When producing liquid products, the cold energy (Q₀) contained in the liquid is directly removed from the unit. To maintain cold balance, the unit must generate an equivalent amount of additional refrigeration. The cold energy of liquid products corresponds to the heat released when cooling from ambient temperature to the liquefaction temperature until reaching saturated liquid state, which is considerable. For example, at 0.1 MPa pressure, the cold energy of saturated liquid oxygen is 407 kJ/kg, and saturated liquid nitrogen is 435 kJ/kg. Converted to per liter of liquid, these are: liquid oxygen 465 kJ/L; liquid nitrogen 353 kJ/L.
For ASUs producing gaseous products, the refrigeration capacity during startup is much greater than that required for normal production, in order to cool down equipment and accumulate liquid as quickly as possible. For instance, in small-scale medium-pressure ASUs, refrigeration can be increased by raising air pressure; in low-pressure ASUs, it can be increased by expanding the expansion volume. Therefore, theoretically, producing some liquid products is always possible. However, in low-pressure ASUs, the air after expansion refrigeration directly enters the upper column to participate in distillation separation. Excessive expanded air entering the column will affect distillation efficiency and consequently impact oxygen production. Generally, the expansion volume is controlled at approximately 25% of the processed air volume. For example, for a 6000 m³/h ASU, when producing gaseous products, the expansion volume is about 19.6% of the processed air volume. When simultaneously producing 250 L/h of liquid oxygen is required, the expansion volume must increase to 27% of the processed air volume. The refrigeration generated at this expansion volume can also be used to produce 300 L/h of liquid nitrogen. However, for a small low-pressure ASU such as 1000 m³/h, the required expansion air volume for gaseous product production already reaches 28% of the processed air volume, and not all of it can enter the column for distillation. If additional liquid products are to be produced, even more expansion air volume would be needed, with more expanded air being bypassed to the heat exchanger, further reducing the oxygen extraction rate from the processed air and decreasing oxygen output. Therefore, for a 1000 m³/h ASU, it is no longer suitable to extract additional liquid products.
Based on experience, for a 50 m³/h small-scale medium-pressure ASU, if 4 L/h of liquid nitrogen is extracted, the air pressure needs to be increased by 0.3–0.4 MPa compared to normal operation.
