| Last time we began our discussion on Preproduction, the final aspect of the Development stage of our systems engineering approach to medical device design. This is the point at which a small amount of devices are put into actual production, then evaluated for full production possibility. It is also the final juncture at which problems will be evaluated and corrected before full commercial production can begin.
Once the medical devices produced during Preproduction are assembled, they’re subjected to rigorous testing in both a laboratory and the field. This testing is necessary to see if stakeholder requirements are satisfied. At this stage devices constructed en masse on the factory assembly line are compared to prototypes built by hand by design engineers earlier in the Development stage.
During Preproduction laboratory test data is gathered and analyzed by engineers to assess how the device will hold up during actual use. Real-life conditions are simulated in the lab environment to facilitate this process. For example, lab testing of a Preproduction kidney dialysis machine can determine whether its blood pump flow rate falls within acceptable range during hundreds of hours of operation. Other factors, such as durability of materials are evaluated during lab testing. In the case of the dialysis machine, there is a component called a dialyzer that filters toxic waste from blood. Over the duration of the lab test, the material used in the dialyzer filter membranes would be inspected and evaluated for durability.
Next week we’ll conclude our discussion on Preproduction to see what happens when testing is moved outside the lab environment into the field.
Posts Tagged ‘pump’
Pumps are all around us. They keep our drinking water flowing, the cooling water circulating in your car’s engine, and even your blood flowing. They’re essential in many aspects of our lives, but most of us don’t think too much about them. For our discussion let’s put them into two categories: positive displacement pumps and centrifugal pumps. This week, we’ll focus on positive displacement pumps.
Positive displacement pumps, as their name implies, displace a quantity of liquid with each complete cycle of movement. This takes place when moving parts of the pump take “bites” out of the liquid at the inlet, then force them to exit through the outlet. A familiar example of a positive displacement pump is the type of hand operated water pump that’s commonly found in campgrounds. See Figure 1.
Figure 1 – A Positive Displacement Pump
This type of pump is known as a reciprocating positive displacement pump. By reciprocating, I mean that the moving parts travel back and forth in a straight line during its operation. Let’s see how it works by referring to the cutaway view in Figure 2.
Figure 2 – Cutaway View of the Pump Shown in Figure 1
In the cutaway view, the pump’s piston and internal check valve are shown, and there’s another check valve in the bottom of the pump housing. When you pull up on the handle, the piston moves down into the water in the pump housing, and the pressure caused by this movement forces the check valve in the bottom to slam closed, while the check valve above is forced open. This causes water movement to flood through the open check valve and fill up the space above the piston. When you push down on the handle, the opposite happens. The piston is made to move upward. The upward acceleration of the water above the piston causes the check valve on the piston to slam shut, and this traps the water above it. As the piston moves back up, a suction is created below, which causes the check valve in the bottom of the housing to pop open and more water is drawn up into the space below the piston. Eventually, when the piston gets high enough, the water trapped on top of it will flow out of the spigot.
Another type of positive displacement pump is represented by a rotary pump. These pumps operate in a circular motion to move a volume of liquid with each revolution of the pump shaft. This is done by trapping liquid between moving parts, such as gears, lobes, vanes, or screws, and the stationary pump housing itself.
To show how this works, refer to the gear pump shown in Figure 3. Its gear teeth mesh together in the middle of the pump, blocking the flow from going straight through and trapping it within the spaces formed by rotating gear teeth and the pump housing. It’s like the water is being forced through a turnstile.
Figure 3 – A Cutaway View of a Gear Pump
Next week, we’ll talk about centrifugal pumps and how they move liquids along using centrifugal force.
Last time we talked about some general concepts in an area of mechanical engineering known as thermodynamics. In this week’s article we’ll narrow our focus a bit to look at a part of thermodynamics that deals with power cycles.
One mammoth example of a power cycle can be found in a coal-fired power plant. You can’t help but notice these plants with their massive buildings, mountains of coal, and tall smoke stacks. They’ve been getting a lot of negative press lately and are a central focus of the debate on global warming, but most people have no idea what’s going on inside of them. Let’s take a peek.
Figure 1 – A Coal-Fired Power Plant
A power plant has one basic function, to convert the chemical energy in coal into the electrical energy that we use in our modern lives, and it’s a power cycle that is at the heart of this conversion process. The most basic power cycle in this instance would include a boiler, steam turbine, condenser, and a pump (see Figure 2 below).
Figure 2 – A Basic Power Cycle
When the coal is burned in the power plant furnace, its chemical energy is turned into heat energy. This heat energy and the boiler are enclosed by the furnace so the boiler can more efficiently absorb the heat energy to make steam. A pipe carries the steam from the boiler to a steam turbine. Nozzles in the steam turbine convert the heat energy of the steam into kinetic energy, making the steam pick up speed as it leaves the nozzles. The fast moving steam transfers its kinetic energy to the turbine blades, causing the turbine to spin, much like a windmill (see Figure 3 below).
Figure 3 – The Inner Workings of a Steam Turbine
The spinning turbine is connected by a shaft to a generator. The turbine works to spin the generator and thus produces electricity. After the energy in the steam is used by the turbine, it goes to the condenser, whose job it is to convert the steam back into water. To accomplish this, the condenser uses cold water, say from a nearby lake or river, to cool the steam down until it converts from a gas back to a liquid, that is, water. This is why power plants are normally found adjacent to a body of water. After things are cooled down, the pump gets to work, pushing the condensed water back into the boiler where it is once again turned into steam. This power cycle keeps repeating itself as long as there is coal being burned in the furnace, the plant equipment is functioning properly, and electrical energy flows out of the power plant.
Thermodynamics sets up an energy accounting system that enables mechanical engineers to design and analyze power cycles to make sure they are safe, reliable, efficient, and economical. When all is said and done, a properly designed power cycle transfers as much heat energy as possible from the burning coal on one end of the cycle to meet the requirements for electrical power on the other end of the cycle. As was mentioned in last week’s blog, nothing is 100% efficient.
Next time we’ll learn about being cool. No, I’m not going to talk about the latest cell phone gadget or who’s connected on Facebook. We’ll be covering refrigeration cycles.