Ground source heat pump underground heat exchanger nested process

Ground source heat pump underground heat exchanger and pile construction of the nesting process

Abstract: The vertical buried geothermal heat pump has the advantages of high heat exchange efficiency, but its application is limited due to the high cost of drilling. Making full use of building pile foundation, laying U-shaped heat exchange tubes in prefabricated pipe piles, bored piles and underground continuous walls can save the drilling process, greatly reduce the construction cost of underground heat exchangers and reduce the initial investment of the system.

Keywords: ground source heat pump; underground heat exchanger; bored pile; prefabricated pipe pile; nested process

Ground source heat pump with its high efficiency, environmental protection, energy saving and many other advantages of the rapid development of energy-saving central air-conditioning on behalf of the development trend. Ground source heat pump with vertical buried tube, surface water, groundwater and other applications. The vertical embedding method has the advantages of high heat exchange efficiency and small floor area of ​​the underground heat exchange system, but the application is limited due to the high initial investment. The high cost of drilling is the main reason for higher initial investment.

The underground heat exchanger is nested with the building pile foundation, that is, the U-shaped heat exchange pipe is laid in the precast pipe pile, the concrete pouring pile and the underground continuous wall to save the drilling process, save the construction cost and make more effective use of the floor of the building Under the area. At the same time, due to the large pile spacing, U-type heat exchange tube mutual thermal impact is almost zero, underground heat exchanger operating conditions more stable. The promotion of this technology will provide new application space for buildings with small green area and high floor area ratio and will surely become a new application model of vertical buried pipe.

First, process introduction

Vertical distribution of soil in Ningbo can basically be divided into four layers: clay layer, silt layer, silty clay layer and silt layer. The soil layer in this area is relatively soft and thick, so the building must have a deep foundation. The foundation of the building is prefabricated pipe piles and bored piles, which are suitable for the nesting process of underground heat exchangers and piles of buildings.

1, prefabricated pipe U-type heat transfer pipe laying process

Precast pipe piles are mainly reinforced concrete solid or empty pipe piles, but also wood or steel piles. The hollow pipe pile diameter is generally 400mm, 550mm, wall thickness of 80mm, the middle cavity diameter of 240mm, 390mm, you can embed a single U-shaped tube in the cavity or double U-shaped heat transfer tubes to save construction The purpose of the fee.

Under the tube is one of the key ground-source heat pump project, because the depth of the next tube to decide how much heat, so we must ensure that the depth of the tube. U tube before the tube should be bundled with the grouting tube and take measures to prevent the U-tube floating. The prefabricated pipe pile mouth placed sacks and the like items to prevent the heat pipe during the tube wear and tear resulting pressure drop performance degradation.

Because when the cap, pipe buried pipe to go out to cap, so the length of the pipe should be greater than the depth of the pile plus the length of the cap height.

The backfilling process is also called grouting and sealing. The purpose of backfilling is to strengthen the heat transfer between the U-shaped heat exchange pipe and the prefabricated pipe pile wall. High pressure of the backfill is sealed up by the grouting pump or mud pump from the bottom of the pile. It is not allowed to contain large particle size particles. During backfilling, the grouting tube must be withdrawn step by step according to the speed of grouting, so that the mixed slurry can be recharged from bottom to top to ensure tight compaction, no cavity and less heat transfer resistance. The backfilling process is completed when the return mud density is the same as the density of the potting material.

2, piles and underground continuous wall U-type heat transfer tube laying process

Cast-in-place piles are deep foundations built into reinforced cages and perforated concrete in drilled holes. Diaphragm wall under the conditions of the mud retaining wall to the underground drilling a long narrow slot, hanging in the tank into the steel cage, and then pouring concrete, built into a section of reinforced concrete wall segments, and connect each wall segment by segment Form a continuous underground wall.

Due to the large diameter of the bored pile, multiple sets of U-shaped pipes can be placed on the steel cages and can be converged in parallel within the pile body (or can be properly connected in series) before the pile body is led out. Damage to the U-shaped heat exchange tube when dealing with the pile head. U-type heat transfer tube can be placed in the inside of the steel cage can also be placed in the outside of the steel cage, nylon cable ties to be fixed on the steel cage, and do a good job on the U-tube protection.

Vertical pipe horizontal connection process: the vertical pipe leads to the platform at the pile mouth bent vertical pipe to pass through the casing in the cap after the construction of the first pressure test and then welded into the level of the Explorer to ensure access The main pipe heat exchange tubes (D32, HDPE) intact, no leakage; until after all the holes on each manifold after the completion of the pressure test and observe the main pipe joints with each branch at the weld seam leakage ; Then the water supply and return were connected to the corresponding sub-catchment; backfill horizontal tube, with sand or soil covered in the pipeline level 15cm thick.

Second, project application examples

The ground source heat exchange system in the form of a coupling pipe is used to lay the U-shaped heat exchange pipe in the pile foundation of the building (prefabricated pipe pile, bored pile and underground continuous wall), a certain number of U-shaped heat exchange tubes Level assembly, the formation of a number of groups with the same way (or range) heat exchange circuit, pooling to set, water separator, to achieve the purpose of providing building air-conditioning system, cold and heat source.

Example 1:

An office building ground source heat pump air conditioning project, with a total construction area of ​​4300m2, air conditioning area of ​​3500m2, the total cooling load of 450KW. The use of double-U hollow tube buried pipe ground source heat transfer system, that is, in this project 241 precast pipe pile (400 ~ 500mm, the depth of 41 ~ 45m) buried in the double U-type coupling pipe, all Coupled with the source of the mainland water pooling to the water header, through the pump to the indoor ground source heat pump unit, the energy exchange, back buried pipeline.

1, pile foundation overview

The project uses prestressed concrete Φ400, Φ500 two types of pipe pile, the selection of the first 8-2 pile tip effective bearing layer pile length, a total of three areas, Ⅰ area is 41 meters, Ⅱ area is 43 meters, Ⅲ area is 45 Meter. Φ400 prestressed pipe 162, Φ500 prestressed pipe 81, a total of: 243. The column is 14 × 6 pieces, the spacing is 9 × 9m.

2, soil properties and backfill thermal parameters

According to the project borehole survey report, the analysis shows that the soil in the range of 0-50 meters can be broadly divided into the following layers, as shown in the following table.

Table 1 Soil distribution

Stratum elevation (m) Main soil types
-2.17 Clay, silt
-21.27 Silt, silty clay
-33.87 Clay
-33.07 silty clay
-43.77 Clay
-49.34 Fine sand, silty clay, round boulders

Mainly in the region of clay, fine sand, pebbles mainly wet and saturated humidity, the soil conductivity of the region 1.6W / mk, backfill selection of fine yarn and saturated clay (or expanded cement + clay), are reaturated Humid soil, thermal conductivity of 1.9W / mk.

3, other thermal parameters of this project

The average temperature of the soil is 660F (19 ℃). In order to keep the heat pump high efficiency (EER≥10 or COP≥3.4), the lowest temperature of the heat pump is 460F (8 ℃) and the highest is 860F (30 ℃).

4, underground heat exchanger heat exchange calculation

According to the heating period in Ningbo is generally 60 days, the summer cooling period of 120 days, calculate the annual soil heat absorption (heating conditions), annual cooling (cooling conditions) as the following data:

Q annual heat absorption = 280 * 106BtuQ annual cooling = 1512 * 106Btu

Calculate the length of the D32 polyethylene vertical U-loop to the coupling pipe (jack depth is 40 meters)

L total endothermic length = 1440mL total cooling length = 7212m

It is learned from the above that the total annual heat absorption length (1440m) is less than the total annual cooling length (7271m), and the total annual cooling length is taken as the design length of the building soil heat exchanger.

According to the table of soil properties and thermal properties table, we know that soil thermal parameters are as follows:

Conductivity = 1.6 W / mk

Diffusivity = 0.020 ~ 0.011m2 / h

Backfill = 1.9 W / mk

Cover thickness of 100%

Check the vertical U-loop length correction factor table, obtained soil length of the heat exchanger correction factor 2.4, then the last soil heat exchanger length correction L = 7271 * 2.4 = 17450m

According to the above parameters, know the required number of pipe pile:

N pile number = 17450 / (40 * 4) = 218 pieces

S column spacing = 9m * 9m

According to the layout drawings of 240 piles on the civil drawings, the design of 8 same-loop loops of 218 coupling pipelines in the inner of the pile holes is concentrated to the set and water separator through the same level header connection. The completely U-shaped heat transfer tube laying inside the pile can greatly satisfy the cold (heat) load requirement of the office building, so the coupling hole is not required to be drilled as a supplement, and the initial investment cost is saved.

Example 2:

Stadtwerke (Bochum) company office building in Bochum, Germany, the use of 104 sets of embedded pipe pile, pipe embedded in the pile in the way of coupling pipe.

Third, the conclusion

The underground heat exchanger and the building pile embedding process have the advantages of saving the drilling process, reducing the initial investment and saving the distribution area of ​​the underground heat exchange system, which opens up a broader prospect for the application of the ground source heat pump air conditioning system. However, due to the limited number of piles available, this kind of nested technology often can only bear part of the air conditioning system load, so the need to drill into the hole as a cold and heat source supplement. Nested construction process requires close cooperation between civil engineering.

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