Putting pressure on steam humidification

Carefully choosing steam humidification equipment can help you reduce or eliminate water waste in the form of condensate. As a result, you can significantly reduce steam humidification system lifecycle costs through water and energy savings and, in pressurized steam applications, also reduce boiler chemical use, installation costs, and maintenance requirements.

09/01/2008


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Carefully choosing steam humidification equipment can help you reduce or eliminate water waste in the form of condensate. As a result, you can significantly reduce steam humidification system lifecycle costs through water and energy savings and, in pressurized steam applications, also reduce boiler chemical use, installation costs, and maintenance requirements.

First, some background: A steam humidification system generates pressurized steam in a boiler or nonpressurized steam in a tank. This steam travels through piping to a dispersion assembly and into an airstream. This airstream, typically 50 to 55 F (or cooler in some specialty applications), blows across stainless steel dispersion tubes, causing some steam in the dispersion tubes to condense and drain from the dispersion assembly.

Second, some physics:

  • Energy used to change water to steam is wasted whenever steam condenses before it gets used for humidification, wasting about 8,000 Btu for every gallon of dispersion-generated condensate.

  • Released heat, from steam changing to condensate in the dispersion assembly, is added to the airstream. This wastes energy by increasing the quantity of steam generated to meet the humidification load. The released heat also can have a ripple effect throughout the HVAC system. Cooling loads increase in applications that humidify and cool simultaneously, fan energy use increases to circulate more air, fans and pumps work harder and add more heat to the airstream, and even cooling tower size may need to increase.

  • The heat transfer rate from a dispersion tube to an airstream is not affected by humidification load. At a steady-state operating temperature, a tube dispersing 10 lb/h transfers the same amount of heat and generates the same amount of condensate as that same tube dispersing 50 lb/h, given the same airstream temperature and velocity.

  • Condensate, not always returnable to the steam generator, is often hot water down the drain. In some jurisdictions, hot discharge water must be tempered with cold water before discharge into a sanitary system, wasting more energy and up to two times more water.

These numbers add up quickly. Reducing dispersion-generated condensate can significantly reduce humidifier and some HVAC system operating costs (see Table 1).

Condensate reduction

The first tip is to reduce condensate with insulated dispersion tubes. Dispersion tube insulation can reduce dispersion-tube-generated condensate production and its associated wasted energy by up to 85%.

There are several ways to insulate dispersion tubes:

  • Steam jacket with insulation: A steam jacket surrounded by factory-installed fiberglass insulation protected by stainless steel (see Figure 1).

  • Ceramic insulation: A factory-applied 0.030-in.-thick thermal insulating coating.

  • Closed-cell foam insulation: A 0.125-in.-thick factory-applied polyvinylidene fluoride (PVDF) insulation with a 0/0 flame/smoke rating.

All of these insulation types withstand the environmental extremes of steam humidification while meeting plenum requirements for smoke and flame.

Steam-jacketed dispersion tubes are a good solution for applications not requiring a short absorption distance. Their profile, especially when insulated, limits the number of tubes that can be installed across a duct or air handling unit (AHU) without obstructing airflow. Absorption distance is, in part, determined by how well air and steam mix. The more steam discharge points or tubes spanning a duct or AHU, the shorter the absorption distance.

Short-absorption dispersion panels can achieve absorption within 3 in. with multiple, closely spaced tubes. However, when not insulated they can condense significant amounts of steam, heating the air stream. Tubes insulated with ceramic or closed-cell foam insulation have a slender profile and, in most cases, do not cause an excessive pressure drop across the dispersion assembly when tubes are closely spaced, making it possible to simultaneously achieve short absorption and energy efficiency. (The typical pressure drop of a short-absorption dispersion panel with closed-cell foam insulation is 0.03 to 0.12 in. water column when tubes are installed 3 in. on-center in airflows of 500 to 1,000 fpm.)

The key difference between ceramic and closed-cell foam insulation is how well they reduce dispersion tube heat loss. As you might expect, closed-cell foam performs better than ceramic insulation. The ceramic coating would have to be about 0.375 in. thick—three times as thick as the 0.125-in.-thick closed-cell foam insulation—to achieve the same insulating capability.

Condensate return

The second tip is to return condensate to the boiler. All steam dispersion tubes, in the simplest terms, are devices with numerous holes through which steam exits, so they operate at just above atmospheric pressure. For this reason, systems delivering pressurized steam to dispersion tubes often waste generated condensate to an open drain, because returning nonpressurized condensate to the condensate return main requires purchasing, installing, and maintaining additional pumps, traps, valves, and controls.

Steam-jacketed dispersion assemblies, when properly applied, do not waste condensate. The steam jacket is a closed loop of pressurized steam that travels from the steam boiler around the dispersion tube to the condensate return main and back to the boiler. Condensate generated within a steam-jacketed dispersion tube is revaporized by heat from the steam jacket. The heat given up to vaporize condensate from the dispersion tube causes condensate to form within the steam jacket. This condensate is sent to the condensate return main without using pumps, thus reducing water and boiler chemical waste.

Short-absorption panels with tube insulation generate much less condensate than panels without tube insulation. However, because all condensate generated is nonpressurized, it is usually piped to a drain. The exception is a short-absorption panel with a heat exchanger in the supply header (see Figure 2). Similar to a steam jacket, the heat exchanger is a closed loop of pressurized steam that travels from the steam boiler through the supply header and back to the boiler. In this type of system, dispersion-tube-generated condensate falls to the hot surface of the heat exchanger and is vaporized back into humidification steam and dispersed into the duct airstream. As with a steam-jacketed system, the pressurized condensate is sent to the condensate return main without using pumps, thus eliminating water and boiler chemical waste, and the need for piping a condensate drain and tempering the condensate. Simultaneously achieving short-absorption distance, energy efficiency, and condensate return without pumps is possible by using an insulated short-absorption panel with a heat exchanger in the header.

Regardless of the pressurized steam humidification system used, proper control is essential. Install a solenoid valve at the steam jacket or heat exchanger steam inlet, and install a temperature switch controlling the modulating humidification steam valve. This will ensure a warm startup (eliminating spitting) and the ability to shut off the jacket or heat exchanger steam when there is no call for humidity, thus saving additional energy.

Managing condensate

To lower operating costs, use dispersion tube insulation to reduce condensate production, and use pressurized return to capture condensate. If you need short absorption and want the energy- and water-saving benefits of tube insulation and pressurized return, use a short-absorption panel with a heat exchanger in the supply header. If short absorption is not a concern, an insulated steam-jacketed system may be a good solution.

Any one of these steam dispersion solutions will save considerable amounts of energy, water, and boiler chemicals. Using the short-absorption panel with the integral heat exchanger also may eliminate the need for installing and maintaining additional valves, traps, pumps, and controls for sending condensate to the condensate return main. These equipment choices will have a significant effect on your equipment lifecycle operating costs.


Author Information

Wasner is a freelance writer who has been writing about buildings and building products for more than 20 years. Lundgreen is a senior mechanical design engineer for DRI-STEEM Corp. with 14 years of professional mechanical design and development experience.


Selecting dispersion panels

Table 1 (below) illustrates the amount of water and energy wasted or saved when using three different types of dispersion panels operating under the same conditions:

Dispersion panel No. 1 (baseline): A 72 in. wide x 48 in. high steam dispersion panel with dispersion tubes fabricated at 3 in. on-center, operating 2,000 h/yr in a duct with 50 F air at 1,000 fpm air speed. There is no tube insulation and no integral heat exchanger in the header on this panel (condensate wastes to drain).

Dispersion panel No. 2: The same conditions as No. 1, except the dispersion assembly has dispersion tubes insulated with closed-cell foam (condensate wastes to drain).

Dispersion Panel No. 3: The same conditions as No. 1, except the dispersion assembly has dispersion tubes insulated with closed-cell foam and an integral heat exchanger in the header (condensate is returned to boiler).

Dispersion panel

Energy wasted in condensate

Energy saved over dispersion panel No. 1

Water wasted in condensate

Water saved over dispersion panel No. 1

Heat added to downstream air by dispersion tubes

Cooling load to offset heat added to downstream air

Dispersion panel No. 1: No tube insulation; no heat exchanger

Total: 162,910,200 Btu/yr (140,208,000 Btu/yr duct air heat gain from condensing steam; 22,702,200 Btu/yr hot condensate)

NA

17,359 gal/yr

NA

2.58 F

140,208,000 Btu/yr 70,104 Btu/h or 5.8 tons

Dispersion panel No. 2: PVDF tube insulation; no heat exchanger

Total: 40,580,600 Btu/yr (34,960,000 Btu/yr duct air heat gain from condensing steam; 5,620,600 Btu/yr hot condensate)

122,329,600 Btu/yr (75%)

4,298 gal/yr

13,061 gal/yr (75%)

0.65 F

34,960,000 Btu/yr 17,480 Btu/h or 1.5 tons

Dispersion panel No. 3: PVDF tube insulation; heat exchanger

Total: 34,960,000 Btu/yr (34,960,000 Btu/yr duct air heat gain from condensing steam)

127,950,200 Btu/yr (79%)

0 gal/yr

17,359 gal/yr (100%)

0.65 F

34,960,000 Btu/yr 17,480 Btu/h or 1.5 tons



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