After deciding to use the ground source heat pump system, in addition to conducting site surveys, the key is to drill the exploration holes to collect soil thermal parameters. The soil thermal parameters mainly include the thermal conductivity of soil, soil temperature and its variation with depth and season. At present, due to the lack of research in this area and the related technology are still immature, the application and development of ground source heat pump are limited.
The heat transfer between the circulating heating medium and the earth in the heating source network of the ground source heat pump heat exchanger is quite complicated. Therefore, the design difficulty of the system is mainly the design of the heat exchanger. The buried pipe form, the spacing of the buried pipe or shaft, the buried depth, the pipe diameter, and the flow rate of the circulating medium are the heating heating nets that should be considered in the system design and construction.
The horizontal buried pipe is divided into single layer and multi-layer, wherein the optimal depth of the single-layer horizontal buried pipe is 0.8-1.0 m, the heat exchange effect varies greatly with the climate, and the lower-layer pipe in the multi-layer pipe is in a lower temperature field, The heat effect is better than the single layer, but it may be expensive. Generally used is a double tube, the optimal depth is 1.2 ~ 1.9m. Regardless of the type, the pipe must be buried below the local freezing depth.
The U-tube heat exchanger has a diameter of less than 50 mm and a buried depth of more than 200 m. It is mainly used in areas with a small buried area. The horizontal spacing of U-tubes is generally indicated to be 4.5 m. There are currently 6 m instances. In practice, the spacing should be related to the depth of the buried pipe, the form of the circuit, the diameter of the pipe and the condition of the system.
The outer diameter of the tube-type heat exchanger can reach 200mm. The increase of the heat exchange area can reduce the number of holes and the depth of the hole, but the heat loss between the inner and outer chambers.
The single-tube type is called “hot well” in foreign countries, which can reduce the installation cost and operation cost, but it is too limited by hydrogeological conditions because it does not have an outer tube.
The construction of vertical buried pipes mainly includes drilling, heat exchanger installation, pressure testing and backfilling. Any one of the steps is extremely critical and should be completed by a qualified and experienced construction team.
Analysis of Key Problems in Site Design of Ground Source Heat Pump System
Proceed as follows:
1. Drilling (1) The machine should have a special geologist to carry out geological cataloging, including lithology description, well depth, groundwater level, mud consumption, etc. (2) After the well is formed, geophysical logging is performed to measure the ground temperature;
2. The heat exchanger should be installed immediately after the completion of the drilling, and take fixed measures to prevent floating;
3. Pressure test must be carried out before backfilling to ensure no leakage;
4. There should be no large particle size particles in the backfill backfill. The backfilling process must be carried out slowly to ensure the degree of solidity and reduce the heat transfer resistance.
The construction of horizontal buried pipes includes trenching, downpipe, pressure test and backfilling, of which backfilling is the key. During the backfilling process, a sand layer of 50-100 mm thick should be laid around each coil to make the joint tightly between the pipeline and the earth to improve the heat transfer effect.
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