Theoretical Analysis on the Design of High-efficiency & Energy-saving Refrigerated Air Dryers

I. Severe Energy Security Situation and Significance of Energy Conservation

Global warming, Sino-US trade frictions, gradual depletion of fossil fuels, and the trend of deglobalization in the world economy… Amid such turbulent global circumstances, 

all countries are strengthening security protection, especially energy security. Since 2014, China has continuously emphasized that energy security is an overall and 

strategic issue bearing on national economic and social development, and it is crucial for national prosperity, improvement of people’s livelihood and long-term social stability. 

As the world’s largest energy producer and consumer, how to safeguard national energy security and support economic and social development remains the primary challenge for the energy sector.

Over the past five years, thanks to efforts at all levels nationwide, remarkable results have been achieved: the average annual growth rate of national total energy consumption stands at 2.2%, 

and energy consumption per unit of GDP has decreased by 20.3% [1]. Nevertheless, a considerable gap still exists compared with advanced developed economies. 

Therefore, energy conservation is an inevitable path for the sustainable development of China’s economy, and also one of the important measures to guarantee energy security for our large-population country.

II. Value of Developing High-efficiency & Energy-saving Refrigerated Air Dryers

Incomplete statistics show that hundreds of thousands of refrigerated air dryers are manufactured each year, and the market inventory has reached several million units. 

Based on this data, estimation is conducted assuming an average power of 10 kW per unit (corresponding to a processing capacity of 69 m³/min) 

and an inventory of 1.5 million units (the power of refrigeration compressors for dryers ranges from 0.4~52 kW with flow capacity of 0.9~269 m³/min):

Annual energy consumption of refrigerated air dryers:

10 kW × 1,500,000 units × 8,600 hours/year = 129,000,000,000 kWh (129 billion kWh). If 20% energy can be saved, annual power saving will reach 25.8 billion kWh. 

Accordingly, operational power consumption of refrigerated air dryers can only be reduced via scientific management and technical improvement.

The larger the pressure differential between the air inlet and outlet of the dryer, the higher the required air supply volume of the air compressor to obtain the same amount of compressed air at the dryer outlet, 

which increases energy consumption. General data indicates that a pressure differential of 1 bar (0.1 MPa) leads to approximately 7% extra power consumption of air compressors. 

At present, the pressure differential of most commercial refrigerated dryers is about 0.03 MPa. If it can be lowered to 0.01 MPa, 

the energy consumption of air compressors can be cut by 1.4%. According to historical data, the annual power consumption of air compressors totals around 800 billion kWh, 

which means a potential power saving of 800 billion × 1.4% = 11.2 billion kWh. Pressure differential arises on the one hand from inherent resistance along the airflow path, 

and on the other hand from unreasonable design of refrigerated air dryers.

Compressed air loss caused by drain devices: The automatic drainer is a vulnerable component of refrigerated dryers. 

Improper operation or long-term service may trigger two opposite malfunctions: continuous draining without closure or permanent closure without draining. In the case of continuous draining, 

massive compressed air leaks and causes air waste. When the drainer stays closed, condensed water accumulates inside the evaporator, pushing up the dew point of treated air or leading to water entrainment in outlet air; 

meanwhile, accumulated water increases the load of the refrigeration system. Both faults exert severe adverse impacts on energy consumption.

Judging from the three aspects above, designing and manufacturing high-efficiency energy-saving refrigerated air dryers carry substantial economic value and social significance.
III. Definition of Energy-saving Refrigerated Air Dryers
Energy efficiency is a relative indicator. Comparisons include: new dryers vs traditional dryers; Nth-generation new dryers vs (N-1)th-generation dryers; and dryers from different brands.
Under the same air processing capacity and identical outlet air quality, lower pressure loss (smaller pressure differential);
Under the same air processing capacity and identical outlet air quality, lower power consumption;
Under the same air processing capacity and identical outlet air quality, less compressed air loss.
IV. Partial Theoretical Basis for Designing Energy-saving Refrigerated Air Dryers
Compressed air encounters different resistance when flowing through straight pipes and various pipe fittings, resulting in varying energy loss (primarily pressure loss).
(1) Frictional Loss
When air flows through pipelines, friction between airflow and pipe wall as well as internal gas friction creates flow resistance. The energy consumed to overcome such resistance is defined as frictional loss.
Where: λ — friction coefficient (varies under different flow regimes);
L — pipe length (m);
d — inner pipe diameter (m); equivalent diameter de shall be adopted for non-circular pipelines;
½ρv² — dynamic pressure head.
(2) Minor Loss

When compressed air passes through pipe fittings, valves, inlets, outlets and other local obstructions, airflow deformation occurs including expansion, contraction and turning. 

Abrupt variation of pipe boundaries redistributes flow velocity, generating intensified friction and collision between fluid particles and consuming energy. In addition, 

vortexes formed during velocity fluctuation also consume energy of the main flow. Energy loss caused by overcoming local resistance is called minor loss.

Where: ζ — local resistance coefficient; other parameters remain consistent with above.

Frictional resistance and local resistance are collectively referred to as flow resistance. Greater flow resistance leads to higher air pressure drop. 

Mass flow rate of compressed air and internal structure of purification equipment are the main factors affecting flow resistance.

(3) Tube Bundle Resistance

Energy loss occurring when compressed air flows through tube bundles inside heat exchangers. It is a combined pressure loss composed of numerous local resistances and frictional resistances. 

When compressed air flows across tube bundles arranged perpendicular to airflow direction, resistance loss depends on the number of tube rows, arrangement mode, Reynolds number and other parameters.

Where: v — gas velocity in narrow gaps of tube bundles (m/s);
ξ — overall resistance coefficient of the tube assembly.
Material Selection for Evaporators (Heat Exchangers): Comparison between Copper Tubes and 304L Stainless Steel Tubes

Pressure loss occurs as refrigerant flows inside the evaporator, lowering the suction pressure of the refrigeration compressor and reducing cooling capacity accordingly. 

To avoid this phenomenon, proper selection of inner diameter of heat exchange tubes, refrigerant flow velocity and flow passes is required during evaporator design. 

Two mainstream materials are available for heat exchange tubes on the market:

The thermal conductivity of copper tube is 100 W/(m·°C), while that of stainless steel tube is 13 W/(m·°C). Under identical other conditions, copper tubes deliver superior heat transfer performance.

Due to strength and abrasion requirements, the wall thickness of copper tubes cannot be less than 1.2 mm, 

whereas stainless steel tubes can be manufactured with wall thickness ranging from 0.5–0.8 mm. According to the law of conductive thermal resistance, 

with fixed thermal conductivity, thinner tube wall brings lower thermal resistance and higher overall heat transfer coefficient. 

Hence stainless steel tubes can narrow the heat transfer performance gap with copper tubes via thinner wall design.

Although thinner walls reduce the performance disparity between stainless steel and copper tubes, copper tubes still possess better overall heat exchange efficiency.

As service time extends, the oxide layer on copper tubes thickens continuously, and internal scale accumulates, gradually degrading heat transfer efficiency. 

Stainless steel barely oxidizes, or oxidation proceeds extremely slowly. Therefore, copper tubes outperform stainless steel tubes initially, 

yet their heat exchange performance declines over long-term operation. After prolonged service, copper tubes deliver poorer heat transfer than stainless steel tubes.

In summary, if 304L stainless steel tubes and copper tubes are put into operation simultaneously, 

the economic performance of 304L stainless steel tubes will become increasingly advantageous with longer operating hours, making them suitable for high-end refrigerated air dryers.

Precooler Design
Essentially, a refrigerated air dryer adopts a composite heat exchanger consisting of two heat exchange sections, where three fluids at different temperatures participate in heat transfer.
Inside the precooler: hot fluid is high-temperature saturated compressed air; cold fluid is cooled dry compressed air, and both have extremely close water equivalent.

Inside the evaporator: hot fluid is saturated compressed air with pre-reduced temperature; cold fluid is low-pressure refrigerant. 

The evaporator obtains cooling capacity from an independent refrigeration system, while the precooler reuses cold energy carried by compressed air cooled by the evaporator and does not feature an independent cold source. 

Therefore, the precooler recovers part of the cooling capacity generated by the refrigeration system during heat exchange, objectively cutting overall energy consumption of the unit, as shown in the heat transfer process diagram.

Nevertheless, heat exchange between cold and hot airflow inside the precooler is subject to operating constraints:

 the temperature drop of hot airflow outlet and temperature rise of cold airflow outlet are both limited. Such restriction cannot be eliminated by simply expanding heat exchange area. 

Proper precooler design is essential to achieve energy saving with controllable costs.

Gas-Water Separator Design

A gas-water separator is normally installed between the evaporator and precooler to remove water droplets and mist entrained in air before compressed air re-enters the precooler.

It is an indispensable key component of refrigerated air dryers. Baffle separators are most widely used in such units. Despite low cost and compact size, 

they introduce pressure loss to compressed air. A rationally designed gas-water separator can effectively reduce the load of air compressors and realize energy conservation and consumption reduction.