Wet and Direct Expansion DX Heating and Cooling Batteries

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Choosing the cooler battery and determining the factors that affect its performance is one of the most important aspects of climate engineering. It should not be forgotten that an incorrectly selected or designed cooler battery will affect the entire climate system.

Cooling Coils (Battery)

Cooling coils are used to cool air with forced convection, with or without moisture. The level of dehumidification depends on the structure of the coil, the raw temperature of the inlet air, the water flow and the water temperatures. The sensible heat ratio of the serpentine, which indicates the ratio of sensory heat change to total heat change, can be used to determine the degree of humidity. Depending on the refrigerant flow used, the cooling coils are divided into two types:

• Water serpentines
• Directly expanding serpentines

To prevent freezing in cold climates in winter, water coolers can be used in watery serpentines to cool them down. Energy such as heating serpentines is spent on the primer side and / or on the air side of the coil.

Watery Serpentines

The movement of the water circulating in the serpentine is provided by one or more circulation pumps. The energy consumption of the circulation pumps determines the hydraulic design of the distribution pipe network and the water side pressure loss in the serpentine. The energy consumption of the pumps is also affected by:

• Pump efficiency (dimensionally linked)
• Serpentine control type (variable water flow, variable water temperature)
• Circulation pump speed control

Direct expansion coils (DX)

The primary energy consumption required to provide flow in the refrigerant coil is also the cost of cooling energy. The compressor in the refrigeration unit provides the necessary pressure to circulate the refrigerant in the coil and pipeline.

Cold Water Battery Selection Criteria

• For choosing the cooler battery, firstly the speed of the cooler battery should be defined.

This parameter directly affects the choice of cooler battery. The following considerations should be considered when defining this value.

1. High air speed increases the battery bypass factor (BF).
2. High air speed increases the air side pressure loss, which causes the fan motor power to increase.
3. High air velocity causes water droplets to be carried on the surface of the battery.
4. High air speed increases the sound level in the battery.

Recommended battery air speed is between 2.0-2.7 m / s.

• The incoming and outgoing weather conditions should be determined as dry and wet thermometer temperature.
• The incoming air flow must be determined.
• The temperature of the cooled water circulating in the cooler battery should be determined. (t sg, t s)
• The increase in the number of batteries increases the pressure loss of the battery air side, as well as the inefficiency of the battery. In order to keep the number of batteries at the optimum, the following must be considered:
1. The water temperature to be sent to the battery should be about 3 ° C lower than the battery average surface temperature (CÇN).
2. The conditions of the air leaving the battery should not be too close to the CWN. (BF should be 8-15%.)
• Keeping the logarithmic temperature difference higher than the recommended value will increase the total capacity from the battery, whereas the chiller will decrease due to the low evaporation temperature, thus reducing the overall energy efficiency.
• In hydraulic calculations, the baseline of the battery fluid should be restricted for pressure loss.For aqueous batteries it is recommended that the fluid side pressure loss not exceed 40 kPa. The loss should be designed to be lower if possible, because of the impact on overall energy efficiency of the pump efficiency.
• Considering the risk of dragging condensate on the battery surface due to the high forehead air velocity, the use of the drop holder should be considered.

Direct Expanded (DX) Batteries

Direct expansion batteries are often used in air cooling applications with cooling units ( outdoor unit ). The direct expansion battery is installed in the duct-mounted or air handling unit and the outer unit is located outside.

– Cooling batteries designed for dehumidification should pay attention to the following points:
• Moisture should not move to the next section or cells from the battery.
• The drip tray of wet batteries must be manufactured from corrosion resistant (eg AISI 316 (stainless 1.4301) or corrosion resistant aluminum alloy (AlMg) material).
• Connection pipes passing through the housing must be insulated to prevent condensation.
• Due to energy and hygienic requirements, the drop holder should only be used if there is a risk of dragging the drop.
• If copper / copper or copper / aluminum batteries are used in terms of corrosion resistance, it is recommended that the battery collector be copper. If a galvanized wet battery is to be used, hot dip galvanizing should be preferred.

Directly Expanded Battery Selection Criteria

• In direct expansion batteries, in addition to aqueous batteries, the type of refrigerant, evaporation temperature and coagulation temperature should be determined. Particular attention should be paid to the copper tube wall thickness for coils using high-pressure refrigerant.
• It is recommended that the refrigerant side pressure loss should not exceed 30 kPa in direct expansion batteries.
• If R-410A coolant is used in direct expansion batteries, copper pipe wall thicknesses should be increased.
• Direct expansion batteries should be targeted at higher evaporation temperatures (to increase compressor efficiency) in order to increase energy efficiency.
• Direct expansion systems are relatively more efficient systems than water circulation systems.

Heating Coils (Battery)

Air heating coils are used to heat forced convection air. Depending on the heating medium, the serpentines are divided into the following categories:
• Water serpentines
• Steam serpentines
• Electric heating coils
Energy is consumed on the primer side and / or the air side of the coil.

Hot Watery Battery Selection Criteria

Due to the lack of choking, the heater speed in the batteries is not as critical as the forehead speed in the cooler batteries. However, since it is placed in the same power plant as the cooling battery, it has practically the same cross section. High speeds in terms of pressure loss are also not recommended here. It is recommended that the air speed should not exceed 3 m / s in systems with heating battery only.

Because the fluid temperature circulating in the battery is considerably higher than the temperature of the air entering the battery, the battery surface area is very low compared to the coolant battery due to the number of batteries and air side pressure loss.

• Heating with low temperature fluids is advantageous in terms of energy efficiency.
• The dry thermometer temperature of the incoming and outgoing air should be determined.
• The heater must indicate the water flow circulating in the battery.
• Battery air and fluid side pressure loss should be specified.
• The incoming air flow and heating capacity must be defined.
• Water pressure loss is not recommended to exceed 40 kPa.

Watery Serpentines

The flow of water circulating in the serpentine is provided by one or more circulation pumps. The energy consumption of the circulation pumps determines the hydraulic design of the heating distribution pipe network and the water side pressure loss in the serpentine. The energy consumption of the pumps is also affected by:
• Pump efficiency (dimensionally linked)
• Serpentine control type (variable water flow, variable water temperature)
• Circulation pump speed control

Electric heating serpentines

The primary side energy losses of the electric heater coils come from the heat generated in the power supply cables. The loss determines the electrical resistance of the current to the heating element and the wires. Voltage drops for heat generation in electrical cables for safety reasons are restricted to national and international standards and regulations.

Steam serpentines

For steam coils, it is assumed that the primary energy consumption is taken into consideration in the thermal heating energy system. The steam at the win provides pressure, steam and condensed water flow.

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