Service-water heating

Nationally recognized and/or code-approved demand flow rates and demand factors must be understood to size service-water-heating systems properly. Hot-water demand flow rates, based on building type and calculated results, should include probable maximum demand, required water-heater output, and any required storage-tank capacity.

Some codes require hot-water supply to satisfy continuous and peak hot-water demand. If ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings, criteria are applied, tempering service hot water from “street” temperatures to 85°F is sufficient. Most chiller-based heat-recovery equipment, however, can produce water temperatures of approximately 120°F to 135°F. Regardless of temperature, a heat-recovery system must accommodate continuous and peak hot-water demand while providing a controlled source of service hot water.

Potable water must be of sufficient quality to serve as drinking water, whether or not it is to be used as such. Thus, it must be protected from contamination. Many codes require the separation of potable hot-water sources from non-potable ones by means of a double-wall vented air gap in a heat exchanger. This minimizes the risk of internal leakage and cross-contamination. Other means of separation exist. An engineer must consider code requirements before proceeding with system design.

Process hot water

As with building- and service-water-heating needs, hot-water flow, temperature, and capacity must be understood to support process-heating needs.

MINIMIZING WASTED HEAT

In a traditional chilled-water HVAC system, heat is transferred from the indoors (at the air-handler chilled-water coil) to the outdoors (at the cooling tower) (Figure 4). Not only is this heat “wasted,” energy is consumed at the cooling tower and condenser-water pumps in the process.

Section 6.5.6.2 of ASHRAE Standard 90.1-2004 requires heat recovery on the condenser side of water-cooled systems for the preheating of service hot water in large 24-hr facilities.¹ System engineers seeking to meet the intent of the standard can do so with a plate-and-frame heat exchanger located in return condenser water (Figure 5). The ability to obtain 85°F pre-heated service water generally is possible, but is a function of the temperature of condenser water leaving a chiller plant. LCWT is a function of outdoor ambient wet-bulb temperature, condenser flow rate, and chiller load. With service water at a minimum of 85°F required to meet the intent of ASHRAE Standard 90.1-2004 during peak conditions, LCWT must be slightly above 85°F. However, elevating LCWT increases chiller lift, reduces chiller efficiency, and increases chiller-plant energy consumption.

Another way to minimize wasted heat is to divert heat to a device that can convert it to a useful heat source, a device known as a heat-reclaim chiller.

HEAT-RECLAIM-CHILLER FUNDAMENTALS

A heat-reclaim chiller (Figure 6) generates within its condenser high-pressure refrigerant that can be used to produce higher-temperature condenser water.

With hot-water requirements varying considerably, several types of heat-reclaim machines are available:

• Single bundle

  The single-bundle heat-reclaim chiller is used for non-potable-water applications, such as building heating and process water. An onboard control system maintains the temperature of “hot” water leaving the condenser while simultaneously producing chilled water.

• Double-wall vented desuperheater
  

  PHOTO A. Double-bundle, double-wall vented desuperheater with fully condensing heat exchanger.

  The double-wall vented heat-reclaim chiller features a heat exchanger that extracts high-pressure, high-temperature heat from refrigerant, “desuperheating” it to a lower-pressure refrigerant. In the process, “hot” potable water is produced. The amount of heat is less than what could be extracted with a single-bundle heat-reclaim chiller. Because full condensing does not occur in the desuperheater, refrigerant vapor must be piped to a separate refrigerant heat exchanger. This process can take place in a remote water- or air-cooled condenser.

  The heat exchanger of a double-wall vented heat-reclaim chiller has an air gap to separate potable water from refrigerant, minimizing the potential for contamination.

• Double bundle with fully condensing heat exchanger

  A double-bundle heat-reclaim chiller with fully condensing heat exchanger (Photo A) can produce hot water and provide full refrigerant condensing through a second heat-exchanger bundle.

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