The design concept of integrated cold storage cold source has attracted much attention. Everyone has talked about this as the future of small and medium-sized cold storage, but I don't know that the underlying logic behind it is not a tall theory, but the word "temperature difference". But before that, many people confused "temperature" with "temperature difference", which can be described as "a word difference, but a thousand miles away". Today we will talk about the temperature difference in cold storage.
Knowledge point 1: The relationship between temperature difference of cold storage and load calculation of cold storage.
Because the owners of cold storage often describe "how many degrees of cold storage do I want, how big it is and what to store", the first thing to talk about when it comes to doing cold storage is temperature. Maybe everyone will ask, to discuss a cold storage, it is not enough to have a temperature. Why should there be a concept of temperature difference?
Although the first step for us to take on a cold storage is the temperature, the design temperature actually involves another problem: the load calculation of cold storage. That is to say, it is necessary to deeply understand how much heat is in the cold storage through calculation. In fact, the so-called refrigeration capacity is the ability to replace the heat in a cold storage.
There are five main sources of heat in the cold storage: Maintain structural heat transfer Q1; Cargo heat Q2; Ventilation heat Q3 (not available in every cold storage); Motor running heat Q4; Operating heat Q5.
These are five calories calculated by cold storage load, but there are 14 factors that affect these five calories. They are:
1, the construction site;
2, the size of the cold storage;
3. Thermal insulation materials;
4. Library temperature;
5. Refrigerated items;
6. Packaging materials;
7, warehouse with and without windproof facilities;
8. Whether there are ventilation and overhead facilities on the ground;
9. Daily purchase volume;
10. Purchase temperature;
11, cold storage evaporator form;
12, other equipment load in the library;
13. Is there an air curtain or a soft door curtain?
14. Daily running time of refrigeration unit.
The key is the construction site. The same cold storage is built in the northeast and in Guangdong, and its heat load is definitely different. Secondly, the daily purchase quantity, if the owner is not clear, then we can make sure that the daily purchase quantity written in the national standard is not less than 8%, as well as the purchase temperature, and whether the incoming goods are precooled or not.
According to the calculation of cold storage load, we can get "cooling equipment load" and "mechanical load", in which the cooling equipment load is the evaporator load and the mechanical load is the compressor load, so the choice of two of the four refrigeration components has a direction. However, whether it is a compressor or an evaporator, the value is a variable under different working conditions; How to accurately select the type under the condition of variables? This involves the "temperature difference". It can be said that if the temperature difference is not set, the compressor unit and evaporator cannot be selected reasonably.
Knowledge point 2: What is the temperature difference of cold storage?
Refers to the temperature difference between evaporation temperature and storage temperature. How is this temperature difference determined? The kind of goods stored in the cold storage determines the relative humidity, and the ideal relative humidity determines the design temperature difference. Many years ago, there were many cold storage rooms for flowers in Kunming, Yunnan, but they were not very satisfactory. Flower bosses complained that they could not be stored or stored well. What is the reason for this? Because in those days, engineers only paid attention to temperature, but not humidity. In order to meet the relative humidity, the temperature difference must be low; If the temperature difference is large, the relative humidity will be dry, because it is similar to a dehumidifying process. It can also be understood from another angle why there can be a temperature difference of 10 to 12℃ in the operation room, because there are people operating inside, and the appropriate degree for the human body is 50% to 65%.
The following four temperature difference ranges are enough to support most cold storage designs.
For example, the cold storage needs -18℃ and the mechanical load is 30KW. What to store? As soon as the table is compared, the design temperature difference will come out. If meat and fish are stored, the temperature difference is-7℃; Then the evaporation temperature is (-18)+(-7) = (-25)℃; Of course, the condensation mode should also be considered here. According to the condensation mode, there will be a recommended value of condensation temperature, that is to say, the refrigeration capacity of the unit with evaporation temperature of -25℃ should reach 30KW, so that the compressor will be selected. How to choose other parts? According to the obtained cooling equipment load, the evaporator is selected, and then the expansion valve is selected according to the heat exchange capacity of the evaporator; Select the condenser according to the refrigeration capacity of the compressor (semi-closed or fully closed compressor should add motor power). After the selection is completed, we should pay attention to the matching degree of the whole system and check the upper and lower range.
Knowledge point 3: How did the big temperature difference appear?
It is said that the failure rate of small and medium-sized cold storage is high and the power consumption is large. After investigation, it is found that the culprit is "large temperature difference". The phenomenon of large temperature difference is basically the reason of large compressor and small evaporator in China. In fact, the actual refrigeration capacity of a compressor is not nominal power or theoretical displacement, but mass flow, that is, the mass of refrigerant transported into the expansion valve in the liquid pipeline per unit time. When the evaporator does not have the ability to vaporize all the mass flow supplied by the compressor; The expansion valve will be turned down, reducing the flow into the evaporator, and the refrigeration capacity of the evaporator will also be reduced; At this time, the suction pressure of the compressor decreases, which leads to the decrease of evaporation temperature. When the evaporation temperature decreases, the refrigeration capacity decreases; However, the size of the cold storage has not decreased, and the heat of the items in the warehouse has not decreased, so the temperature of the cold storage has increased; In this way, a large temperature difference is formed.
Therefore, the bigger the compressor, the better. Matching is the key. Many times, more money is spent, but higher efficiency is not achieved.
Knowledge point 4: What harm does the large temperature difference have to the refrigeration system?
Here we have to understand the chemical characteristics of refrigerant. The lower the evaporation temperature, the higher the exhaust temperature, which is especially obvious in R22 refrigerant. When the evaporation temperature is lower than -20℃, the exhaust temperature will be very high instantly. Excessive exhaust temperature and increased mechanical wear will definitely affect the service life of the compressor. In addition, at high exhaust temperature, the shell paint of scroll machine will carbonize, and the valve plate of piston machine will have carbon deposition, and the bearing of screw machine will wear and eventually the rotor will bite, which is the first damage of large temperature difference: the damage to the compressor.
Secondly, the large temperature difference will cause the evaporator to frost too quickly and lead to the low efficiency of the refrigeration system. According to our data, when the temperature difference is 7K, the refrigeration capacity of the system is 32.8KW, when the temperature difference is 10K, it is 25.7KW, and the efficiency is reduced by about 22%. When the temperature difference is 15K, the refrigeration capacity is only 18.4KW, which is almost reduced by half.
Knowledge point 5: How to measure the actual temperature difference in cold storage operation?
A needle valve can be installed at the outlet of the evaporator; Whether it is an engineer or a manufacturer of evaporator air coolers, it is recommended that everyone install them. When you want to know the real operating temperature difference of your designed cold storage, use a 0.5-level precision pressure gauge (either pointer or electronic) to measure the pressure value there, and compare it with the refrigerant gauge to get the corresponding temperature value, which is the temperature difference minus the temperature on the display screen of the cold storage. It should be emphasized here that the measurement should be made at the outlet of evaporator, not at the suction port of compressor, because there is pressure loss on the gas return pipe.
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