Wednesday, February 13, 2013

History of Air Motion and GSA “Peach Book” Specification

In the early 70’s, The GSA (Government Services Administration) developed a set of performance based specifications for buildings, referred to as the “Peach Book”. One of these specifications was that the measured air speed throughout the space shall lie between 20 and 50 fpm at all points. Originally, they specified an air velocity meter that was incredibly inaccurate below 100 fpm. We located an anemometer that was accurate down to 5 fpm but was directionally sensitive. Nonetheless, carefully oriented (using smoke to assure the predominant direction), we developed a repeatable test procedure that eventually became the basis for ASHRAE Standard 113. Using this procedure, we discovered that the specification was not able to be met with any air distribution system. Eventually, a modified specification was agreed upon that allowed 20% of the measured points to exceed 50fpm, and 40% to be below 20, as long as the average was between 20 and 50fpm. 

The measurement devices have greatly improved, and if anything, they are more sensitive than what we used in the GSA tests in '78. The ASHRAE Fundamentals Handbook ADPI data was taken with heated sphere anemometers at Kansas State in the 60's. All those had been calibrated so that they were accurate at low airspeeds, using careful measurements. In the GSA tests, we had a number of distributed 100 watt loads and the designs called for either 0.6 or 0.9 cfm/sf, and resulted in meeting the new GSA specification. The testing was conducted by Dr. Paul Miller (the ADC Engineering Consultant at the time) and myself, with GSA personnel witnessing all the tests.

The diffuser was a continuous ½”, 2 slot linear diffuser that resulted in a two dimensional air pattern throughout the space. The highest air speeds were recorded under the diffuser, in an upward direction, as expected. In attempting to meet the original specification, we also tested an array of ceiling diffusers and found that at some location, air speeds were always less than 20, and at others (under the diffusers) exceeded 50 fpm.

In the course of running over 1000 air distribution tests (reported in two ASHRAE Technical papers), we found that average room airspeed in the comfort zone is essentially proportional to the room load with any ceiling diffuser type, as long as the primary jet doesn't enter the occupied zone. Therein lays the problem. Some air outlets at low flows are unable to maintain sufficient "Coanda" to prevent the negative buoyancy of the air jet from falling into the space. This is often referred to as "dumping" and we find that perforated face diffusers are the most likely to exhibit this behavior. A number of other types of air outlets are seldom prone to this behavior.

At design flow on the other hand, if the diffusers are too close together, throws collide and drop into the occupied zone. By using ADPI analysis, it is possible for the designer to identify in advance, using published throw data, how different diffusers respond to changes in airflow rate, inlet size, and diffuser separation. If the predicted ADPI is greater than 80% at a given set of conditions, the primary jet(s) are not entering the occupied zone. We have also seen that when the ADPI is greater than 80%, average room air speeds are never greater than 40 fpm at flow rates less than 1.2 cfm/sf. This analysis, of course, is for interior zones in cooling mode.

Perimeter zones are another matter, and while average air speeds in heating are typically 20 fpm or lower, there are always high air speeds at the floor near cold windows. These will always exceed 30 fpm somewhere. (Unadjusted linear slots at the window, which is almost always the case, are likely to deliver jets exceeding 100 fpm into the occupied space, in either heating or cooling mode).

I would expect that anyone taking data today, using modern omnidirectional anemometers and meeting the requirements of either ASHRAE 113 (or the ISO 7726) specification, would get similar results.

Authored by: Dan Int-Hout, Chief Engineer Krueger

Wednesday, February 6, 2013

ASHRAE Review

Well, another ASHRAE meeting is now behind us. This last meeting was here in Dallas. As usual, I was fully committed to a number of technical meetings. I won’t bore you all with the machinery of managing Technical Committees (I am a new member of the Technical Activities Committee, TAC). There were a couple of interesting things that happened at the meeting that I can report on, however.

The committee to rewrite the Underfloor Design Manual finally finished its work and voted a document out for publication. While not pleasing everyone (naturally), it contains a great deal of what we have learned in the several years since the first Design Guide was published. This document is the first time a committee has ever attempted to write a design guide. In the past, design guides are commissioned by a TC and ASHRAE pays a contractor to prepare it. This one was different, as the GSA had complained they had a number of buildings with UFAD systems which are “underperforming”. We have learned this means high energy use and difficult maintenance. Many of the problems stem from a number of buildings being built by first-time contractors without experience in some of the unique construction details required to maintain pressurization in the plenum. Other problems involve the need to manage infiltration when the system is shut down at night and condensation occurs. The guide also describes the issue of heat gain, or “thermal decay” and recommends several ways to minimize this effect. Sadly, there is still little data on actual energy consumption, and claims of energy savings with this technology are still mostly anecdotal and unproven.

The Thermal Comfort Standard (ASHRAE Standard 55) is proceeding towards a revision with most of the informative stuff moved into an informative appendix, leaving only mandatory requirements in the main body. The issue of compliance to a 40 fpm allowed maximum “average air speed” (30fpm when the setpoint is below 72.5) is still an open issue.  

A committee representing the VRF (Variable Refrigerant Flow) suppliers requested to be exempted from meeting the Standard 62.1’s requirement for a MERV 6 filter (ASHRAE 189 is requiring a MERV8) upstream from “:any wetted surface”, which means their condensing coils. This is actually a code requirement wherever ASHRAE 62.1 is included in code (like the 2009 International Mechanical Code). If granted, I see a lot of other devices wanting to be included as well. Frankly, I don’t see the justification. 

The final report from the research project at the Yahoo campus was presented. Interior loads were reported to be less than 6 btuh/sf (vs. the typical design of 22 at 1cfm/sf). This is essentially the ventilation load. At the same time, 62.1 is discussing variable ventilation rates for occupied, temporarily unoccupied, and truly unoccupied spaces. I predict that this will require pressure independent ventilation supply systems for any type of equipment, be it fan coil, VRV, WSHP or even Chilled Beam. It makes one wonder why someone doesn’t just install a VAV system, doesn’t it?

Authored by: Dan Int-Hout, Chief Engineer Krueger