Pressure Transducers

On my last post I discussed common refrigeration pressure switches. As the name implies, these are switches which are opened and closed by pressure changes. They can make or break circuits, but they cannot indicate pressure. Pressure transducers are often used for electronic controls because they can actually indicate system pressures.

The word “transduce” means to change from one form to another. A pressure transducer turns pressure changes into analog electrical signal changes. This is most often a change in a DC voltage, typically 0 – 5 volts DC. This changing voltage can then be interpreted as a pressure by the electronic control to which it is connected.

The most common pressure transducers used in HVAC use a small stainless-steel diaphragm with strain gauges bonded to it. A change in pressure causes the diaphragm to bend, which causes the strain gauges to change resistance. These transducers have three leads: two are wired to DC+ and DC- and the third carries the signal. Pressure transducers ohm out like a potentiometer. On diagrams this looks like a potentiometer with a pressure bellows connected to the wiper arm. The resistance between the two leads that connect to DC voltage should stay the same regardless of the pressure. The third lead changes resistance relative to the two other leads as the pressure changes.

When the two other leads are connected to 5 volts DC, the signal connection will vary between 0 and 5 volts DC depending on the pressure. The control then interprets this voltage and controls the system based on the board’s program. If you want to check the transducer signal, read the voltage between the signal lead and DC- and then compare this voltage to a chart published by the manufacturer. Here are a couple of links to more information on presure transducers

Omega Transducers

Emerson Climate Technologies

 

Pressure Switch Lingo

Pressure Range
The range of a pressure switch is the  minimum to maximum pressure at which the switch can be set. Because refrigeration systems have two basic pressure areas, high side and low side, pressure switches are often described as either high or low pressure switches, based on which side of the refrigeration system the switch is designed to operate. However, this is a bit of an oversimplification. It is possible for two pressure switches with identical ranges to behave very differently if they have different switching actions.

Switching Action
The switching action  describes what occurs when pressure rises above the switch set-point. Basically, only two things can happen: the switch either closes or it opens. So, pressure switches are classified as either close on rise, or open on rise. For refrigeration system safety applications, close on rise switches are used to protect against low system pressure, and open on rise switches are used to protect against high system pressure. But there are many other applications for pressure switches. For example, a close on rise pressure switch can be used for condenser fan cycling to maintain head pressure. The condenser fan is energized when the condenser pressure rises to the switch cut-in point. Similarly, an open on rise pressure switch can be used to control the harvest cycles on an ice machine. When the evaporator pressure drops to the switch cut-in point, the harvest cycle is initiated. The safest way to describe pressure switches is by both their switching action and their pressure range.

Cut-in, Cut-out, and Differential
It is important to understand that the switch contacts cannot both open and close at exactly the same pressure – there has to be a difference between the pressure when the switch closes and the pressure when the switch opens. This difference is called differential. Three terms are used when describing pressure switch settings: cut-in, cut-out, and differential.  Cut-in is the pressure when the switch closes, cut-out is the pressure when the switch opens, and differential is the difference between the two.





















High Event and Low Event
Sometimes the terms high event, low event, and differential are used. In the case of a close on rise pressure switch, the high event would be the cut-in and the low event would be the cut-out. In the case of an open on rise switch, the high event would be the cut-out and the low event would be the cut-in.

What Difference Does it Make?
There are both physical and practical reasons for having a differential. Physically, pressure switches are mechanical devices which use levers that are controlled by springs and pressure bellows. There must be some mechanical motion to open or close the electrical switch contained within the pressure switch. Since this motion is created by a change in pressure, the opening and closing point cannot be the same point. Practically, you really would not want the control system to respond so quickly. For example, once the compressor stops running, the high side pressure almost immediately drops a little, even on systems with hard shut-off expansion valves. If a small drop caused the high pressure switch to close again, the compressor would quickly cycle on and off repeatedly, which would cause more damage than the high pressure alone. I hope this helps explain why there are multiple settings on a pressure switch and what they all mean.

Using Manual D Speed Sheet

The free ACCA Manual D speed-sheet makes properly sizing residential duct work really easy. However, you need to complete three important steps BEFORE you are ready to use the Manual D speed-sheet. You must first do a Manual J room by room load study, select the specific equipment you will be installing, and draw out your duct system in stick form. The speed-sheet will help you size your ducts according to the required heat load in each, the unit output, the unit airflow, and the external static pressure requirements of the unit. There are three basic steps: determining the total effective length of the duct system, determining the design friction rate, and finally sizing the duct.

Effective Length
Manual D looks for the worst case duct run and bases the design friction rate on that longest run. The idea is that if the blower can move the air through the longest run, it can easily push the air through the other ducts. The assumption is made that each run will have a balancing damper, and that the balancing dampers will be used to balance the system airflow once the system is installed. The speed sheet gives you four columns to use for determining the longest effective length. You don’t have to use all of them if the worst case run is obvious. Note that you are NOT entering data for every run, just looking for the longest run.

There are three rows labeled Trunk. They are there for systems which have multiple branching trunks. Most systems will only use one.  Enter the length of the trunk duct from the plenum to the branch takeoff in one of the Trunk rows. Then enter the length of the branch run beside Runout Length. The second set of rows on this tab are for entering the equivalent length of all the fittings. They are arranged in groups of fittings with similar functions. Click on a group to go to the tab showing the different fittings. Choose a fitting that best matches the fittings you will use. You will need to remember it, or jot it down. Click return to return to the Effective Length tab. You will probably NOT have a fitting for every group. Just leave spaces blank which do not apply to your system. Repeat the process for the return. Note that the group numbers change bit because the equivalent length of return air fittings varies from supply air fittings.

Friction Rate
The second tab is for determining the design friction rate. You need to know the specific unit for this part because you will be entering the unit airflow and external static pressure. The idea is pretty simple. All things that the air moves across cause a pressure drop. You list the pressure drop in wc for all the air components. This is totaled and subtracted from the system external static pressure, and what remains is available static to be used for moving air through the ducts.

Since friction charts are based on 100 feet, the friction rate, or wc friction drop per 100 feet, needs to be determined. For example, if your available static was 0.12 and your total effective length is 200 feet, the design friction rate would be 0.06. A duct which would cause a pressure drop of 0.06 per 100 feet would create a total pressure drop of 0.12 by the time the air traveled 200 feet. The speedsheet does this for you based on the total effective length calculated on the Effective Length tab.

Duct Sizing
You need to have a Manual J calculation of the heating and cooling loads for each room before using the final tab, Duct Sizing, Simply list the room name, heating BTUs and cooling BTUs and the speed sheet calculates the duct size based on the friction rate and CFM from the Friction Rate tab. Note that it re-sizes the ducts every time you enter more data – so don’t be alarmed if it tells you the first room you enter requires a 16 inch run. The sizes are not accurate until you have all the room information in.

Trunk sizing is as easy as clicking a box for each branch duct that t trunk feeds. Note there are probably more rows for trunk ducts than you will need.  Returns are also sized the same way. You click the box of each supply run which you believe will be served by that return. This is obviously not an exact science. However,  it is important that each supply run is selected in a return. If you have return trunks, you size them by selecting the return branches which feed into the return trunk.

Creative Application of Manual D Speed Sheet
You can use the Manual D Speed Sheet as a teaching/learning tool by varying some of the entries. For example, play with different equivalent length fittings to see the effect between best case and worst case fittings. Try different external static pressures and airflows to see the effect on duct sizing. Thi is a great way to see the effect different design decisions can have on the end result.