling of this (and others) refrigerator systems including for transient operation. This model may be used to analyze the reliability of the system under varying conditions, such as: determining the amount of power the refrigerator requires when chambers are loaded; preventing condensate discharge into compressor while reducing the evaporator capacity; and to change cooling water parameters, etc.