Why do direct-fired engines often freeze?

1. Precautions for selecting water chillers:

★ The purpose of the building.

★ Performance and characteristics of various chillers.

★ Local water sources (including water quantity, temperature and water quality), power sources and heat sources (including types, properties and grades of heat sources).

★ Distribution law of cooling load (heating load) of building air conditioning throughout the year.

★ Initial investment and operating expenses.

★ Limit period of Freon refrigerant and possibility of using alternative refrigerant.

2. Matters needing attention in the selection of water chillers:

After fully considering the above factors, the following points should be paid attention to when selecting water chillers:

★ The cold source of a large central air conditioning system should be a water chiller with compact structure and small floor space. The compressor, motor, condenser, evaporator and automatic control components are all assembled on the same rack. For small air conditioning systems, direct evaporative compression condensing units should be adopted.

★ For places with suitable heat sources, especially places with waste heat or waste heat or power shortage, absorption chillers should be used.

★ Generally, 2 ~ 4 refrigeration units, 2 small and medium-sized units, 3 large units and 4 extra-large units should be selected. The possibility of standby and inter-unit switching should be considered. In the same computer room, different types and capacities of unit combination schemes can be adopted to save energy consumption. Among the units running in parallel, at least one unit with high degree of automation, good regulation performance and high efficiency under partial load should be selected. When selecting piston chillers, multi-head automatic combined control chillers should be preferred.

★ When selecting electric water chillers, when the cooling capacity of a single air conditioner is φ > 1 163 kW, centrifugal type should be selected; When φ = 582 ~ 1 163 kW, centrifugal or screw type should be selected; When φ < 582 kW, piston type should be selected.

★ The COP of electric refrigerator is higher than that of absorption refrigerator, and the former is more than twice as high as the latter. The order of energy consumption from low to high is: centrifugal type, screw type, piston type and absorption type (screw type of foreign units is before centrifugal type). However, various units have their own characteristics and strengths.

★ When choosing a refrigerator, environmental pollution should be considered: First, noise and vibration should meet the requirements of the surrounding environment; Second, the harm of refrigerant CFCs to the atmospheric ozone layer and the size of greenhouse effect, with special attention to the timetable for banning CFCs. Absorption refrigerator has obvious advantages in preventing CFCs pollution.

★ If there is no special computer room or air conditioning renovation and installation project, modular water chillers can be considered.

★ Try to choose domestic units. China's refrigeration equipment industry has developed rapidly in recent ten years, and the performance of most products is close to the international advanced level, especially small and medium-sized water chillers, which can compete with imported products and have unparalleled advantages in price. Therefore, under the same conditions, domestic chillers should be preferred.

(2) Heat pump unit

★ The calculation method of cooling load of heat pump unit is the same as that of conventional air conditioning system, and the calculation method of heating load is basically the same as that of heating system, but the heat consumption of fresh air should be considered;

★ Pay attention to whether the local water source (including water quantity, water temperature and water quality), power supply and heat source (including the nature and grade of heat source) are sufficient when selecting the type;

★ The water supply temperature of air-cooled heat pump units is generally 45℃, and the heating provided in samples such as fan coil units and combined air conditioning units is usually based on 60℃ water inflow, so the heating of these devices must be revised;

★ When selecting a heat pump unit, it should be based on the heating load in winter and the cooling load in summer;

★ For buildings with particularly heavy internal load and fresh air load such as shopping malls and restaurants, the heating load is generally only 60% ~ 70% of the cooling load. Therefore, it is appropriate to adopt the mode of combined energy supply of heat pump unit and single cooling unit, such as "30 1" mode, that is, three air-cooled heat pump units plus 1 single cooling unit;

★ The rated heat supply of air-cooled heat pump unit is usually the value under standard working conditions (ambient temperature t0 = 7℃, outlet water temperature ts = 45℃). When the ambient temperature is below 7℃, the heat supply will be greatly reduced. The general decline range is as follows: when T0 = 5℃, the percentage of decline is 5% ~ 8%; When t0 = 3℃, the percentage of decrease is 12% ~ 14%, and when t0 = 0℃, the percentage of decrease is 25% ~ 32%. When t0 =-3℃, the percentage of decrease is 45% ~ 50%; When t0 =-5℃, the percentage of decrease is 55% ~ 65%. Note: Air-cooled heat pump units designed according to standard operating conditions cannot operate normally below -3℃.

★ Air-cooled heat pump unit has small capacity and is suitable for small and medium-sized projects;

★ In winter, the outdoor temperature is always higher during the day than at night. Therefore, in areas where the calculated outdoor heating temperature is TW =-3℃ for a long time, air-cooled heat pump units can be used for buildings that are only used in the daytime, such as office buildings and shopping malls. For buildings that need all-day (24-hour) heating, air-cooled heat pumps should be treated with caution;

★ Water source heat pump system is more suitable for multi-tenant apartment buildings and large villas with larger area. When designing, ensure the accurate calculation of system water flow, so as to facilitate the selection of cooling towers, pumps and other equipment;

★ When selecting a heat pump in an area with high relative humidity, special attention should be paid to analyzing the operating conditions and taking effective defrosting measures.

(3) The heat pump unit in the ground source heat pump room.

1. The heat pump unit in the ground source heat pump room can be selected according to the following steps:

★ The capacity of water source heat pump unit should not be too large. When the cold and heat source equipment of central air conditioning is selected, the refrigeration (heat) capacity of the equipment is about 1.05 ~ 1. 10 of the designed cold (heat) load.

★ When selecting a water source heat pump unit, it should be as close as possible to the design cooling (heating) load. If the unit is too large, the running time is short and the startup is frequent. The unit capacity is appropriate, and the operation time is long, which is beneficial to dehumidification.

★ Selection of water temperature of closed water system requires lower water temperature in summer to improve energy efficiency and reduce power consumption. The water temperature should not be too high in winter, because when the water temperature is high, the cooling capacity is high, but the power consumption is also high, and the energy efficiency coefficient does not change much.

★ The simultaneous use coefficient of the heating and air conditioning target building should be considered in the design. The value of simultaneous use coefficient is related to the type and quantity of buildings, which needs to be determined by theoretical calculation and actual measurement. The data listed in "Determination of Simultaneous Use Coefficient in Air Conditioning Load Calculation of Residential Buildings" are: when the number of households is less than 100, the coefficient is 0.7; 0.65 ~ 0.7 when the number of households is 100 ~ 150; 0.6 when the number of households is 150 ~ 200.

2. The outdoor underground heat exchange part can be selected with reference to the following steps:

The selection of geothermal heat exchanger includes the selection of type and structure. For a given construction site condition, the design should be as low as possible and meet the operation needs. The selection of geothermal heat exchanger mainly involves the following aspects:

★ Layout mode of geothermal heat exchanger, including buried pipe mode and connection mode, as shown in the figure. Buried pipelines can be divided into horizontal and vertical types. The choice mainly depends on the site size, local soil type and excavation cost. If the site is large enough and there is no hard rock, horizontal style is more economical. If the site area is limited, vertical layout is the only choice in many cases. If there are hard rocks in the field soil, you can successfully drill holes with a rock drill. There are two connection modes: series connection and parallel connection. In series connection, there is only one fluid channel, while in parallel connection, there can be more than two fluid channels in the pipeline. Whether to use series or parallel depends on the cost. The diameter of series system is larger than that of parallel system, and the cost of large-diameter pipeline is higher. In addition, due to the larger pipe diameter, the system needs more antifreeze, and the weight of the pipeline increases accordingly, resulting in higher labor costs for installation.

★ Selection of plastic pipe, including material, pipe diameter, length and head loss of circulating fluid. Polyethylene is the most commonly used pipe material in geothermal heat exchangers. This kind of pipe has good flexibility and can be heated and melted to form a joint with better strength than the pipe itself. The selection of pipe diameter should follow the following two principles: first, the pipe diameter is large enough to make the energy consumption of circulating pump smaller; Secondly, the pipe diameter is small enough to make the fluid in the pipe in the turbulent region, so that the heat exchange effect between the fluid and the inner wall of the pipe is good. At the same time, the installation cost should be considered in the design.

★ Selection of circulating pump. The selected circulating pump should ensure that the driving fluid can continuously flow through the heat pump and geothermal heat exchanger with low power consumption. Generally, in the design, the circulating pump should be able to reach the energy consumption level of each ton of circulating liquid 100W.