Ground source heat pump system design method steps
Editor's Note: This article describes the design of ground-source heat pump systems and steps, focusing on the underground heat exchanger design process. And give an example to illustrate.
With the continuous development of China's construction industry, the energy-saving requirements for building are getting higher and higher, while the heating system and air-conditioning system are the main components of building energy consumption. Therefore, it is very significant to try to reduce the energy consumption of these two parts. Ground source heat pump heating air conditioning system is a use of renewable energy efficient, energy saving, environmentally friendly system <1>. In winter, energy is supplied to the building by absorbing the energy of the earth, including natural energy such as soil, well water and lakes. In summer, heat is released to the earth to cool the building. Correspondingly, ground source heat pump system is divided into three forms: ground source heat pump system, groundwater heat pump system and surface water heat pump system.
The heart of a ground-source heat pump system is a soil-coupled heat exchanger.
Groundwater heat pump system is divided into open, closed two: open groundwater directly to the heat pump unit, and then well water back to the ground; closed groundwater is connected to the plate heat exchanger, the need for secondary heat exchange .
The surface water heat pump system is similar to a ground-source heat pump system and replaces the soil heat exchanger with a groundwater heat exchanger consisting of potentially underwater parallel plastic pipes.
Although groundwater and surface water heat pump systems have good heat transfer performance, low energy consumption and a higher coefficient of performance than ground-source heat pumps, groundwater and surface water are not available everywhere and water quality may not necessarily meet the requirements. Therefore, The scope is limited. The groundwater source heat pump systems that are mainly studied and applied abroad (such as the United States and Europe) and the theoretical and experimental studies in China focus on the ground source heat pump system. The current lack of system design data and more specific design guidance, this paper conducted a preliminary discussion for reference.
1 ground source heat pump system design of the main steps
(1) The thermal load of buildings and underground heat exchange calculation in winter and summer
Building thermal load calculation and conventional air conditioning system cooling load calculation method is the same, refer to the air conditioning system design manual, not repeat them here.
The amount of underground heat transfer in winter and summer respectively refers to the heat released to the soil in summer and the amount of heat absorbed from the soil in winter. Can be calculated by the following formula <2>:
kW (1)
kW (2)
Where Q1 '- summer heat to the soil, kW
Q1 - summer design total cooling load, kW
Q2 '- Heat absorbed from the soil in winter, kW
Q2 - Total heat load in winter design, kW
COP1 - Design Coefficient of Cooling Water Source Heat Pump Unit
COP2 - Design conditions of water source heat pump unit heating coefficient
In general, the water source heat pump unit samples are given under different inlet and outlet water cooling capacity, heating capacity and cooling coefficient, heating coefficient, calculation should be selected from the sample design conditions COP1, COP2. If there is no desired design condition in the sample, interpolation can be used.
(2) underground heat exchanger design
This part is the core of ground source heat pump system design, including the form of underground heat exchangers and pipe selection, pipe diameter, pipe length and the number of shafts, spacing determination, pipe resistance calculation and pump selection. (Described in detail below)
Editor's Note: This article describes the design of ground-source heat pump systems and steps, focusing on the underground heat exchanger design process. And give an example to illustrate.
(3) other
2 underground heat exchanger design
2.1 Select the heat exchanger form
2.1.1 horizontal (horizontal) or vertical (vertical)
Based on the site survey results, consider the available surface area available on the site, the type of soil in the area, and the cost of drilling to determine if the vertical heat exchanger is to be arranged vertically or horizontally. Although the horizontal arrangement is usually shallow buried pipe, manual excavation may be used, the initial investment will be cheaper, but its heat exchange performance is much smaller than the vertical pipe <3>, and often limited by the available land area, so in The actual project, the general use of vertical buried pipe layout.
There are three types of vertical pipe jacking according to different pipe laying methods: (1) U-tube (2) Sleeve type (3) Single-tube type (see <2> for details). There is heat loss in the fluid heat exchange between the inner and outer tubes of the casing type. The use of single-tube type is limited by hydrogeological conditions. U-tube is the most widely used, the diameter is generally below 50mm, the deeper the tube, the better heat transfer performance, the data show that: the deepest U-tube depth has reached 180m. There are three typical U-shaped loops (see <1> for details). The most common use is for single-lumen U-tubes in each shaft.
2.1.2 series or parallel
Underground heat exchangers in the form of fluid flow loop in series and parallel in two, the larger the diameter of the series system, the pipeline costs are high, and the length of the pressure drop characteristics of the system capacity. Paralleling systems have smaller pipe diameters, lower pipe costs, and are often arranged in the same way. When the flow balance between each parallel loop is the same, the heat exchange capacity is the same, and the pressure drop characteristic is conducive to improving system capability. Therefore, the actual project are generally used in parallel with the program. Combination of the above, that is often used in a single U-tube heat exchanger in parallel with the form.
2.2 Select the pipe
In general, once the heat exchanger buried in the ground, the basic impossible to repair or replacement, which requires the buried pipe buried under the chemical stability and corrosion resistance. The metal pipe used in the conventional air-conditioning system is seriously deficient in this respect, and there are many pipelines that need to be buried in the ground. Therefore, the use of lower-priced pipe materials should be given priority. Therefore, the general use of plastic pipe in the ground source heat pump system. At present, the most commonly used polyethylene (PE) and polybutylene (PB) pipe, they can be bent or hot melt to form a more solid shape, you can guarantee the use of more than 50 years; and PVC pipe due to not bending, pressure endurance Poor, easily lead to leakage, therefore, is not recommended for underground buried pipe system.
2.3 determine the diameter
In practical engineering to determine the diameter must meet two requirements (2): (1) the pipe is large enough to maintain the minimum delivery power; (2) the pipe is small enough to maintain turbulence in the pipe to ensure that the fluid and the pipe wall Heat transfer between. Obviously, the above two requirements are contradictory and need to be considered together. Common parallel with the small diameter of the loop, the collector with a large diameter, underground heat exchanger buried pipe diameter commonly used 20mm, 25mm, 32mm, 40mm, 50mm, the control of the flow in the tube below 1.22m / s, for larger diameter Of the pipe, the pipe speed control in the 2.44m / s below or generally the pressure loss of each pipe control 4mH2O / 100m equivalent length below the <1>.
2.4 to determine the shaft buried pipe length
The length of the underground heat exchanger to determine in addition to the established system layout and pipe, but also need to have local soil technical information, such as underground temperature, heat transfer coefficient.
Editor's Note: This article describes the design of ground-source heat pump systems and steps, focusing on the underground heat exchanger design process. And give an example to illustrate.
Literature <2> introduced a calculation method is divided into 9 steps, it is cumbersome, and some of the data is not easy to obtain. In the actual project, you can use the pipe "heat capacity" to calculate the pipe length. The heat transfer capacity is the vertical depth of the unit or the heat transfer per unit length of the pipe. Generally, the vertical pipe is 70-110 W / m (well depth), or 35-55 W / m (pipe length), and the horizontal pipe is 20-40 W / m (pipe length) around <3>.
The design may take the lower limit of heat capacity, that is, 35W / m (pipe length), the specific formula is as follows:
(3) Where Q1 '- the total length of the shaft buried pipe, m
L - Heat released to the soil in summer, kW
The denominator "35" is the heat dissipation per m tube length in summer, W / m
2.5 to determine the number of shafts and spacing
Abroad, most of the vertical shaft depths are 50 ~ 100m <2>. The designer can select a vertical shaft depth H within this range and calculate the number of shafts by the following formula:
(4) Where N - the total number of shafts, a
L - shaft buried length, m
H - shaft depth, m
The denominator "2" takes into account that the length of buried pipe in the shaft is about twice as long as the shaft depth.
Then calculate the results round, if the calculation results are too large, you can increase the shaft depth, but not too deep, or drilling and installation costs increased significantly.
Information on the shaft spacing points out that: The horizontal spacing of the U-tube shaft is generally 4.5m <3>. There are also U-shaped tubes with DN25 mentioned in the examples, and the horizontal spacing of the shafts is 6m. The DN20's U- Pitch 3m <4>. Triangular layout (see <2>) saves floor space if series connection is used.
2.6 Calculate pipeline pressure loss
In the same system, the loop where the heat pump with the largest pressure loss is located is selected as the most adverse loop to calculate the resistance. Equivalent length method can be used to convert the local resistance component into equivalent length and add the actual length of the pipe to obtain the total equivalent of each pipe section of different diameters and then multiplied by the pressure drop per 100 meters of pipe sections with different flow rates and different pipe diameters The sum of all tube pressure drops gives the total resistance.
2.7 pump selection
Based on the pipeline pressure loss resulting from the calculation of the most adverse cycle, plus the pressure loss of heat pump units, balancing valves and other equipment components, the lift of the pump is determined by taking into account a certain safety margin. According to the total system flow and pump head, choose to meet the requirements of the pump model and the number of units.
2.8 check the pipe pressure capacity
The maximum pressure pipe should be less than the pressure capacity of the pipe. Excluding the static pressure caused by grouting in the shaft, the maximum pressure that the pipe can withstand is equal to the atmospheric pressure. The sum of the static pressure of gravity and the half of the pump head is <1>
Where p - the maximum pressure of the pipeline, Pa
Editor's Note: This article describes the design of ground-source heat pump systems and steps, focusing on the underground heat exchanger design process. And give an example to illustrate.
p0 - the local atmospheric pressure where the building is located, Pa
Ï - underground buried pipe fluid density, kg / m3
g - local gravitational acceleration, m / s2
h - the lowest point of underground pipe and closed loop system, the highest point of the height difference, m
Ïh - pump head, Pa
3 other
3.1 Similar to conventional air conditioning system, it is required to design expansion tank or expansion tank, air release valve and other accessories at the highest point (usually 1m) higher than the closed cycle system.
3.2 In some GSHP systems, the cooling capacity of the system is much larger than that of the heat supply, resulting in a huge underground heat exchanger which is expensive to save investment or land available, The pipe can be designed according to the maximum heat absorption under the designed heating conditions, meanwhile, the auxiliary heat exchange device (such as cooling tower + plate heat exchanger, plate heat exchanger is mainly used to make the inner loop of the building run independently of the cooling tower ) Undertake the part of cooling capacity that exceeds the heat exchange capacity of underground buried pipe under the condition of cooling. The method can reduce the installation cost and ensure that the ground source heat pump system has a larger market prospect, and is particularly suitable for the reconstruction project <1>.
4 design examples
4.1 Design parameters
Shanghai, a duplex residential air conditioning area 212m2.
4.1.1 outdoor design parameters
Summer outdoor dry-bulb temperature tw = 34 ℃, wet bulb temperature ts = 28.2 ℃
Winter outdoor dry-bulb temperature tw = -4 ℃, relative humidity φ = 75%
4.1.2 interior design parameters
Summer indoor temperature tn = 27 ℃, relative humidity φn = 55%
Winter indoor temperature tn = 20 ℃, relative humidity φn = 45%
4.2 Calculation of air conditioning load and choose the main equipment
Considering the calculation method of the cooling load and heating load of the conventional air conditioning building, the cooling load and heating load of each room are calculated and the fan coil model is selected. Taking the common coefficient of the room (taking 0.8), the total cooling load of the building in summer design is 24.54 kW, Heat symbol load of 16.38kW, choose WPWD072 type water source heat pump unit 2, the design example case COP1 = 3.3, COP2 = 3.7.
4.3 Calculate the underground load
According to the formula (1), (2) calculated kW
kW
Take summer to the soil heat Q1 'design calculations.
Editor's Note: This article describes the design of ground-source heat pump systems and steps, focusing on the underground heat exchanger design process. And give an example to illustrate.
4.4 determine the pipe and pipe diameter
Polyethylene pipe PE63 (SDR11) is used. The diameter of the parallel loop is DN20, and the pipe diameters of the pipes are DN25, DN32, DN40 and DN50, respectively.
4.5 to determine the shaft buried pipe length
According to the formula (3) calculated
m
4.6 determine the number of shafts and spacing
Select shaft depth of 50m, calculated according to formula (4)
A
After rounding take 10 shafts, shaft spacing 4.5m.
4.7 Calculate buried pipe pressure loss
With reference to the calculation method introduced in 2.6, the pressure loss of each pipe segment of 1-2-3-4-5-6-7-8-9-10-11 / 11'-1 'was calculated respectively, and the total pressure loss of each pipe segment was 40kPa. Coupled with the loss of line pressure to the heat pump unit and the pressure loss of heat pump units, balancing valves and other equipment components, the pump head selected was 15 mH2O.
4.8 check the pipe pressure capacity
Shanghai summer atmospheric pressure p0 = 100530 Pa, water density Ï = 1000 kg / m3,
The local gravitational acceleration g = 9.8 m / s2, height difference h = 50.5 m
Gravity effect static pressure Ïgh = 494900 Pa
Half of the pump head 0.5 Ïh = 7.5 mH2O = 73529 Pa
Therefore, the maximum pipe pressure p = p0 + Ïgh + 0.5 Ïh = 668959 Pa (about 0.7Mpa)
Polyethylene PE63 (SDR11) rated pressure capacity of 1.0MPa, the pipe to meet the design requirements.
5 Conclusion
Ground-source heat pump systems have broad application prospects in the Yangtze River basin and its surrounding areas in our country, but there are still limited studies on the main factors influencing the widespread application of ground source heat pump systems (such as heat transfer enhancement and soil properties of underground heat exchangers) , The design can generally follow the following principles:
(1) Where sufficient surface area is available around the building, first consideration should be given to the use of a more economical method of horizontal pipe jacking; conversely, vertical U-shaped pipe jacking should be used if the available surface area around the building is limited.
(2) Although it is possible to connect buried pipes in series and in parallel, the small diameter is used in parallel, the initial investment and operating costs are low, so it is commonly used in practical projects. In order to maintain the balance of resistance among the parallel circuits, The best design into the same program.
(3) When choosing the pipe diameter, the pressure loss of each pipe should generally be controlled below 4mH2O / 100m (equivalent length), in addition to the installation cost. At the same time, the flow in the pipe should be in the turbulent transition zone.
Fully Electric Container Spreader
Gantry Crane Spreader,Electric Lifting Spreader,Electric Container Spreader,Fully Electric Container Spreader
Jiangsu OUCO Heavy Industry and Technology Co., Ltd , https://www.cranemarine.com