【Daily Question】12.8 Short Answer: How do heat exchangers prevent freezing in winter?

Use insulation and steam tracing; the steam drain at the bottom should be kept open at all times

1. Electric heating 2. Thermal insulation and heat tracing 3. A small-capacity circulation pump connected in parallel to the chilled water system (to prevent freezing of fan-coil units) 4. Bypass flow guidance method

1. It is achieved by using a fan, a circulating hot water pump, and an electric insulation valve in a interlocked manner. 2. Use a heat source (such as steam) to prevent coil freezing ； 3. Electric heating.......

How to prevent freeze damage to heat exchangers in winter? 1. The same surface-cooled heat exchanger is used for both heating and cooling, but the temperature difference for chilled water is small (usually 5°C), while that for hot water is larger (15–30°C). Due to variable flow regulation, when the temperature difference is large, less water is required; as a result, the flow velocity of water in the heating tubes is low, resulting in a laminar flow state, which causes the coils in contact with the cold outdoor air to freeze. 2. An excessive design margin was used when selecting the heater, resulting in a large temperature drop of the heating medium; the return water temperature is low, making freezing likely in corners. 3. When two or more sets of heat exchangers are connected in parallel, the water circuit system becomes unbalanced: one set of heat exchangers has a high flow rate while another has a low flow rate, and the one with the lower flow rate may freeze.

1. All cooling water heat exchangers, whether in use or not, should be connected to the cooling water supply; the water flow must not be interrupted, and the upper and lower bypass valves should be kept slightly open. 2. For unused steam heat exchangers, the steam valve should be kept slightly open to allow a small amount of steam to flow in, and the condensate valve should also be kept slightly open.

Winter anti-freezing of heat exchangers mainly involves maintaining circulation, draining condensate, and using heat tracing for insulation when the equipment is shut down.

1. The same surface-cooled heat exchanger is used for both heating and cooling, but the temperature difference for chilled water is small (usually 5°C), while that for hot water is larger (15–30°C). Due to variable flow regulation, when the temperature difference is large, less water is required; as a result, the flow velocity of water in the heating tubes is low, resulting in a laminar flow state, which causes the coils in contact with the cold outdoor air to freeze. 2. An excessive design margin was used when selecting the heater, resulting in a large temperature drop of the heating medium; the return water temperature is low, making freezing likely in corners. 3. When two or more sets of heat exchangers are connected in parallel, the water circuit system becomes unbalanced: one set of heat exchangers has a high flow rate while another has a low flow rate, and the one with the lower flow rate may freeze.

1. The excess margin in the heat exchange area of the heat exchanger results in a reduced flow rate and velocity of the hot water. The heat exchanger circuits are connected in parallel for water flow, with variable water flow rates used to control the heating process, while there is a substantial surplus in heat exchange area. It is mainly manifested by incorrectly assuming the outlet temperature of the heat exchanger, rather than using the calculated outlet temperature. The amount of excess heat exchange area reflects the magnitude of the difference between the actual outlet water temperature and the assumed outlet water temperature. The greater the excess surface area of the heat exchanger, the lower the outlet water temperature compared to the assumed value, and thus the actual flow rate of the heat transfer fluid becomes smaller. Therefore, increasing the excess surface area of the heat exchanger results in a lower outlet water temperature, reduced water flow rate, and lower flow velocity within the pipes – all of which increase the risk of freezing. 2. In heating systems equipped with automatic protection, the water temperature used for heating being higher than the value specified by the heating curve is often one of the main causes of freezing. This is because when the outdoor temperature is close to 0°C, the water temperature tends to be high. It features variable flow automatic control; as the water supply temperature rises, the flow rate is reduced, and thus the flow velocity also decreases, which leads to freezing. 3. Manufacturing issues with the heat exchange coils result in air accumulating inside the coils, forming air pockets that hinder normal water circulation. Additionally, when the system is not in operation, cold air can seep in through gaps in the air valves, and since the water valves are fully closed, ice can form as well. In short, the main reason for heat exchanger freezing is the too low flow rate of water in the coil.

1. The same surface-cooled heat exchanger is used for both heating and cooling, but the temperature difference for chilled water is small (usually 5°C), while that for hot water is larger (15–30°C). Due to variable flow regulation, when the temperature difference is large, less water is required; as a result, the flow velocity of water in the heating tubes is low, resulting in a laminar flow state, which causes the coils in contact with the cold outdoor air to freeze. 2. An excessive design margin was used when selecting the heater, resulting in a large temperature drop of the heating medium; the return water temperature is low, making freezing likely in corners. 3. When two or more sets of heat exchangers are connected in parallel, the water circuit system becomes unbalanced: one set of heat exchangers has a high flow rate while another has a low flow rate, and the one with the lower flow rate may freeze.