[Mechanical Equipment Technology Edition] Daily Question 20180704: Refrigeration Systems – Refrigeration Parameters (15)

“\"Small temperature difference\" is the common term used in the industry, while its scientific name is \"approaching temperature difference\". For example, taking a water-cooled chiller as an instance: the difference between the \"saturation condensation temperature of the refrigerant\" in the condenser and the \"temperature of the cooled water exiting the system\" is referred to as the \"small temperature difference\", which is usually 1-2°C. Similarly, in an evaporator, the difference between the \"saturated evaporation temperature of the refrigerant\" and the \"temperature of the chilled water outlet\" is also referred to as the small temperature difference. For full-liquid evaporators, this value is around 1°C or a little over 1°C; for dry evaporators, it is higher, at around 5°C or more. On air-cooled units, this value is relatively high, for example 15°C; therefore it is not referred to as a small temperature difference. Instead, it is called the \"heat transfer temperature difference.\" In fact, the term \"heat transfer temperature difference\" has a different definition in strict terms, and those who are not familiar with it may easily get confused. (For example, the temperature difference between the inlet and outlet water; some also refer to this as the heat transfer temperature difference.) A large \"small temperature difference\" indicates that the heat exchange efficiency of the heat exchanger has decreased, and it is time to take action. If cleaning is needed, then clean; if water treatment is required, carry out the water treatment; if oil extraction is necessary, do that, and so on. It is an intuitive indicator. Things like the temperature difference between incoming and outgoing water become less straightforward to understand if it decreases; this could be caused by an excessive water flow rate or by low heat exchange efficiency.

It refers to the heat transfer temperature difference. An increase in the temperature difference indicates a decline in the heat exchange efficiency of the heat exchanger

This post was last edited by “Hasty Passerby” on July 4, 2018, at 22:38. “Small temperature difference” is a commonly used term in the industry; its scientific name should be “approach temperature difference”. There are two instances of small temperature differences in a refrigeration unit: the small temperature difference in the evaporator and the small temperature difference in the condenser. The small temperature difference in the evaporator refers to the difference between the saturated temperature of the refrigerant in the evaporator and the temperature of the chilled water at the outlet, while the small temperature difference in the condenser refers to the difference between the saturated temperature of the refrigerant in the condenser and the temperature of the cooling water at the outlet. The evaporation approach temperature = Temperature of chilled water at outlet – Evaporation temperature of the refrigerant ； Condensation approach temperature = Refrigerant condensation temperature - Cooling water outlet temperature ； Taking a water-cooled chiller as an example: the difference between the \"saturation condensation temperature of the refrigerant\" in the condenser and the \"temperature of the cooled water at the outlet\" is known as the \"small temperature difference\", which is generally 1-2°C. Similarly, in an evaporator, the difference between the \"saturated evaporation temperature of the refrigerant\" and the \"temperature of the chilled water outlet\" is also referred to as the small temperature difference. For full-liquid evaporators, this value is around 1°C or a little over 1°C; for dry evaporators, it is higher, at around 5°C or more. “A large \"small temperature difference\" indicates that the heat exchange efficiency of the heat exchanger has decreased; this could be due to scaling, accumulation of foreign substances, etc. In such cases, cleaning should be carried out, water treatment measures should be applied, oil removal procedures should be followed, and so on. “If the temperature difference between the inlet and outlet water decreases, it could be caused by an excessive water flow rate or by low heat exchange efficiency; further analysis is required to identify the cause and take appropriate actions.

I really don’t know how to do this...... Looking forward to your guidance

After reading the previous post, can a small temperature difference indicate whether the condensation performance of the chiller is good?

Yes, it is mainly the difference between the water temperature and the saturation temperature of the refrigerant. For example: the temperature of my hand is 30 degrees, while the temperature of an iron piece is -1 degree； I wanted to heat this piece of iron with my hand. After some time, the temperature of my hand rose to 20 degrees, and the temperature of the iron also reached 20 degrees. In fact, 30 degrees on my hand corresponds to the temperature of the refrigerant in the system, while -1 degree for the iron corresponds to the temperature of the water entering the system. After a while, the 20-degree temperature of the iron corresponded to the temperature of the water exiting the system, and the 20-degree temperature of my hand corresponded to the saturated temperature of the refrigerant. The reason why eventually the temperature of my hand and that of the iron became the same is because they were in direct contact with each other, with no copper pipes in between. External factors were not taken into account; if there had been copper pipes between my hand and the iron, then the temperature of the iron would never have been able to reach that of my hand. Can you understand?

How can one find out the saturation temperature of the refrigerant water in the condenser and evaporator? Does the equipment come equipped with a display showing this temperature difference?