Performance of direct expansion coils

In direct expansion coils (DX) such as cooling coils used in air conditioners, the internal panel of split air conditioners, etc., refrigerants such as R-22 and R-134a evaporate and expand directly inside the pipes until the air Cool and dehumidify the environment. In this refrigeration method, the refrigerant enters a distributor and is distributed uniformly on the copper pipes. The blade density is usually around 12 to 18 blades per inch in length. And mostly direct expansion coils (DX) are used in four rows. Sometimes, micro fins with a density of about 60 fins per inch and a height of 0.008 inches are installed on the inner surface of the copper tubes to increase heat transfer efficiency.
The arrangement of airflow and refrigerant in direct expansion coils (DX) is often a combination of counter flow and cross flow, and the discharge header is usually located on the air inlet side. Refrigerant distribution and its placement on different circuits are critical in determining the performance of the coils. In direct expansion (DX) coils, the evaporated refrigerant vapor is supersaturated by about 10 to 20 degrees Fahrenheit to prevent any liquid refrigerant from flowing into the reciprocating compressors and damaging them. Finally, the refrigerant vapor enters the suction line through the header.
Direct expansion coils (DX) are designed to achieve maximum heat transfer efficiency in the refrigeration process. Direct expansion coils (DX) are also referred to as evaporator coils. These coils can be used in the structure of the evaporator (Cooling Coil) and condenser (Heating Coil). Compared to water-cooled systems, in a system equipped with direct expansion coils (DX), the airflow passing over the direct expansion coils (DX) due to the expansion of the refrigerant flowing in the coils directly and without the use of cooling water becomes.

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Figure 1 - Direct expansion coils (DX)

The operation and application of direct expansion coils (DX)

In systems equipped with direct expansion coils (DX), the air is cooled by the refrigerant passing through the pipes of the finned cooling coils. The image below shows a conceptual view of an air conditioning system with direct expansion (DX) coils.

Figure 2 - Conceptual view of air conditioning systems equipped with direct expansion coils (DX)

In this image, heat is taken from the air and discharged to the outside through 3 heat transfer rings (from left to right).
In the left loop, an air fan built into the system moves ambient air over the evaporator, whereupon the air transfers its heat to the refrigerant.
In the middle ring, a refrigeration compressor takes the refrigerant vapor from the evaporator to the condenser and returns it to the evaporator as a liquid refrigerant. The cycle continues in a closed loop with copper piping.
In the ring on the right, the condenser fan circulates the outside air along the condenser, where heat is transferred from the refrigerant to the outside air. Finally, after passing through an expansion valve located between the condenser and the evaporator, the refrigerant cools down and turns into a liquid.
Window air conditioning systems, package units, and split systems are examples of direct expansion (DX) systems. These systems include a hermetically sealed compressor, evaporator (with copper cooling coils and aluminum fins), intake air blower, filters, and outdoor unit. Direct Expansion Systems (DX) are necessarily supplied as factory-assembled units and are also known as Local Systems.
The applications and capacity ranges of these units are as follows:
Room air coolers (capacity range between 0.5 to 3 tons of refrigeration, suitable for an area of ​​less than 1000 square feet) such as air conditioners, etc.
Packages of units equipped with air-conditioning condensers (capacity range from 3 to 50 tons of refrigeration, suitable for a space with a maximum area of ​​1000 to 10000 square feet) such as rooftop packages, special air conditioners, etc.
Split systems equipped with an air-conditioning condenser in the external unit (capacity range of about 0.5 to 50 tons of refrigeration, suitable for a space with an area of ​​100 to 10,000 square feet) such as splits, split ducts, etc.
Note: Each building has special and different characteristics and the design conditions differ significantly from one area to another. In humid and hotter areas, the cooling demand may reach 1 ton of refrigeration per 150 square feet, and in cooler areas, the cooling demand is even estimated to be 1 ton of refrigeration for about 500 square feet! To provide comfort and air conditioning, and in the absence of heating load calculations, it seems reasonable to consider 200 square feet per ton of refrigeration as an initial estimate and as a rule of thumb for temperate climates.
Systems with direct expansion coils (DX) that work based on the reverse vapor compression cycle (Reverse Vapor Compression Cycle) are classified as heat pumps. By adding a special control valve, the heat flow in the mechanical refrigeration cycle can be reversed in such a way that the heat is taken from outside and transferred inside the building. Such equipment is needed during the cold season to heat the interior using the same physical components. Due to the added heat of condensation, the efficiency of the heat pump is higher compared to the refrigeration cycle.
In general, air conditioning systems are divided into four categories: All Air Systems, Air-Water Systems, All Water Systems, and All Refrigerant or Direct Expansion Systems. They become mel
The use of direct expansion coils (DX) in air conditioning systems is rapidly increasing due to the elimination of most of the ducting and piping. The reason for the popularity of these coils is the ease of installing the system and consequently reducing the overall costs of operating the system. As mentioned before, among air conditioning systems, direct expansion coils (DX) in air conditioners, splits and duct splits, central split systems (VRF), compression chillers, package units, and heat pumps are used with gas engines (GHP). Therefore, air conditioning systems with direct expansion coils (DX) can be used in spaces such as residential, commercial, and office buildings, hotels, industrial units, chemical factories, research laboratories, etc. In the pictures below, the main components of some systems equipped with direct expansion coils (DX) can be seen.

Figure 3 - Location of the main components of the air conditioner equipped with direct expansion coils (DX)

Figure 4 - Placement of the main components of the split equipped with direct expansion coils (DX)

Figure 5 - Placement of the main components of the central split equipped with direct expansion coils (DX)

Figure 6 - Placement of the main components of the water compression chiller equipped with direct expansion coils (DX)

Figure 7 - Positioning of the main components of the combined system of air compression chiller and air generator equipped with direct expansion coils (DX)

Figure 8 - Positioning of the main components of the unit package equipped with direct expansion coils (DX)

Figure 9 - Placement of the main components of the heat pump equipped with direct expansion coils (DX)

Advantages of systems equipped with direct expansion coils (DX)

Systems equipped with direct expansion (DX) coils have several advantages. The insignificance of waste compared to conventional systems is the most remarkable feature of these coils. Other benefits are discussed below:

• Strong cooling performance
• Low operating costs
• Set up in a very short time
• Very low maintenance costs compared to other systems
• Easy assembly and low assembly cost
• Reduced operating costs for heating and cooling due to the elimination of piping and water pumps
• Minimum ceiling and wall space required
• Enabling heating and cooling with a central split system (Variable Refrigerant Flow: VRF)
• Providing a new, easy, fast solution that requires only one investment for small and medium-scale buildings
• Can be used in applications requiring medium power
• Eliminating the possibility of air conditioner coils freezing
• Low energy consumption
• Good control of relative humidity
• The absence of energy loss in refrigerant pipes is a problem of conventional systems.
• Longer service life than conventional systems
• Less noise

Disadvantages of systems equipped with direct expansion coils (DX)

Systems with direct expansion coils (DX) have their advantages as well as disadvantages, which are discussed below:

• Investment costs are sometimes higher than conventional systems.
• Condenser efficiency loss during system operation in cooling mode
• Loss of capacity during the operation of the system in cooling mode and when the ambient air temperature exceeds 43 degrees Celsius
• Loss of capacity during the operation of the system in the heating mode and during the operation of defrosting required in the external unit
• Although these disadvantages can be improved by adding some items (electric heating coils or hot water coils) to the system. It should be noted that even though the defrosting activity is unwanted, it cannot be avoided and if other solutions are not considered for this issue, the airflow of the system may stop during the defrosting time (about 5 minutes).

Why are systems equipped with Direct Expansion Coils (DX) only suitable for SingleThermal Zone applications?

In air conditioning systems with direct expansion coils (DX), system control is necessary for their successful and efficient operation.
Capacity control in systems with direct expansion coils (DX) is mostly realized by turning off and on the compressor in response to the signal received from the thermostat. So the two rooms overlap with the control thermostats set to 22°F and 28°F, or in other words,, the rooms cannot reach the set conditions unless the two rooms are cooled by different units. To cool a zone (Single Zone), the air conditioning system equipped with a direct expansion coil (DX) has only one control device or thermostat.
Air conditioning systems with direct expansion coils (DX) cannot be properly networked.
Refrigerant piping plays a key role in connecting different components in terms of size, length,, and pressure drop. It is not recommended to use multiple evaporators in a direct expansion system, because in this case,, it is difficult to adjust the exact flow of refrigerant to the evaporators. Therefore, each direct expansion system is an independent unit that includes a compressor(s), evaporator coils, fan, external unit (Condensing Unit), and filtration unit (Filtration Unit). For large buildings that include several zones, air conditioning systems equipped with direct expansion coils (DX) are a collection of independent units that are placed in different places in a network, and each unit operates in isolation. , are seen.

You can also read the article TWO PIPES OR FOUR PIPES FAN COIL UNITS ! for more information.

Final speech

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