Thursday, June 30, 2011

Montreal ASHRAE Meeting Updates

Well, another ASHRAE meeting is behind us. There are a few interesting items to report:

Standard 55 (Thermal Comfort) is being made to be “more mandatory”. As it is referenced in many codes, there has been a concern that it is a bit wishy washy in its requirements. To clear up the concerns, changes are proposed that will make it clear that Standard 55 is intended to standardize how one calculates thermal comfort, not which conditions will be required. It may help the code guys, but will likely still be misinterpreted.

A study ongoing through the Thermal Comfort Technical Committee (TC 2.1) is showing, yet again, that buildings are operating at far lower loads than they are designed for. In many cases, VAV systems are showing satisfactory performance at less than 0.3 cfm/sq. ft. Airflows greater than that are causing systems to run in reheat in interior/cooling zones. This is further evidence that part load operation is the predominant condition and that we need to be designing for that eventuality.

The results of the Texas A&M study on Series vs. Parallel / ECM vs. PSC motor study has been donated to both ASHRAE and AHRI at the summer meeting. It is hoped (indeed expected) that these results (partly funded by Krueger and two other manufacturers) will result in a rewrite of Appendix G of 90.1 which currently favors Parallel boxes, to the exclusion of Series flow types.

AHRI’s ACDD section, which covers VAV terminals, has updated their Operations Manual. We will be seeing a change in everyone’s discharge sound performance in the next year to account for end reflection in the reverb room. Other standards take this phenomenon into consideration, but VAV boxes, which have been using test methods developed before ASHRAE, had a full understanding of End Reflection, do not. The result will be between 3 to 7 NC increase, depending on unit discharge duct size, in reported, and certified, discharge sound levels. The units won’t be any louder, but the sound data will increase. Typically, discharge sound is seldom critical, especially when there is lined duct present between the box and the diffuser.

TRG7 / UFAD which is a committee rewriting the Underfloor design manual (no longer available from ASHRAE) has uploaded all the first draft documents to a website where committee members can propose revisions and review changes to the entire draft. We should be able to have an approved document to turn over to ASHRAE (which will do an independent review) after the Chicago meeting. This is the first design guide written by a technical committee, not an independent contractor. Fortunately, I am rolling off as chair of this committee after this meeting and hand the reigns over to the very capable Jim Mergerson of Burns and McDonald of Kansas City, who has many successful UFAD projects to his credit.

I’ll be glad to get back to the USA, where at least I can read signs directing me where things are!

Authored by: Dan Int-Hout, Chief Engineer Krueger

Monday, June 20, 2011

Accurate Sound Data Matters

For years, sound has been specified in terms of NC, or noise criterion. ASHRAE has published tables of recommended NC levels for many years, which are often the basis if sound specifications. A few years back some acousticians were promoting RC, or Room Criteria, and for a while, ASHRAE replaced the NC table with RC, which had the same numerical ratings, but added a letter (always an “N”) which described the shape of the sound curve. RC never took off in engineer’s specifications; it was flawed in that it didn’t really properly describe the sound shape of devices which contained fans (but worked ok for air outlets).

Fast forward to today where ASHRAE lists both NC and RC; however, most specifications still require NC. The problem with NC is that it is both a single number rating and an estimate based on assumptions. Manufacturer’s don’t actually measure NC. We measure octave band Sound Power in a reverb room and then use standard assumptions (outlined in AHRI Standard 885, 2008, Appendix E) to estimate NC (or RC or dBA or whatever is specified), as required by AHRI when data is presented in a catalog that also has certified data.

We can only guarantee Octave Band sound power as measured in a reverb room. In fact, VAV box sound levels are certified by octave band at a certification point and checked annually. At present, only VAV boxes are certified for sound generation through AHRI. Fan Coils, WSHP’s, and Chilled Beams are proposing to do so in the future, also in a reverb room.

Specifications, however, seldom require meeting stated maximum sound power levels, as would seem logical, since they could be both validated and verified. Instead, we typically see schedules based on manufacturer’s calculated NC values, and sometimes the required assumptions are listed, but more often not. This, of course, allows the suppliers to use whatever assumptions they wish, with the potential to underestimate the sound that will result in the space.

Worse, we see schedules generated by software that list NC for a single manufacturer’s device (the “basis of design”), often at very low NC levels for small units at low flows. Requiring an NC=21 in an office, because that is what was spit out of a manufacturer’s computer program, is pretty silly, as sound levels less than an NC=30 can’t likely be heard in most environments. There is often a note requiring the addition of a silencer if “NC falls below scheduled values”. Silencers are likely ineffective at the low frequencies that set the NC, add cost and pressure drop, and simply don’t make sense.

Finally, with fan powered terminals, which have an induction port open to the plenum, radiated sound levels will be significantly affected if the insulation inside the units is covered up by foil, or worse, sheet metal. We have seen increases as great as 12 NC in some reverb room tests. When the specification calls for the insulation to be covered up, and someone submits data with exposed insulation, there is likely to be a big surprise when the job is commissioned.

And the surprises will be discovered far more readily these days, as one can now download a real time sound analyzer, often for free, as an iPhone app! In the past, in order to validate a space sound level, it was necessary to hire someone with the real time sound analyzer equipment, which often costs at least $4,000. That also meant hiring the services of the guy who owned the equipment. Today, however, it seems everyone occupying the space can do their own sound analysis.

So in short, we need for engineers to produce meaningful and verifiable sound specifications, based on real needs (not some manufacturer’s computer output) and based on real product options, such as interior linings and expected inlet pressures. The AHRI 885 spreadsheet is available on the Krueger website (http://www.krueger-hvac.com/lit/xls/885_08_calc.xls) to get realistic sound estimations, as well as a reverse spreadsheet (http://www.krueger-hvac.com/lit/xls/885_08_spec.xls) to determine realistic octave band sound power limitations.

Authored by: Dan Int-Hout, Chief Engineer Krueger

Monday, June 13, 2011

Control Stategies for ECM Fan Powered VAV's

There are lots of opportunities to take advantage of ECM motors. Assuming you have the ability to custom program a DDC controller on a series (or parallel) fan powered box, the ECM motor can easily be controlled using an analog output on the controller and the analog input control card on the ECM motor.

An ECM motor's efficiency increases, in terms of watts/cfm, as it is turned down. A PSC motor doesn't share this advantage. At full flow, however, a PSC motor may use less real power than an ECM, so the secret is to minimize the fan airflow rate. With a series fan box, however, it is important to keep the fan flow equal to or greater than the primary air flow to avoid over pressurizing the unit and causing primary air to spill into the return air plenum. The ECM motor can be (and typically is) programmed for pressure independent operation, making the fan airflow directly controllable with an analog signal, without the need for a feedback control loop.

There are limits, of course. The main limit is the ability of the supply diffuser to properly provide uniform conditions in the space, which degrades at very low airflows as the cold air supplied at the ceiling falls into the space instead of hugging the ceiling. (Often referred to as "dumping", we have recently learned of alternate terms including "premature ceiling separation" and "failure of coanda". My marketing department prefers the term "horizontally challenged").

The best way to determine a given diffuser's turndown is to use ADPI analysis. I have a couple papers on this process on our website.

We are going to present some ideas on increasing occupant productivity at Greenbuild in Toronto in October. Without giving it all away in advance, occupant productivity has been shown to be greatly affected by ventilation, so one should strive to maximize ventilation. There is a trade off in energy, however. Conditioning outside air is expensive when it is humid, too hot, or too cold. Often, economizer operation has been abused by allowing space temperature and humidities to wander outside acceptable (or optimal) conditions, in an effort to minimize energy use.

Fan boxes are likely required in cold climates to manage perimeter zones when heating from the ceiling, if one is to comply with both Standards 62.1 and 90.1 (and many codes). By utilizing the variability, and pressure independence, of ECM motors, one can provide high ventilation rates when conditions permit, and reduce them when outside conditions are less favorable. Further control options can be realized if the terminal is supplied with a non-condensing (sensible) cooling coil on the induction port and DOAS air provided to the primary air valve. Alternately, a second DOAS inlet can be provided on a series fan box. Controlling wither two inlets or two coils can be a challenge, so Krueger offers a contact closure / 4 position, fresh air, pressure independent actuator option.

We also have a couple engineers using a parallel box with ECM motor where the fan is designed to operate at all times. This allows the induced air to be independent of the primary airflow rate, offering a number of advantages in controlling room conditions.

In my opinion, occupant satisfaction (and productivity) will be optimized when ventilation air is provided at the highest possible rate, outside conditions permitting, and when the system is operated within the limitations of the air distribution system installed in the space. Maximizing ventilation in the morning, then reducing it as the temperature rises during the day, is an excellent strategy. It will require a clever control strategy, taking advantage of both sensors and equipment options.

Authored by: Dan Int-Hout, Chief Engineer Krueger

Monday, June 6, 2011

A Visit with Engineers in Philly

I recently had the opportunity to visit a number of engineer’s offices in Philadelphia. We saw over 50 engineers in 7 different offices on this trip. It was great to get back to Philly where we hadn’t had a strong engineering presence for some time.

Philly is experiencing the same construction doldrums that the rest of the country is having, but I was glad they made time for me to visit and bring them up to date on the latest proposed LEED changes along with the impact of Standard 62.1 VRP (Ventilation Rate Procedure) being adopted as code by the IMC, and Pennsylvania as well.

Once again, I asked how many knew the recommended maximum delta-t when heating from the ceiling. I estimate I have asked 12,000 engineers this question over the past 10 years. The response in Philly was on par with the rest of the engineers I have questioned. Out of 57 engineers questioned, only 6 knew the answer (or were willing to guess, and guessed correctly). The answer, of course, is 15F delta-t (difference between room and discharge). Higher than this and two bad things happen:

1) ASHRAE 62.1 requires that 25% more outside air be brought into the zone being heated, to compensate for the ventilation air short circuiting that is sure to be happening as hot supply air is drawn into the ceiling returns and

2) ASHRAE 55 (Thermal Comfort) vertical stratification limitation will be compromised, and one cannot claim compliance to 55 in a LEED project.

ASHRAE 62.1 is a prerequisite for any LEED building, and code in Pennsylvania, so that is not optional. In LEED for 2012, outside air will likely require to be monitored (at least with VAV systems, so compliance can be verified.

We also discussed the impact of oversizing diffusers with VAV systems and the result at low flows. While the common term for the dropping of cold air into the occupied zone is “dumping”, I informed them, as I usually do, that diffusers may exhibit “excessive drop”. I was informed of a couple alternative terms for this phenomenon. These include:
- Premature Ceiling Separation (“PCS”)
- Failure of Coanda (“FOC”)

I have since been informed by our marketing department that the politically correct description is “horizontally challenged airflow”.

I’m glad that confusion is finally cleared up.

Authored by: Dan Int-Hout, Chief Engineer Krueger