How much oxygen do you need? (Part 2)

In part 1, we talked about the density of oxygen, gas measurement standards, and how to calculate gas volume for a container of a given size at a given pressure. In this post, we’re going to use that knowledge to configure an HVO system that is capable of producing the amount of oxygen that you require.

Quick Summary

Oxygen compresses in proportion to pressure. To accurately size your HVO system, you must know both the maximum flow rate and the regulator pressure that your application requires. The oxygen that you generate at low pressure will become more dense at a higher pressure. Input is not equal to output when the pressures differ.

The HVO Pressure Cycle

HVO systems attempt to maintain the oxygen tank pressure within a range between the low and high setpoints, which can be either 30-100 psig or 100-150 psig, depending on the type of compressor in your system.

Figure 1. Normal Usage Pattern (psig)

The graph above comes from a veterinary clinic that uses an HVO system with a Mighty Mite compressor (setpoints of 100-150 psig). In this facility, oxygen usage varies throughout the day because of surgeries that start and end at various times, and animals that are in and out of oxygen cages — it’s fairly random. Look at the second peak from the left, which shows a period in which usage decreased, and then increased (note the drop-off in the 9th interval). Despite these variations in usage, this pattern shows that the HVO system is able to maintain the tank pressure between the high and low setpoints, which is an indication that the system is properly sized.

Contrast the above graph with the one in Figure 2 (below), which indicates over-utilization, meaning that output exceeds input. This HVO system also has pressure setpoints of 100-150 psig, but usage was so high in this instance that the tank pressure (blue line) dropped to 50 psig before rebounding as usage decreased. The trajectory of the rebound indicates that the tank pressure will never reach the high setpoint of 150 psig, assuming that usage continues at the same rate.

Figure 2. Over-utilization Pattern (psig in blue)

It’s perfectly acceptable to over-utilize oxygen for a time. In fact, it’s quite common in some applications. For example, glass blowers have a highly variable usage pattern that is based on whether or not they are actively blowing glass, the nature of the work being done, and the number of torches that are running.

In a setting where there is periodic over-utilization, attaching an extra storage tank to the HVO system will extend the time that work can continue before the supply runs out. This strategy is, in some cases, more practical than purchasing additional oxygen concentrators. [Another option is to use a backup supply of high-pressure oxygen cylinders and tie this in using our “Backup Oxygen Supply Valve” aka the “Peak Patch Kit”.]

However, constant, around-the-clock over-utilization means that your system needs more input. Fortunately, HVO systems are modular and expandable, so you can easily add components to increase your oxygen-generating capacity.

Achieving Constant Output Pressure

Most oxygen-consuming devices require a constant pressure and flow. A regulator will reduce the pressure to whatever your device requires, whether that’s a glass torch, a respirator, a Venturi injector, or a nano-bubbler. You should be able to find the oxygen pressure and flow information in your device documentation.

As a rule-of-thumb, never set the oxygen regulator on your HVO system to a pressure that is higher than the low setpoint (Figure 3). If you do, the output pressure will drop below the regulator setting at some point in the charging cycle.

Figure 3. Constant Pressure Zones (in Green)

NOTE: The HVO Pro Series system comes pre-configured with setpoints of 100-150 psig. However, it can be reprogrammed to use any setpoints that the compressor is able to support. This feature is used for applications that require a constant pressure greater than 100 psig.

Here are some examples of devices with their maximum pressure and flow specifications:

  • Glasswork Bench Torch, 20 psig, 40 LPM
  • Veterinary Anesthesia Machine, 50 psig, 20 LPM
  • Venturi injector, 30 psig, 10-15 LPM
  • Ceramic Oxygen Diffuser, 50 psig, 1 – 7 LPM
  • Nano-bubbler 100-130 psig, 1 – 5 LPM

Glancing at this list, we see a wide range of output pressure requirements from a low of 20 psig to a high of 130 psig. We also see flow rates ranging from 1 to 40 LPM. Until you consider the effect of output pressure, there’s no way to know how much input an HVO system must have in order to drive these devices effectively.

An Illustration: 1 Liter of Compressed O2

If you know the output flow rate but not the output pressure, you can’t determine how much input you require, as the density of oxygen increases in proportion to pressure. Thus, a flow of 40 LPM @ 50 psig delivers half as much oxygen as 40 LPM @ 100 psig. Same flow rate, significantly different volumes of oxygen being dispensed.

To illustrate, imagine a 6 foot long, 1 inch diameter pipe that is connected to an oxygen regulator on an HVO system. The far end of the pipe has a ball valve that is closed so no oxygen can escape. We can read the pressure gauge on the regulator to know the pressure in the pipe.

Assume that the pipe contains 1 standard liter of oxygen when filled to a pressure of 0 psig (which is equivalent to 14.7 psi — remember that psig is “gauge” pressure). What happens if we continue to add oxygen to the pipe? As shown below, we can use the ideal gas law formula V1 x P1 / P2 = V2 to find the volume of oxygen at a given pressure.

Notice how the volume of oxygen increases in proportion to pressure:

Gauge Pressure (psig) Ideal Gas Formula*Liters of Oxygen
00 / 14.7 =1
3030 / 14.7 =2.04
6060 / 14.7 =4.08
9090 / 14.7 =6.12
Table 2. Oxygen Volume in the Pipe at Various Pressures

This illustrates that LPM-in is not equal to LPM-out when the pressures differ. As you crank up the pressure on your regulator, you are increasing the density of oxygen, which increases the volume of oxygen that you will consume.

* Note that, since V1 will always be 1 liter in this case, we can simplify the formula to P1 / P2 = V2.

How to Account for Pressure

When estimating your oxygen requirements, start by determining the maximum flow rate and the maximum output pressure needed. If there are multiple devices that use oxygen from one HVO system, use the device with the highest pressure requirement. You may need to add downstream regulators to prevent over-pressurizing some devices. The high pressure becomes your HVO regulator setting aka the output pressure.

  • Divide the required output pressure by 14.696 to get the number of atmospheres.
  • Divide the maximum flow rate by the number of atmospheres. This gives you your flow rate at pressure.

For example, if your regulator setting is 50 psig and your input is generated by four 10 SLPM oxygen concentrators, you will have input of 40 SLPM. To derive your output, do this calculation:

40 SLPM x (50 psig / 14.696 psi) = 40 / 3.4 = 11.76 ALPM

Thus the output of four 10 SLPM oxygen concentrators yields 11.76 actual liters per minute at 50 psig. Without taking pressure into account you might have underestimated your requirements. It’s better to be aware of this fact during the planning phase than to find out after you’ve started running your system.

Summary

It is important to size your HVO system with your output pressure and flow requirements in mind. If your regulator setting will be greater than 30 psig, choose a Mighty Mite or a MAX compressor, both of which have a low pressure setpoint of 100 psig. Do the math and make sure that you have enough storage and generating capacity to ensure a constant flow of oxygen for your application. If you’re uncertain, feel free to contact us for assistance!

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