Helpful Tips for Complying with the National Energy Building Code

Helpful Tips for Complaying with the National Energy Building Code

In this article, we will discuss two aspects of the National Energy Building Code relating to hydronics: variable flow in pumping systems and system balancing (minimizing throttling losses and trimming the pump impeller).VARIABLE FLOW IN PUMPING SYSTEMS

The language of ASHRAE 90.1 Standard (Section 9.5.5.3) pertaining to variable flow is quite specific: “Pumping systems that serve control valves designed to modulate or step open and closed as a function of load shall be designed for variable fluid flow. The system shall be capable of reducing system flow to 50% of design flow or less. Flow may be varied by one of several methods, including but not limited to variable- speed driven pumps, staged multiple pumps, or pumps riding their characteristic performance curves.”

The new code does, however, allow four possible exceptions to this variable flow requirement. They include: 1) systems already requiring minimum flow to be greater than 50~o of design flow in order to operate; 2) systems that only serve one control valve; 3) systems that already include temperature reset controls that meet another specific ASHRAE standard; and 4) alternative design systems that can show no higher energy demands than those afforded by variable flow designs.
Variable volume/variable speed design systems that satisfy the ASHRAE 90.1 requirement generally have the following characteristics:

Two-way control valves are used to throttle flow.
Pump speed is reduced in response to decreased demand.
Control valve pressure differential is significantly reduced.
Reduced pump speed consumes less horsepower; hence, the overall system’s energy requirement is significantly less.

Figure 1 illustrates the ability of variable speed pumps to reduce flow and pump head as the speed is reduced.

When the flow requirement is 600 GPM (1/2 of the 1200 GPM base condition), the pump is operating at 70% speed, resulting in a 43% reduction in head and a 68% reduction in pumping horsepower.

Primary/secondary and primary/secondary/tertiary pumping systems are advanced variable volume pumping that satisfy the new energy code’s 50% flow reduction requirement. Specific information about the designs of such systems are found in a number of source materials, besides the text of ASHRAE Energy Standard 90.1 – 1989. These materials include: ASHRAE’s 1969 HVAC Systems Equipment Handbook Chapters 11 & 12 (District System Hydronic System) and Chapter 38 (Pumps) and ITT Bell & Gossett’s bulletin TEH-685, among others.

In looking at the variable flow, primary/secondary/tertiary hydronic system shown in Figure 2, consider the “official Bell & Gossett guidelines”: 1) size common for 3 pipe diameters of straight pipe 2) size common for a maximum pressure drop of 18″ wc, based on the flow of the largest chiller pump through the common pipe; and 3) follow ASHRAE Standard 90.11989 and B&G System Syzer, which suggest a maximum pressure drop of 4 ft./100 ft.. in the primary, secondary, and tertiary piping.

Figure 3 illustrates primary/secondary/tertiary hydronic pumping design with constant flow through the chillers and variable flow distribution which would satisfy the ASHRAE 90.1 requirement.

SYSTEM BALANCING – MINIMIZING THROTTLING LOSSES

Section 9.4.10.3 of the new National Energy Building Code specifies that: “Hydronic system balancing shall be accomplished in a manner to first minimize throttling losses, and then the pump impeller shall be trimmed or pump speed shall be adjusted to meet design flow conditions. A balancing report from the installer may be required by the Code Enforcement Official and a copy included in the operating and maintenance manual.”

Basically, pump discharge valve throttling normally should be done in conjunction with impeller trimming, except in cases where throttling alone is preferred and permitted. These instances would include: 1) when pumps have motors of 10 hp or less; 2) if pump discharge throttling results in no greater than 3 hp of pump draw above that required if the impeller were trimmed; 3) to reserve additional pump pressure capability in open-circuit systems subject to fouling, but pump discharge valve throttling pressure drop cannot exceed what is expected for future fouling; and 4) wherever you can show that pump discharge throttling won’t increase the building’s overall energy costs. Essential components of successful valve throttling include such items as circuit setters, balance valves and Triple Duty valves.

Balancing a hydronic system by means of circuit balancing valve throttling is a process that must take into consideration the following design factors and control procedures:

Steel and copper pipe size diameter selection,
Friction losses and velocity values
Corrections for viscosity (temperature and Glycol corrections)
Proportional balancing valve/flow meter test reports
Circuit setter pre-sets, proportional balancing and gauge readings.

Fortunately, help in calculating all these factors is available to you in the form of an easy-to-use software program known as the Electronic System Syzer (available from your local ITT Fluid Handling representative). This software package includes a “Circuit Setter balancing wheel,” a “balancing report,” and appropriate specifications for Circuit Setters and Triple Duty valves.
Printed materials that may help you balance hydronic systems by using this method include B&G’s Bulletin A-508F Circuit Setters; Curve A-560A Large Circuit Setters; and Bulletin B-821B Triple Duty Valves.

TRIMMING THE PUMP IMPELLER
The new energy code also allows for balancing the hydronic system by trimming pump impellers, sometimes called proportional balancing, except in the four cases noted previously where valve throttling may be used exclusively.

Pump heads often are oversized to assure terminal flow rates. A 10% to 100% safety factor frequently is added to compensate for higher than planned head loss equipment, or for unexpected piping changes. But a 100% head safety factor, for example, increases power requirements by approximately 2.5 times, depending on pump curve characteristics (flat or steep). Proportional balancing (impeller trimming) can eliminate unnecessary power consumption.

By establishing the true system curve, you can determine the new impeller diameter providing design flow to each terminal with minimal power consumption resulting in a significant power savings. You should realize that each hp saved represents about 0.75 KW, and a reduction in power draw of 46 hp, for example, would save 34.3 KWH. Assuming that energy costs $.075/KWH, such a simple thing as trimming an impeller could save a building owner $2.57 an hour or $61.74 a day or $22,535 a year!

For specific information on impeller trimming, you may want to review Bell & Gossett’s End Suction Pump specifications and the B&G ESP-Plus Software Selection Program. In order to pull and trim an impeller quickly, you should specify a “True Back Pull Out” End Suction Pump. This requires a self supported volute and a center drop-out spacer coupler, both of which happen to be standard features on Bell & Gossett Model 1510 pumps.

At the very least, if the system design you are considering might require balancing by means of trimming the pump impeller, you should add the words “center drop-out spacer couplers” to your equipment specifications.

If you would like more information about the impact of the new National Energy Building Code, contact your local Bell & Gossett Representative.