Is Moisture Content high for water injected compressor?

1. The relationship between air dew point and moisture content

1) It is rumored that water-lubricated oil-free screw compressors have a high moisture content in the compressed air because the compression chamber is lubricated with water, thereby increasing the post-processing load. This is an excuse for unprofessional people to attack water lubricated compressor. Because compressed air is a gas containing saturated steam under the exhaust pressure, its moisture content can be represented by pressure dew point, as shown in the figure below. It can be seen from the figure that the moisture content of compressed air is only related to the exhaust pressure and exhaust temperature. When the exhaust pressure is set, the moisture content is only related to the exhaust temperature.

Air Dew Point Moisture Content Relationship Table (Dry Saturated Air)

Dew point (℃)	Moisture content （g/m3)	Dew point (℃)	Moisture content (g/m3)	Dew point (℃)	Moisture content (g/m3)	Dew point (℃)	Moisture content (g/m3)	Dew point (℃)	Moisture content (g/m3)
64	153.8	39	48.67	14	12.07	-11	2.186	-36	0.2597
63	147.3	38	46.26	13	11.35	-12	2.026	-37	0.2359
62	141.2	37	43.96	12	10.66	-13	1.876	-38	0.2141
61	135.3	36	41.75	11	10.01	-14	1.736	-39	0.1940
60	130.3	35	39.63	10	9.309	-15	1.605	-40	0.1757
59	124.7	34	37.61	9	8.819	-16	1.483	-41	0.1590
58	119.4	33	35.68	8	8.270	-17	1.369	-41	0.1438
57	114.2	32	33.83	7	7.750	-18	1.261	-43	0.1298
56	109.2	31	32.07	6	7.260	-19	1.165	-44	0.1172
55	104.4	30	30.38	5	6.797	-20	1.074	-45	0.1055
54	99.83	29	28.78	4	6.360	-21	0.9884	-46	0.09501
53	95.39	28	27.24	3	5.947	-22	0.9093	-47	0.08544
52	91.12	27	25.78	2	5.559	-23	0.8359	-48	0.07675
51	87.01	26	24.38	1	5.192	-24	0.7678	-49	0.06886
50	83.06	25	23.05	0	4.523	-25	0.7047	-50	0.06171
49	79.26	24	21.78	-1	4.487	-26	0.6463	-51	0.054
48	75.61	23	20.58	-2	4.217	-27	0.5922	-52	0.040
47	72.10	22	19.43	-3	3.930	-28	0.5422	-53	0.029
46	68.73	21	18.34	-4	3.660	-29	0.4960	-54	0.021
45	65.50	20	17.30	-5	3.407	-30	0.4534	-55	0.014
44	62.39	19	16.31	-6	3.169	-31	0.4141	-56	0.011
43	59.41	18	15.37	-7	2.946	-32	0.3779	-57	0.008
42	56.56	17	14.48	-8	2.737	-33	0.3445	-58	0.005
41	53.82	16	13.63	-9	2.541	-34	0.3138	-59	0.003
40	51.19	15	12.83	-10	2.358	-35	0.2856

Pressure MPa	0.1	0.2	0.3	0.5	0.7	0.8	0.9	1.1	1.3	1.5	1.7	2.0
Correction coefficient	0.38	0.53	0.65	0.85	0.95	1.0	1.13	1.25	1.36	1.46	1.56	1.7

We all know that there is moisture in the air. The higher the temperature, the greater the relative humidity. In other words, the higher the temperature, the more moisture there is in the air. From the above table, we can also know the temperature and the moisture content in the air.

For example:

1. When the maximum working pressure of compressed air is 0.8Map and the air outlet temperature is 45℃, the maximum moisture content is 65.50g/m³.

2、When the maximum working pressure of compressed air is 0.8Map and the air outlet temperature is 64℃, the maximum moisture content is 153.80g/m³.

3. Therefore, the compression temperature and outlet temperature of the air compressor determine the moisture content of the compressed air. The higher the temperature, the higher the moisture content in the outlet (liquid moisture can accumulate in the air compressor systems due to evaporation into gaseous moisture, and the overall moisture content remains unchanged).

4. The maximum operating temperature of the water-lubricated oil-free screw air compressor air end does not exceed 55°C (generally only 45°C), and the exhaust port temperature is even lower than 45°C, which is far lower than the 100~200°C of the dry oil-free air end, depending on the pressure

The following conclusion can be drawn from the comparison of dew point tables: the water content of water-lubricated oil-free machines is lower than that of dry oil-free machines. For detailed analysis, see the table below:

Comparison of water content between water-lubricated oil-free screw air compressor and dry oil-free screw air compressor

Compare items	Water lubricated oil-free screw air compressor	Dry oil-free screw air compressor
Compression temperature	Host operating temperature (30-55) ℃ Water vapor separation barrel (25-50) ℃ Exhaust temperature (25-50) ℃	The operating temperature of the air end (the first stage is about 100°C, the second stage is 200°C), the temperature is high, and moisture will not separate out The compressed air is forced to cool down (an additional cooler is required). When the temperature drops, water will break out. However, the cooler is not easy to collect and discharge the water. Long-term drainage will have an impact on the gas production of the compressor, so it is generally designed to discharge it directly.
Amount of water vapor released	Centrifuge the water vapor barrel to separate the air and water. The water will settle at the bottom. When the water is full, the water in the separation barrel will be discharged out of the machine.	No water vapor separation equipment
Air outlet temperature	(25-45) ℃, at 0.8Map, the maximum moisture content is 65.50g/m³	It is about 160℃ before cooling and about 65℃ after cooling. At 0.8Map, the water content is 153.8g/m³.
air cleanliness	Due to the low temperature, the high-efficiency centrifugal separation water vapor barrel can keep the incoming moisture at the bottom of the water vapor barrel, ensuring that the water content of the outlet gas is low, and the temperature is low without after-cooling. Water also freshens the air, and impurities in the air stay at the bottom of the water vapor barrel along with the water.	Impurities in the air and equipment operating impurities are discharged from the compressor outlet together.

From the above comparison, we can see that it is not true that the moisture content of water-lubricated oil-free screw air compressors is high. Not only is it not high, it is also lower than other types compressor such as dry screw air compressor)

PREV: Will there be Bacterial Issues for water injected compressor?

NEXT: CMN 40bar oil-free screw compressor VS Oil-free reciprocating piston compressor