Designing a duct system

So recently I was tasked with designing a duct system for a small dental practice. I learnt a few things that bear repeating. The terminals where ceiling diffusers, so I set about selecting them, looking for which sizes would best suit my flow rates. However I ignored the fact that at these differing flow rates, the diffusers would have to be adjusted to varying pressure levels. Now in a system where all the terminals have balancing dampers, its OK for you to have differing pressure drops in your diffusers, but as was pointed out to me, if I have a diffuser in one room with a low pressure drop, and another in the next room with a high pressure drop, the flow of air will tend towards the lower pressure, and your flow rate there will increase, while your flow rate in the other diffuser will decrease. Thus when designing a system, it is important to insure that all your terminal outlets have similar pressure drops, unless there are individual balancing dampers at each outlet.

CAD tips and tricks 4

So I recently opened a drawing where the UCS was set at a very awkward angle, and tried rotating and redefining, and all sorts of things to get it right. Turns out there is a simple answer to the problem, just type in UCS in the command line, and then use your cursor to designate three points to where the UCS should be. Its almost too simple to comment on, but since I found lots of ways not to do it, I thought it was worth the comment!

CAD tips and tricks 3 - Text Frame

So sometimes its good to have a frame around your text, to highlight it from the surrounding text. I searched high and low for how to do this, thinking that it must be to do with Mtext options. However the only solution I came up with was to use a multileader and to remove the arrow. you can do this by hovering your mouse over the endpoint of the multileader, and a text box will pop up with 3 options, one of which is "Remove Leader" once selected this leaves you with text in a frame, that will update accordingly if you edit the text!

CAD Tips and Tricks 2 - Rev Cloud

So while trying to find out how to make my revision clouds look the way they are supposed to, I discovered a nifty trick that when creating revclouds, one can select an object (ie a polyline) and change it into a revision cloud. No more messy revision clouds for me!

Valve Types

While at a training session recently a valve type was mentioned that I found unfamiliar. Here is some clarification:

Globe valve - this is a valve which is named for the shape it used to be constructed in, round like a globe. I was confused as to what the difference between the Globe and Ball valve was, as I would image the similarity in names. however the globe is named by its OUTSIDE shape, whereas the ball is so named for it inner shape. the flow is controlled by a ball with a hole in it, when the ball turns, so flow is restricted. In a globe valve, there is a chamber, which has a physical partition in the middle with a horizontal plate with a circular hole in it. A cylindrical plug is what causes the reduction in flow, which will be controlled by a screw in manual valves, or an actuator assembly in automatic valves. The plug moves perpendicular to the flow, coming from the wall of the pipe to the centre of the chamber, to sit on the plate when the valve is fully closed.

CAD tips and tricks 1 Publishing

So recently I had to print a whole batch of drawings to pdf at one time. It took a long time, but afterwards I was told this trick. The publish option in AutoCAD, which I have never used, can be used to print multiple documents at once to pdf, DWF and to the plotter that is specified in the page setup of that drawing. This means you don't even need to have the drawing open to plot it, you can insert it into the publishing sheet set, and the publishing option will print it automatically. This has already been a great help to speed up the process when printing multiple documents, especially when they need to be printed to pdf!

Technology is not always our friend

Recently we have been reviewing the way things work in our office, to try to do what every business want, that is to increase productivity without having to work more hours. It was with this in mind that we reviewed a project that was done about 8 years ago. An engineer and his technical assistant sat down, armed with heat load calculations, airflow volumes and unit selections, and drew out the design for a large commercial venture. We now look at these marked up drawings, and clearly marked are the duct routes, the air flow amounts, the duct sizes, as well as structural elements such as columns and penetrations which would affect our services. This was all neatly laid out in a fashion that could be drawn up by any draughtsman. This was a systematic approach, requiring some time, and some effort to produce a drawing which could then be interpolated into a working CAD drawing.

Conversely what we have found in recent years is we have been trying to use our technology, which can be very useful. However, for the most case, we do this bit by bit, never bothering to collate the full calculations, and thus a marked up drawing is impossible. We tend to feel this saves time, and in the short term, that may be the case. Sitting down, making sure we have all of the information, may be time consuming at the start of the project, however this simplifies the process further down the line significantly. When starting a project, we should collate all of the data and compile a drawing that clearly shows the design, such that further down the track, we can clearly see where our design originated from, rather than having multiple sketches and revisions, so no one know where it all started. In this technological age, where we try to minimise our paper useage. we must still remember the effectiveness of having a blueprint for our design in hardcopy.

HVAC design tips - Duct Design

So today I was working on a project where some design alterations where necessary. We are changing the AC in a shopping centre from individual units, to combined AHU's (Air handling Units). Basically what was happening was that each shop had its own small unit, which as we all know is vastly less efficient than a combined unit. In this instance, we are bringing a duct in from the plantroom, which is situated on the roof, and feeding multiple shops along the pedestrian mall via ducting. the fire consultant has declared that each shop is its own Fire Zone, thus when ducting goes between two shops, a fire damper is required. Now a fire damper is a mechanical device that will shut off at a signal from the fire panel, preventing the flow of smoke through the system. It also creates a barrier for flammable materials being transported through the duct. These are fairly costly pieces of equipment, and thus avoiding using them is advantageous. When a duct passes from the pedestrian mall into a shop, no fire damper is required, so the obvious solution is to have the main duct situated in the ceiling void of the pedestrian mall and to feed the shops from there.

In some instances this cannot happen, unless a complete redesign of the internal ducting is done, which would require removal of the ceiling, and much disturbance to the shop. As this centre is currently operating, and shut-down for the retail areas must be minimised, we have tried to keep the original ducting as much as possible, so sometimes it is necessary to come in through another shop.

A useful tip that I picked up in doing this redesign, was when I was working at the end of a stretch of duct, where a large duct feeds off the main duct, and a small duct continues on to feed a small shop. I was going to make a transformation after the large duct had fed off, and then make a bend into the shop. However I was advised that I should rather oversize the small duct and bring it to the end, and then take a shoe off that duct. this would cause a balance of pressure in the end of the duct, which would allow for the correct amount of air to travel down both the large duct and the smaller one.