1.Freeze-up protection
Any heat exchanger could freeze up when the temperature is below zero. Freezing up inside the heat exchanger will definitely result in structural damage and leaking. If leaking happens in the evaporator, even the compressor could be destroyed. Such grave consequence should be paid special attention to.
The freeze-up risk is divided into two basic areas:
(1) External freezing. If evaporator or condenser is placed outdoor, the heat exchanger could freeze up in winters. To protect it from low temperature, it is imperative to install a drainage. Discharge the water when the exchanger is left unused and keep heating the water when in use. Adding glycol in the water is also an efficacious method.
(2) Internal freezing. This only happens in the water side of evaporators. The freezing cause is the refrigerant’s temperature being below zero owing to low evaporating temperature or suction pressure, low evaporator water entering temperature, fouled evaporators causing low water flow rates and loss of refrigerant charge. Analyse by cases to solve the problems.
Methods of evaporator freeze-up protection:
- A LP control switch should be fitted to prevent operation at low suction pressure conditions.
- A temperature sensor fitted behind the expansion valve can be used to stop the compressor when the valve temperature is below a certain level.
- Temperature sensors can also be used to stop the compressor when the leaving water temperature approaches freezing. They must be as responsive as possible and fitted directly in the water flow as close as possible to the water outlet. Fitting on the pipe wall is not advisable for possible slow response leading to cease of effect in sensors.
- It is recommended to turn on the cooling water flow in advance and start the compressor with an appropriate delay. A reliable flow switch to detect low water flow rates should be fitted so that it could give out warning or stop the compressor at a low water flow rate.
- Additionally, pay attention and avoid control sensor failure or control lag.
2.Scale treatment
In daily use, scale accumulates at the water side of heat exchangers therefore regular scale treatment is a must.
Variations in water quality from place to place and variations in applications of plate heat exchangers make it difficult to define in simple terms the water quality requirements for minimum maintenance. It is important to ensure the right water quality and reduce scale by establishing the requirements for local conditions or utilizing specialist water treatment companies.
The following should be paid attention to in daily maintenance.
(1) Undissolved solids (turbid water)
If undissolved solids like sand, weeds, leaves and other fibres are in the water, the channels are easily blocked. Fibrous litters can not pass the heat exchanger and must be disposed of. To reduce undissolved solids blocking the unit, granules should kept smaller than 0.8mm and is recommended to fit a 40-70 mesh strainer. Regular maintenance of this unit is required however, to ensure continued performance.
(2) Dissolved products
Under certain circumstance crystallized deposits of calcium and magnesium ion in the water will attach to the heat exchanger surface and scale – the higher the concentration of Ca2+ and Mg+ and the water temperature, the greater the possiblity of scale formation. Thick scale will not only gravely affect heat transfer capability but even block the channels. As a result, to overcome nuisance tripping at start up never use this kind of water directly and always carry out water treatment beforehand.
(3) Cleaning
Due to the diverse applications of brazed plate heat exchangers various methods of cleaning may be used. The following method is often used for heat exchangers applied as refrigeration condensers in cooling tower circuits where poor quality water is used or poor water treatment results in heat exchanger fouling. This fouling can be corrected by chemical cleaning, reverse flushing or a combination of both. Regular reverse flushing in situ is the simplest option and may be the only treatment necessary if fouling is predominantly caused by sedimentation. If however, scaling has occurred, chemical cleaning will be necessary. This should be performed using a weak acid cleaning liquid pumped through the heat exchanger in reverse flow direction at approximately twice the normal flow rate. Remember that the cleaning acid should be circulated in reverse flow for usually 24 hours. At the completion of the cleaning process, it is important that the unit be flushed with clean water for at least 30 minutes.
3.Corrosion protection
Stainless steel has a high resistance to most chemicals but is sensitive to chloride ion concentration so this must be limited to 280ppm or less (lower than 55°C). The copper used in the brazing process is mainly sensitive to strong acid and ammonia so the pH must be maintained between 6 and 8. In practical application, for example, hydrochloric acid should never be used to clean the exchangers; if heat exchangers are applied in swimming pool heating, limite chloride ion concentration to 280ppm in the water; what’s more, in chemical, pharmacy and other industries, solutions containing strong acid like hydrochloric acid or nitric acid and hartshorn won’t be proper fluid medium. |
