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Investigating Reduced-Length Alternative Inlet Duct Configurations

Investigating Reduced-Length Alternative Inlet Duct Configurations

From ASHRAE Journal Newsletter, Nov. 9, 2021

The U.S. Department of Energy (DOE) requires installing ASHRAE Standard 37-2009 (RA 2019), Methods of Testing for Rating Electrically Driven Unitary Air-Conditioning and Heat Pump Equipment, inlet duct when testing a coil-only indoor unit, a ducted blower coil indoor unit or a single-package system. However, many existing testing facilities have space constraints that make this testing difficult.

A recent Science and Technology for the Built Environment article explored an ASHRAE research project that investigated reduced-length alternative inlet duct configurations. ASHRAE Journal talked with one of the article’s authors, Khaled Alghamdi, Student Member ASHRAE, about this research.

What is the significance of this research?

Increasing energy efficiency requirements lead to substantial increases in residential air-handler unit’s length. But that increase in the unit’s length makes it difficult for testing facilities to use the required inlet ductwork, which is specified by ASHRAE 37 inlet duct, for rating tests due to space constraints. Therefore, the main goal of ASHRAE Research Project 1743, Effect of Inlet Duct Design on Fan Performance and Static Pressure Measurements of Indoor Air Handling Units, was to investigate reduced-length alternative inlet duct configurations compared to ASHRAE Standard 37 inlet duct in terms of fan performance.

The project also aimed to enhance the repeatability of rating tests of electrically driven unitary air-conditioning and heat pump equipment. Aside from duct geometry, the project also investigated the effects of changes in atmospheric pressure, approach direction to the fan operating point, side flow from a destratification fan and floor distance.

Why is it important to explore this topic now?

The U.S. Department of Energy (DOE) requires installing ASHRAE Standard 37-2009 (RA 2019), Methods of Testing for Rating Electrically Driven Unitary Air-Conditioning and Heat Pump Equipment, inlet duct that is specified and depicted in Figure 7b, Figure 7c and Figure 8 of the standard when testing certain units. However, many existing testing facilities have space constraints.

Reduced-length alternative ductwork developed by ASHRAE RP-1743 can enable these facilities to achieve equipment airflow rates and fan power consumption values very close to that obtained with the ASHRAE Standard 37-2009 (RA 2019) inlet duct.

What lessons, facts and/or guidance can an engineer working in the field take away from this research?

Maintaining the repeatability of the rating tests is difficult due to the variability of environmental parameters found in ASHRAE RP-1743, with atmospheric pressure to be covered in greater detail by ASHRAE RP-1824, Accounting for the Barometric Pressure Impacts on Psychrometric Performance Testing of Unitary Air-Conditioning and Heat Pump Equipment.

In addition, an inlet duct might impact flow distribution of air into unit under test, so the fan performance of the unit can be inconsistence if the unit is tested with different inlet ducts. ASHRAE RP-1743 provided a range of inlet ductwork that reduces the impact of flow distribution, reducing the risk of the fan performance inconsistency if the standard inlet duct is not used.

The reduced-length alternative inlet duct configurations considered for ASHRAE RP-1743 generally maintained fan performance within ±5% for fan power and ±2.5% for air volumetric flowrate. The hysteresis effect associating with adjusting external static pressure through unit under test was significant for the push-through electronically commutated motor fan unit and was noticeable for the pull-through constant torque motor fan unit at a low air flow rate. Atmospheric pressure associated with daily fluctuations of atmospheric pressure in a location affected only the electronically commutated motor fan units significantly. Side flow from external “destratification”-fans had a significant effect on the push-through units in some of the configurations. Floor distance on ASHRAE Standard 37 inlet duct had an insignificant effect on fan performance during testing the standard inlet duct.

How can this research further the industry's knowledge on this topic?

Findings of ASHRAE RP-1743 can help the industry obtain consistent ratings of equipment by increasing the repeatability of fan performance between testing labs. Industry members will likely be inspired by the provided ductwork options to design ductwork that fits better to their testing lab’s manufacturing capabilities. Additionally, the project pointed out the necessity of quantifying the impact of other effects, such as the atmospheric pressure, which is covered by ASHRAE RP-1824.

What opportunities exist for future work?

The goal of ASHRAE RP-1743 was to provide insight for updates to ASHRAE Standard 37’s inlet ductwork as well as federal standards.

While ASHRAE RP-1743 identified promising ductwork assemblies, we also discovered that atmospheric pressure effects need further investigation as U.S. testing sites are not located at the same altitude. In addition, with different types and configurations of units reacting differently, it appears that the programming of fan motors may contribute substantially to fan flow rate and power consumption for a given condition and inlet duct geometry. This effect may require reassessment as programming changes and as electronics with different (internal) back-electromotive force (back-EMF) measurement accuracies are used.