Capturing Waste Heat From the Condenser

June 7, 2010
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Equation: Total COP for heat recovery + chiller water for heat reclaim chiller

Hot water has been created in very much the same way for many years. Fossil fuels are combusted around a metal vessel filled with water. More recently, electric elements have been immersed in a water vessel to heat the water.

When heating with fossil fuels, efficiencies can reach more than 90 percent in a condensing-type boiler and upwards of 100 percent with electric immersion water heaters. These efficiencies represent a coefficient of performance (COP) of 0.90 and 1.0 respectively.

Is it possible to generate hot water at a COP greater than 1.0? The answer is yes.

Heat recovery captures energy that would otherwise be wasted to the atmosphere and converts this energy into useful heat. It is possible to capture this “Heat Out” from the condenser and use it to generate hot water. By capturing this heat that would otherwise be wasted, overall system efficiencies can be significantly increased.

Unlike the process of generating heat from combustion or electrically driven water heaters, capturing waste heat from the condenser can result in efficiencies greater than 100 percent. This achievement is the result of using the heat captured from the condenser plus the cooling effect from the evaporator. When both sources of heat are captured, efficiencies greater than 100 percent can be realized. In fact, the total COP for heat reclaim from a chiller system for both water heating and cooling purposes can reach over 5.0.

If we look at the performance of a nominal 120-ton air-cooled chiller with heat reclaim we can see the energy advantages based on the total COP.

Consider chiller performance at 44°F leaving chilled-water temperature, 2.5 gpm per ton evaporator flow with heat reclaim in operation to produce 121° entering heat reclaim water temperature and 131° leaving heat reclaim water temperature (see equation above).

COOLING CAPACITY: 93.7 tons (329.6 kW)

INPUT POWER: 159.9 kW

HEATING OUTPUT: 1,642 MBH (480.7 kW)

HEAT RECLAIM CAPABILITY

An air-cooled chiller produces chilled water while simultaneously transferring significant quantities of heat to the outdoors through its air-cooled condenser.

If this heat could be captured and redirected to a water-cooled condenser, the system could produce not only a controlled source of chilled water but also a significant amount of useful heat to generate hot water.

How does it work? This can be explained by using a Carrier air-cooled chiller with heat reclaim capabilities as an example. Such a unit produces chilled water controlled to the necessary temperature while generating hot water as a by-product of the chilled water system.

Under the normal cooling only mode, the chiller produces a controlled source of chilled water using its air-cooled condenser. In this mode, the system performs as a typical air-cooled chiller to produce a controlled source of leaving chilled water according to its set point temperature while rejecting heat to the environment through its air-cooled condenser.

When an air-cooled chiller is operating in the heat recovery mode, there must be a simultaneous need for chilled water and tempered hot water. It is important to note that this chiller will always maintain the leaving chilled-water temperature. As a result, the chiller will produce as much hot water as possible while controlling the leaving chilled-water temperature. The leaving hot-water temperature is a by-product of the cooling cycle. To generate hot water, the entering hot-water temperature is compared to the hot-water set point, also known as the heat reclaim set point, to determine the number of circuits necessary to maintain entering hot-water temperatures.

If the entering hot-water temperature is below the customer adjustable set point, one refrigeration circuit will automatically change over to heat recovery mode. The chiller is now operating much like a water-cooled chiller with one circuit and as an air-cooled chiller with the other circuit. Depending on the entering hot-water temperature and deviation from set point, the second circuit may also be switched to heat recovery mode through the integrated controls.

The entering hot-water temperature is controlled by the cycling of each refrigerant circuit from the cooling to the heat recovery modes. When the hot-water set point is satisfied, the chiller will then transition back to the cooling only mode to operate as a conventional air-cooled chiller. The leaving hot-water temperature is a function of the entering hot-water temperature, hot-water flow, and chiller capacity.

APPLICATION CONSIDERATION

The heat recovery condenser can transfer 100 percent of the chiller’s total heat of rejection to the hot-water loop.

The leaving water temperature can reach a maximum temperature of 131° under steady state and constant hot-water flow conditions. The allowable leaving hot-water temperature range is 68 to 131°. Since the hot-water loop is used to condense refrigerant into a subcooled liquid, head pressure control may be necessary to ensure stable chiller operation when entering-water temperatures (EWTs) are between 59° and 104°. A modulating three-way control valve will be necessary when the EWT is within this range.

Since the three-way valve must respond to chiller head pressure control needs, the distance between the three -way valve and the chiller should be minimized to ensure a sufficient response time for stable chiller operation.

As a result, this distance and minimum loop volume should be confirmed with the manufacturer’s recommendations.

This material was prepared by Carrier Corp., www.carrier.com.

Sidebar: Heat-Water Applications

It is important to understand the type of building or process and how captured heat will be used for a given application. Hot water can be used to heat a building or pre-heat ventilation air, for zone reheat coils, to maintain pool conditions, pre-heat make-up domestic water, or to heat a process.

Each of these applications requires thorough consideration of the simultaneous need for chilled water and hot water.

This requirement can be present in each of the building types listed below. In all these cases, chilled water can be used for space cooling and air conditioning purposes.

Hotels, motels, resorts: Hot-water make-up for potable and nonpotable purposes such as laundry, kitchen, washroom, and swimming pool heating;

Athletic facilities (college, high school, community): Shower and washroom make-up water and swimming pool heating;

College residence halls: Hot-water make-up for showers, laundry, kitchen, and dining facilities;

Commercial laundries: Hot-water make-up;

Hospitals and elderly care facilities: Patient room make-up hot-water, laundry, and kitchen facilities;

Condominiums and residential buildings: Hot-water make-up for showers, laundry, kitchen, and dining facilities.

- Carrier Corp.

Publication date: 06/07/2010

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