1、 Core judgment indicators (sorted by ease of operation)

1. Check the concentration effect: whether the material processing efficiency meets the standard (most intuitive)

The decrease in heat exchange efficiency will directly lead to insufficient heat transfer, which cannot effectively evaporate the moisture in the material, manifested as:

The concentration of the concentrated solution does not meet the standard: under the same feeding amount and operating time, the concentration of the concentrated solution is lower than the set value (such as setting the concentration to 15%, but actually only reaching 8-10%); It may be necessary to extend the running time by more than 20% to reach the target concentration (for example, it used to take 4 hours to complete, but now it takes more than 5 hours).

Reduced production of distillate: The amount of distillate (condensate) collected per unit time decreases, and the factor of increased feed concentration is excluded (such as a decrease of more than 15% in distillate production if the feed concentration remains unchanged, most likely due to heat exchange efficiency issues).

Example: Treating industrial wastewater containing 10% salt, with a designed production capacity of evaporating 500L of water per hour. If the actual evaporation rate is less than 400L per hour and there is no change in feed flow rate and concentration, it can be preliminarily judged that the heat transfer efficiency has decreased.

2. Check the temperature difference: whether the temperature difference between the heat exchange medium and the material is abnormal

The heat transfer efficiency of a low-temperature evaporator depends on the difference between the "heating medium temperature" and the "material evaporation temperature" (referred to as the "heat transfer temperature difference"). Under normal circumstances, the temperature difference should be stable within the set range, and abnormal manifestations include:

Heat transfer temperature difference increases: The temperature difference between the inlet and outlet of the heating medium (such as steam, heat transfer oil) exceeds the design value, and the material temperature cannot rise.

Example: If the designed steam inlet temperature is 60 ℃ and the outlet temperature is 50 ℃ (temperature difference of 10 ℃), and the actual inlet temperature is 60 ℃ and the outlet temperature is 45 ℃ (temperature difference of 15 ℃), it indicates that the heat released by the heating medium is not effectively transferred to the material, and it is most likely due to scaling on the heat exchange surface (increased thermal resistance).

Material temperature fluctuation or low: Setting the evaporation temperature to 25 ℃, the actual material temperature remains below 20 ℃ for a long time, and the vacuum degree is normal (excluding the influence of the vacuum system), indicating insufficient heat transfer efficiency and inability to heat the material to the set evaporation temperature.

Practical steps: Use a thermometer to measure the inlet and outlet temperatures of the heating medium, as well as the temperature of the material inside the evaporator. Record continuous data for 30 minutes. If the temperature difference fluctuates by more than ± 3 ℃ or deviates from the design value by more than 5 ℃, be alert to a decrease in heat transfer efficiency.

3. Observing pressure changes: whether the system pressure rises abnormally

Scaling and blockage on the heat exchange surface can increase fluid flow resistance, manifested as:

Heating medium side pressure increase: The pressure in the steam or heat transfer oil pipeline is 10-15% higher than normal operation, and the flow rate decreases (if the steam pressure rises from 0.3MPa to 0.35MPa or above, the original heating effect cannot be achieved).

Material side pressure increase: The circulating pressure of the material in the evaporator increases, and the current of the circulating pump increases (exceeding 10% of the rated current), indicating that the flow of the material in the heat exchange tube is obstructed and the effective utilization of the heat exchange area is reduced.

Key troubleshooting points: After eliminating pipeline valve blockage and pump body failure, if the pressure still rises abnormally, it can be basically determined that the heat exchange efficiency has decreased due to scaling on the heat exchange surface.

4. Check the condition of the heat exchange surface: whether there is scaling, frosting, or attachment of pollutants

The heat exchange surface is the core of heat transfer, and its cleanliness directly affects efficiency. Through visualization or disassembly inspection:

Scaling: Open the equipment observation window or disassemble the heat exchanger. If white, yellow, or black deposits (calcium magnesium salt scale, organic scale, corrosion products) appear on the surface of the heat exchange tube (or plate heat exchanger plate), even a thin layer will significantly increase the thermal resistance (such as 1mm thick scale thermal resistance is equivalent to 40mm thick steel plate).

Frosting: Under low temperature conditions, if the heat exchange surface is frosted (especially the condenser heat exchange surface), it will hinder heat transfer and lead to a decrease in heat transfer efficiency on the evaporation side (it is necessary to distinguish between "normal frosting" and "excessive frosting": normal frosting is uniform and thin, while excessive frosting will cover the entire heat exchange surface, causing a sudden drop in temperature).

Attachments: Solid particles and suspended solids in the material adhere to the heat exchange surface, forming a "fouling layer". Even if there is no obvious fouling, it will reduce the heat transfer coefficient.

5. Comparing operating energy consumption: whether the energy consumption per unit processing capacity has increased

The decrease in heat exchange efficiency can lead to equipment being "laborious and unrewarding" - in order to achieve the same processing effect, more energy is required, manifested as:

Increase in electricity/steam costs: Processing materials of the same volume may result in longer operation time of motors (circulation pumps, vacuum pumps) or an increase in steam consumption (such as originally requiring 50kg of steam to treat 1 ton of wastewater, but now requiring over 60kg).

Abnormal energy consumption ratio: Calculate the "unit distillate energy consumption" (such as kWh/L water, kg steam/L water). If the value increases by more than 20% compared to the initial operation of the equipment, and excluding changes in feed composition and environmental temperature effects, it can be determined that the heat transfer efficiency has decreased.

Example: When a new device is running, it consumes 0.8 kWh of electricity for every 1 L of water evaporated. After 6 months of use, the energy consumption increases to over 1.0 kWh/L without any other abnormalities. It is highly likely that the efficiency is reduced due to scaling on the heat exchange surface.

2、 Industrial scene practical judgment process (5-minute quick troubleshooting)

Record current operating parameters: feed flow rate, concentration, evaporation temperature, inlet and outlet temperature of heating medium, vacuum degree, energy consumption data;

Compare historical data (parameters during the initial or normal operation of the equipment): focus on "distillate production", "heat transfer temperature difference", and "unit energy consumption". If two or more items do not meet the standards, it is preliminarily judged that the efficiency has decreased;

Visual inspection: Check whether there is obvious scaling or frosting on the heat exchange surface through the observation window;

Pressure investigation: Check the pressure of the heating medium pipeline and the current of the circulation pump for any increase in pressure or current;

Accurate verification: If all of the above are abnormal, disassemble the heat exchanger or inspect the heat exchange surface with an endoscope to confirm the scaling/attachment situation, and ultimately determine the cause of the decrease in heat exchange efficiency.

3、 Key supplement: Eliminate interference from non heat exchange efficiency factors

Some situations may seem like a decrease in heat exchange efficiency, but in fact, it is caused by other faults that need to be eliminated first:

Insufficient vacuum degree (causing a decrease in the boiling point of the material, which appears to evaporate slowly, but is actually a problem with the vacuum system);

The feed concentration is too high or contains a large amount of non condensable gases;

Insufficient supply of heating medium (such as insufficient steam pressure, malfunction of thermal oil heater);

Equipment leakage (causing heat loss).

Summary: To determine the decrease in heat transfer efficiency of low-temperature evaporators, priority should be given to rapid screening through the three core indicators of "concentration effect+temperature difference+pressure change", and then precise verification through "heat exchange surface status+energy consumption comparison". This is not only suitable for on-site operation and maintenance personnel to quickly judge, but also serves as a standardized process for fault diagnosis in customer service.