Wet filters

I just read the article by Gerald J. Williams, PE, LEED AP, about wet filters on a blow-through air-conditioning system (“The Case of the Wet Filters,” April) and found it interesting. However, I was disappointed in Mr. Williams' failure to describe the “target plate” that was installed to ensure even distribution of air across the cooling coil. It has been my experience that few designers understand the difficulty in installing target plates.

This problem is compounded when a coil is located close to a blower's discharge. It is compounded further because of the uneven velocity at the blower outlet. The highest velocity is at the top of the discharge, with a possibility of backward flow at the extreme bottom of the outlet. Therefore, average velocity calculated across a blower outlet means nothing to a designer.

For this discussion, let us assume the air being handled can be considered standard air — that is, it weighs 0.075 lb per cubic foot. If 40,500 cfm of air were handled at maximum conditions, a designer would have to work with more than 3,000 lb of air per minute at a great range of velocities. In a very short space, the designer would have to equalize the velocities at 400 fpm or less and deliver them evenly over the entire coil. While it is obvious that Mr. Williams knows how to do this, I am sure many readers do not.
Kenneth E. Robinson, CIH
Mears, Mich.

Author's response:

The target plate is one of those curious things in our industry that many people are aware of, but relatively few have actually designed. The best reference on the topic is “Fans and Their Application in Air Conditioning” (available for purchase at www.trane.com/Commercial/DNA/View.aspx?i=482). I have followed this manual's recommendations for years with good results. Basically:

  • A target plate should have openings that yield about 50-percent free area uniformly over its surface.

  • A target plate should be placed two-thirds of the distance from the discharge of a fan to a coil.

  • A target plate should have an area about four times the size of, and approximately the same shape as, the fan-discharge area.

  • A target plate should be centered over the fan discharge and secured rigidly with adequate bracing.

The manual provides a formula I have used with success in determining when a target plate is required, which, as you might guess, turns out to be a function of the distance from a fan to a coil, fan outlet velocity, coil face velocity, and coil pressure drop.

In the article, I say that a target plate will improve air distribution across a coil, but not make it uniform. The accompanying figure (above) shows the air distribution we measured on a coil after a target plate was installed. The highest face velocity was reduced from 58 percent to 30 percent above the average face velocity, while the lowest face velocity was reduced from 43 percent to 23 percent below the average face velocity. The velocity distribution was improved, but anything but uniform. In this case, the solution was not a target plate, but reheat provided by the last two rows of the cooling coil.
Gerald J. Williams, PE, LEED AP
McClure Engineering Associates
St. Louis, Mo.

I just read the article on filter wetting with blow-through units and wanted to offer another scenario that can produce similar results when airflow varies, whether by design or because of the dynamics of a system.

Generally, the lowest flow will produce the coldest air, chilling downstream components. When the flow increases, the dew point of the leaving air increases, and the chilled surfaces produce condensate. This is particularly interesting when an electrostatic precipitator provides downstream filtration.

I enjoyed the article and agree that a fog-region mixture will produce more water than downstream condensation.
John W. Hodoway III
Cromwell Architects Engineers Inc.
Little Rock, Ark.

Author's response:

I see no reason why the scenario you describe would not produce condensation on downstream surfaces exposed to colder air off low-velocity areas of a coil. The key is the presence of enough airflow variation across the coil to produce varying-dew-point discharge air. With an electrostatic precipitator (a solid object) close to the coil discharge, condensation would occur, but only on the colder surface locations. This process very likely has played out with many blow-through cooling coils equipped with baffle-type downstream moisture eliminators. I have never witnessed it — I have seen moisture coming off eliminator sections and assumed it was carryover, but perhaps it actually was the process you describe.
Gerald J. Williams, PE, LEED AP
McClure Engineering Associates
St. Louis, Mo.

I had a similar experience a few years ago, when we were hired to do a peer review of a hospital's HVAC-system design.

After seeing the problems caused by wet filters on that project, I would not recommend a blow-though fan arrangement for a hospital application. The hospital's initial response was to remove some final air filters to move air into the building. As a result, the inpatient areas were closed by the health department until the problem was resolved.

Initially, the problem appeared to be poorly crafted sheet-metal safing at the ends of the coils and air leakage between the vertical cooling-coil banks causing moist mixed air to condense on the underside of the elevated coil drain pans and the interior walls and cold filter frames downstream of the cooling coils. After we submitted the report of our findings, we received a white paper regarding wet filters in HVAC systems.
Philip S. Leader, PE
Kahn Detroit
Detroit

Great article. Although I do not have the psychrometrics to back up my intuition, I suspect at least some of the moisture on the filters is condensate resulting from uneven airflow, which allows some leaving air to exist at saturation, or even supersaturation. I agree that reheat is the only guaranteed means of preventing both deposition of blowoff (fog or otherwise) and condensate on filters.
Lee Weatherby
Colwill Engineering Consultants
Tampa, Fla.

District-steam design

According to the article “District-Steam Design for University Campuses” (March), “District steam systems are a viable thermal-energy source, as confirmed by the hundreds of universities across North America that utilize them.” More likely, district steam systems have been in place for many years, and universities cannot afford wholesale replacement.

Formerly, I was the energy manager for a military installation in a moderately cold climate. A 100-psi district steam system that served approximately 25 buildings (barracks, administrative buildings, dining facility, etc.) was replaced with distributed heating during the early 1990s. The project included the installation of multiple modular hot-water boilers in each building and a new natural-gas distribution system to supply each building. The life-cycle cost was roughly 50 percent of the cost of steam-plant/distribution-system replacement, and the energy savings were huge. Because of the 200°F difference between the steam temperature and ground temperature, the ground loss actually was larger than the combined building loads, even with a preinsulated direct-buried steam system with no leaks.

In my opinion, a district steam system would be a viable thermal-energy source only in an urban setting with zero-clearance buildings.
Russ Goering, PE

Author's response:

Mr. Goering is correct: An individual-building hot-water system utilizing high-efficiency modular boilers provides better thermal performance than a district steam system. Just the boiler-efficiency difference (75 to 82 percent for steam boilers vs. 90 to 95 percent for high-efficiency modular boilers) makes a hot-water system attractive.

For a military post with low thermal-quality requirements (space and hot-water heating), there are advantages to a building-dedicated hot-water system. However, most universities have higher thermal-quality requirements for their research facilities. Instead of taking up high-dollar-per-square-foot space in each building and costing more to maintain, a centrally located boiler plant concentrates high-maintenance equipment in one location and frees up space in each building.

Many colleges and universities do have district steam systems that would be cost-prohibitive to replace. The article's intent is to provide a logical methodology for designing a new or modifying an existing district steam system.
Vincent A. Sakraida, PE, LEED AP
Merrick & Co.
Aurora, Colo.

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