Formaldehyde and Tracer Gas Transfer Between Airstreams in Enthalpy-Type Air-to-Air Heat Exchangers

Abstract

Enthalpy exchangers are frequently employed to transfer heat and water between the supply and exhaust airstreams of mechanical ventilation systems.Concern has been expressed that some indoor-generated air pollutants, especially formaldehyde, may be transferred between airstreams by this type of heat exchanger and, thus, returned to the indoor space. This paper describes an experimental study in which the formaldehyde, tracer gas, and water vapor transfer rates in two enthalpy exchangers were measured. The first exchanger uses a crossflow core fabricated from a treated paper. The core of the second heat exchanger is a rotating heat wheel coated with lithium chloride.To reduce the transfer of gases by air leakage each core was installed in a specially fabricated case. Only 5% to 8% of the two tracer gases and 7% to 15% of the formaldehyde injected into the exhaust airstream was transferred to the supply airstream. Therefore, formaldehyde transfer between airstreams by processes other than air leakage does not seriously compromise the performance of these enthalpy exchangers.Theoretical calculations indicate, however, that the transfer of water vapor between airstreams in enthalpy exchangers can significantly diminish their ability to lower indoor formaldehyde concentrations because of the positive coupling between indoor humidity and the emission rates of formaldehyde from building materials.

The Volatile Organic Compound (VOC) Removal Performance of Desiccant-Based Dehumidification Systems: Testing at Sub-ppm VOC Concentrations

“…The investigation occurred during the winter months, so the net enthalpy transfer (moisture and heat) was from the building exhaust to the building inlet. They measured the ambient formaldehyde concentration across the enthalpy wheel, and calculated reentrainment or transfer rates in the range of 1%-9%, similar to the results of Fisk et al (1985). However, since they were working with commercial units installed in buildings, they did not add formaldehyde to the inlet streams to the enthalpy wheels.…”

“…Fisk et al (1985) investigated the transfer of formaldehyde between airstreams by two different enthalpy wheels. Enthalpy wheels are similar to desiccant wheels in that they are rotary honeycomb devices containing an adsorbent, but, unlike desiccant wheels, the regeneration stream is not heated; enthalpy wheels are used to directly contact building inlet and exhaust streams to transfer enthalpy (both heat and moisture) either from the exhaust to the inlet (during winter months) or from the inlet to the exhaust (during summer months).…”

Formaldehyde transfer in residential energy recovery ventilators

“…Typically, a tracer gas is used to determine the transfer efficacy via air leakage and entrainment of air within the wheel, whereas formaldehyde is measured to determine transfer via adsorption/desorption as well as air leakage and entrainment. Fisk et al [14] found that the transfer efficacy of formaldehyde was 7-15% through a rotary wheel enthalpy exchanger, with 5-8% transfer of tracer gases. Similarly, Andersson et al [15] reported a transfer efficacy of 1-9% for rotary wheel heat exchangers installed in commercial buildings in Sweden.…”

Mass Transfer Of Contaminants In Rotary Enthalpy Exchangers

“…was reentrained in the supply air (Khoury et al, 1988). In a similar laboratory study, SF, and formaldehyde were used as tracer gases (Fisk et al, 1985). In this study, where the rotor was coated with a layer of lithium chloride, 543% of added SF6 and 7-15% of added formaldehyde were measured in the supply air stream.…”