Archive for February, 2012

Variable Refrigerant Flow Systems

Wednesday, February 29th, 2012

Variable Refrigerant Flow (VRF) systems or Variable Refrigerant Volume (VRV) systems are a new kind of air conditioning in the US. They were originally developed in Japan in the early 80’s, but have only recently started to be used here. The systems take advantage of high tech controls to allow more than one hotel room to be connected to a single condenser mounted on the roof. The VRF units are very efficient and quiet, making them the most comfortable hotel room temperature control you could ever imagine.

But there is one catch. Multiple hotel rooms are connected to a single condenser and they share a common piping system for the refrigerant. If a leak occurs in a single unit, all the refrigerant in the system goes into one room. The resulting concentration of refrigerant can be lethal.

This danger has prompted many hotel chains, including Marriott and Hilton, to ban these systems from use in their hotels. There is an exception to this rule if costly refrigerant leak detection alarms are added to the rooms.

In this comic, the refrigerant began filling the room when the VRF unit was struck with the throwing star. The leak detection alarm might have saved the ninjas had it not been cut in half. Unfortunately, there is no NEMA rating that corresponds to “provides protection against ninja sword fights.”

Mark Robison, PE

Vertical Wires

Wednesday, February 15th, 2012

The issue shown in this comic is a real concern in the electrical industry. The wires in a tall building are incredibly heavy. If they fall through an open conduit, they can trap and crush anyone in the electrical room. Only after they are terminated in a switchboard is it safe to be in the room.

In a finished building, the conductors are supported every few floors to protect against this possibility and to reduce the strain on the wires. During construction, if the wire is pulled in a single run from the bottom of the building, there is significant danger if the pulling rope fails.

There are two ways of reducing this danger during construction. The preferred method is to pull the wire from the roof down to the lower levels. Of course, care must be taken to make sure that the wire never gets loose at the roof. The alternative method is to pull the wire from the bottom a few floors at a time to a junction box, lay the extra wire out on the floor, then secure it in the conduit. The extra wire is then fed back into the conduit for several more floors and the process is repeated.

What happens to the ninja in this comic really should not have occurred. The supports should have held the wires in place and prevented them from falling into the room. The ninjas would have been foiled, but not dead.

Mark Robison, PE

BIM vs As-Builts

Wednesday, February 1st, 2012

Oh, how naive our ninja is! He thought that the plans for the building would match what was actually built. Silly ninja.

There is a term in the construction industry called “as-builts”. These are drawings that have been updated to match what actually got built. Hence, “as-built”.

With the advent of BIM (building information modeling) and 3D modeling, the theory is that the model is so realistic that there will be no need for “as-building” the plans. The model and the building will be exactly the same.

But real life is still messy, and changes during construction happen all the time. Sometimes there is an error in the design that needs to be fixed. Sometimes designers and contractors come up with ways to improve a design. All of these reasons to change a design apply to both traditional plans and BIM. Using BIM does not prevent these changes, though it hopefully reduces the number and allows them to more easily be made.

As-building the plans is a necessary step at the end of the project in order for the final plans and the building to match. This extra step costs extra money that owners are seldom willing to pay. As-building is a very tempting process to skip to save money after construction. Using BIM might make creating the as-built model easier, but it still takes time and money.

In the comic, like in most buildings, as-building the model did not happen. The ductwork in the stolen BIM file and the ductwork in the building don’t match, and the ninja ends up dumped into an alley rather than his foe’s office.

Or maybe what really happened is that the Green LEEDer planted false plans to foil the ninja. Oh, how clever those USBGC folks are.

Mark Robison, PE