How diaphragm pumps can work for sensitive devices
Diaphragm pumps offer a number of features that could be valuable to designers of products that require the movement of gases and fluids: they are relatively inexpensive; capable of flow, pressure and vacuum levels suited to mobile or stationary applications; and they are configurable, efficient and durable, with no sliding seals.
These pumps are used in ventilation and could work in other compressor applications that require non-pulsing input. One of their biggest advantages is that the fluid path is completely sealed from the environment, making them ideal for handling sensitive gases and fluids. The smallest ones are about 30mm (1”) long and fit in the palm of your hand, weighing only a dozen grams (under a half ounce) for moving or sampling small amounts of air or gas. Heavy-duty industrial diaphragm pumps can weigh hundreds of pounds for process applications involving chemicals, fluids and gases.
How they work
The way they work is fairly simple: A variable-speed electric motor converts rotational motion to linear (pumping) motion by driving a connecting rod from an off-center location much like an automotive crankshaft and the connecting rod to a piston. The resulting displacement of the free end of the connecting rod is used to push and pull on an elastomeric diaphragm, much like pushing and pulling on one flexible wall of an otherwise rigid box.
This box is commonly referred to as the “head” of the pump. The motion of the diaphragm causes a volumetric change in the head and thus alternately creates a vacuum (when the diaphragm is pulled outward) and pressure when pushed inward. An intake valve ensures that during the outward stroke of the diaphragm, the gas or fluid enters the head. On the inward stroke, the gas or fluid exits the exhaust valve. Thus, these pumps can be used to create vacuum or pressure, depending on how they are plumbed. They are also inherently self-priming.
Diaphragm pumps have a huge range of uses, from coffee makers to medical aspirators to air sampling systems and blood pressure measurement instruments. But they also have disadvantages:
- Their output is inherently pulsatile.
- They are typically noisy due to the rapid opening and closing of the valves.
- They can vibrate if not properly balanced.
- Their service life and operating efficiency depend on, among other things, diaphragm design
- Low-speed cogging of the motor can limit their dynamic range.
For most applications, the pulsatile nature of the flow is not an issue, but it is in medical ventilators, which assist patients who may not be able to breathe on their own. Dynaflo was presented with the challenge to design a compressor for a ventilator with the advantages of diaphragm pumps but without the pulsing, and with the ability to operate over a wide range of output flow to suit a wide range of ventilation patients.
The solution was a multi-head diaphragm pump with 12 radially oriented pumps driven off of a central, common eccentric. This approach causes each pump to go through its usual cycle once per rotation, as in single-head pumps. However, with 12 heads connected in series, any of the 12 pumps is just 30 degrees apart from its neighbor at any point in time, thus creating a 12-point averaging effect on the output flow. The symmetrical, balanced design virtually eliminates vibration and presents a relatively constant torque load on the motor.
The pump’s radial 12 heads were optimized for the output flow and pressure required: 140 l/min (4.9 cfm) and 140 mbar (~2 psi), respectively. This radial, symmetrical design effectively solved the pulsation and vibration issues, and made things easier for the implementation of the motor: a low-profile, long-life brushless DC motor that can operate over a wide range of speeds. Symmetrical loading of the motor also allows it to run more efficiently than it would against the uneven loading of a single- or dual-head design.
The pump’s non-pulsing output flow and pressure also make it easier for closed-loop control, in which a downstream flow or pressure sensor can be used as input to a control circuit to carefully control how the patient is ventilated. This, plus its light weight (1.5 lbs./0.7 kg) makes it ideal for mobile applications in which battery-powered devices need to last as long as possible, especially in field-based medical devices such as ventilators.
Operating efficiency is critical for battery-powered applications. In diaphragm pumps, this means focusing the electric drive motor’s power on creating as much pressure or vacuum as possible, as opposed to overcoming mechanical impediments such as friction, accelerating masses, or stretching the diaphragm. Careful attention to the profile of the diaphragm also yields high efficiency: instead of a flat elastomeric part that would have to be stretched for each cycle, the diaphragms are designed to flex by rolling instead of by stretching. This dramatically reduces the work required by the motor and extends the life of the diaphragm.
Thousands of these compressors are deployed today in mobile ventilator systems for the U.S. military, and they may have other applications in sensitive medical devices.
From: Medical Design & Outsourcing