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2024-07-31 11:36:33

【Technical News】Is hot or cold better for refrigerated air dryer outlet?

There are currently two common types of air dryers on the market. One has a cold outlet, while the other has a warmoutlet. So which one is better? In fact, it need to be considered among the inlet, outlet temp. and dew point pressure. The air dryer with the smallest inlet-outlet temp. difference and the lowest dew point is the best performing.
 

Air dryers with low air outlet temp. are very common. Although it gives people the intuitive feeling that the refrigeration effect is better, the treated compressed air will experience condensation in the pipeline during the process of being transported to the air-using end, as well as secondary condensation, producing condensed water, resulting in impure compressed air. When the air outlet temp. of this kind of air dryer becomes high, it may be the reason for the failure of the refrigeration and heat exchange system, and the compressed air can not be effectively cooled.
 

However, there is also a kind of air dryer under normal working conditions, the output temp. is relatively high, the temp. difference between the import and export is small, but the dew point effect is very good. This kind of air dryer is relatively rare on the market, because the cost is higher, and it is generally a high-end brand. The air pre-cooler heat exchange area is sufficient, the refrigeration capacity is high, so the return temp. effect is good, and the dew point is stable. However, its air outlet is warm, which will make many people mistakenly think that the machine refrigeration effect is poor.

Many users have a common cognitive misunderstanding in the judgment of the PDP temp. of the air dryer where the cooler the air temp. of the air dryer, the better the effect. In fact, the PDP temp.of the air dryer is not directly related to the air outlet temp.. The PDP temp. indicates the unit content of water vapor in the gas, and the outlet temp. indicates the surface heat of the gas, and the relationship between the two is mainly related to the relative humidity.


 

Technical tips:

Relative humidity ψ is the absolute ratio of the absolute humidity of the wet air (defined in the last chapter) to the saturated wet air at the same temp., indicating the ability of the wet air to absorb water vapor and reflecting the moisture layer of the air.

ψ=ρv/ρs X100%

Ρv represents the density of absolutely wet air and ρs represents the density of water vapor in saturated air.

The higher the value of ψ, the higher the humidity of the air, the closer it is to saturation, and the stronger its ability to absorb water. The opposite is true.

When ψ = 100%, the air is saturated moist air, and the gas temp. of the moist air can be considered the dew point temp..

When ψ=0, it means that the air is dry air, when ψ=1~99%, it means that the air is unsaturated wet air, and the gas temp. of the wet air is slightly higher than the dew point temp..
 

The diagram shows the flow chart of the plate fin type of the air dryer, which is divided into the refrigeration process and the air heat transfer process. The compressed air undergoes a secondary heat transfer process in the air flow of the air dryer. The first is the heat exchange of the gas-gas heat ex-changer (referred to  the pre-cooler), and the air source is the untreated high temp. and high humidity compressed air flowing through the transverse path of the pre-cooler (the shell path of the tube and shell). The cold source is the processed low-temp. saturated compressed air (90% of the water vapor in the compressed air has been condensed and discharged in the evaporator) that flows through the longitudinal process of the pre-cooler (tube and shell). 
 

The result of a heat exchange is that the heat source air is cooled, the cold source air is heated, and the saturated air becomes unsaturated air. In this case, when the relative humidity of air ψ=1~99%, the actual dew point temp. is less than the gas temp..



In summary, the dew point temp. and the gas temp. of the wet air are not the same numerical concept, and the level of the air outlet temp. of air dryer can not be used as the basis for judging the quality of the dew point of the machine, which should be judged by the dew point meter test.


 

The same processing capacity, the same intake air temp. and relative humidity, the same PDP, the high air outlet temp. of the air dry will be more energy saving. Specific reference cases as follows:

Analysis of two different design conditions of plate-fin heat exchanger:
Intake 45℃, relative humidity 65%, dew point 7℃, 0.7MPa
Outlet temp. 40℃and 30℃


 
Case 1

The outlet temp. is high, small temp. difference between inlet and outlet, the pressure drop is small, the ratio of the pre-cooler and the evaporator load is close to 1:1, the refrigeration system is low in power consumption, and the system is high in energy saving.
 

Case 2

The outlet temp. is low, large temp. difference between inlet and outlet, the pressure drop is large, the ratio of the pre-cooler and the evaporator load is close to 1:2, the refrigeration system has a high power consumption, and the system has a low energy saving degree.

In summary, the performance of air dryer good or bad.Temperature difference between inlet and outlet and dew point should be considered together. Small temp. difference between inlet and outlet, and the air dryer with low dew point has the best performance. Because this kind of air dryer although the outlet temp. is high.At the actual operation of the pressure drop is small, the refrigeration system power consumption is low, so as to achieve the real energy-saving effect.


Appendix:
Definitions of relevant professional words
 
Dew point temp. is short for dew point, refers to the temp. at which air cools to saturation without changing its water vapor content or pressure. Generally speaking, the temp. when the water vapor in the air becomes dew is called the dew point temp..
 
If the temp. is lower than the dew point, liquid water will be formed. If the temp. is higher than the dew point, water will remain in a gas state.
 
When the water vapor in the air has reached saturation, the temp. is the same as the dew point temp.; When the water vapor has not reached saturation, the temp. must be higher than the dew point temp.. So the difference between the dew point and the air temp. can indicate the degree to which the water vapor in the air is saturated.
 
Absolute humidity refers to the mass (kg) of water vapor contained in 1m³ of wet air. Absolute humidity is equal to the density ρv of water vapor at the temp. of wet air and the partial pressure of water vapor Pv
 
ρv=mv/V
 
Mv is the mass of water vapor and V is the volume of wet air
 
From the ideal gas equation of state: Pv*V=mv*Rv*T - ρv=Pv/(Rv*T)
 
Rv is the water vapor gas constant
 
In the case that the temp. remains unchanged, the water vapor content in the wet air increases, the partial pressure of water vapor Pv will increase until the water vapor content increases to the maximum (wet air is saturated), the density ρvmax of water vapor is equal to the density ρs of saturated water vapor in the wet air.

The higher temp. of the wet air, the higher the density of the saturated water vapor of the wet air.

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