Melody Baglione

Associate Professor of Mechanical Engineering

Building Sustainability into Control Systems > Hot Water System

Introduction

Background

Control


­­­Introduction

The heating system uses two hot water heaters, sometimes also referred to as boilers (although technically, boilers involve phase changes to steam), to supply hot water to the air-handling units and heat exchangers. These hot water heaters are of the water-tube variety, as depicted below, employing the combustion o­­f natural gas as a heat source. Each hot water heater is capable of producing 9.0 MBtu/hr (2.64 MW) of heat with a thermal efficiency of 80%. These two water heaters are designed to supply the heating system with water at roughly 180 °F (82.2 °C) during the winter months [1] .

Background

Water Tube Boilers:

The purpose of a boiler is to transfer heat from hot combustion gases to the working fluid (water). In a water tube boiler, water-filled pipes are passed through the combustion chamber, or furnace. At the bottom of the furnace, there are several natural gas burners that produce hot combustion gas. This gas is passed over the water tubes, consequently heating the flow. Figure 2 illustrates the manner in which water tube boilers operate.

Figure 1. Water Tube Boiler at 41 Cooper Square.
Figure 2. Water Tube Boiler Schematic. [2]

 

 

 

 

 

 

 

 

 

 

 

 

 

The schematic in Figure 2 shows the combustion gases passing over the array of water tubes only once before being exhausted. This configuration is not very effective, as the hot gases are not given a chance to transfer all of their heat to the fluid. In Cooper Union’s 41 Cooper Square building, the boilers are of the multiple pass variety, in which the combustion gases are circulated over the water tubes many times. This allows more of the heat from the combustion to be transferred to the fluid, making the boiler more efficient and consequently allowing it to consume less fuel than with a single pass. (Note: condensing boilers are water heaters that can achieve even higher efficiency (typically greater than 90%) by extracting additional heat by condensing water vapor in the exhaust gas to liquid water.)

Forced Draft Combustion:

The combustion process inside the furnace is also controlled in order to boost efficiency and reduce fuel consumption. Each boiler is outfitted with blowers that force air into the combustion chamber. The rate of air supplied to the combustion chamber by these blowers is balanced in relation to the rate of natural gas being fed to the burners. In this manner, an optimal air-to-fuel ratio is constantly maintained, producing combustion that yields high heat with minimal fuel consumption.

Control

During the winter months, the BMS controls the boilers so that they are operating about their nominal set point, providing the heating system with water at 180 °F [3]. In order to achieve this constant set point temperature, the BMS actuates various valves, pumps, and burners that regulate the temperature and flow of the primary water flow. The figures below are screen shots of the BMS Hot Water System and BMS Boiler Control Panel that regulate the operation of the boilers.  

Figure 3. BMS Hot Water Boilers Panel with Outside Air Temperature of 62° F and 61% Relative Humidity.

 












 

Figure 4. BMS Boiler Control Panel Screenshot with Outside Air Temperature of 62° F and 61% Relative Humidity

 

  


 

 

 

 

 

 

 

 

 

 

Boiler Sequence of Operation*

The flow of water to each boiler is controlled by a single valve that is either fully opened or fully closed. The BMS takes frequent measurements (about every 15 minutes) of the supply temperature and return temperature. Based on these readings, various control decisions are made.

Once the BMS gives a heating command to the boilers, the lead boiler is turned on. In order to protect the boiler’s water tubes, the system verifies that proper flow is established before firing the burners. If the burners are activated while the tubes are dry, the tubes will overheat, expand, and crack because they are not being cooled internally by a constant flow of water. Thus, after taking a differential pressure reading across the water tubes that verifies proper flow rates, the burners are ignited.

The burners are regulated in order to maintain a nominal set point temperature of 180 °F.  The Boiler Control Panel shows that each boiler has two burners, or ‘Flames’, that can be turned on together or individually to control primary water temperature. If the nominal set point temperature of 180 °F is reached or exceeded, the burners are turned off and the pumps switched off 5 minutes later. This sequence allows the water in the system to cool back down to the desired temperature range.

The BMS also monitors the water temperature returning from the heat exchanger and air-handling units back to the boiler. If this return water flow temperature is below 160 °F (71 °C), this indicates that the primary flow is not meeting heating demands, and the BMS turns on a second feedwater pump (HHWP-2), shown in Figure 3. Turning on this pump increases the flow rate through the primary water loop, and consequently increases the rate of heat transfer occurring in the heat exchanger and air-handling units. This increase in heat transfer rate allows the primary return water flow’s temperature to rise, lowering the boiler’s heating load. During times of large heating loads, the return water flow temperature can dip to 150 °F (65.6 °C). When the return flow reaches this temperature, the BMS activates the building’s second boiler. With the second boiler turned on, the gas flow to both sets of burners is regulated to maintain a supply temperature of 180 °F while using only a single feedwater pump. These control sequences, amongst others, allow the temperature of 41 Cooper Square’s primary hot water flow to be regulated efficiently [3].


[1] GMP Set - The New Academic Building of Cooper Union – Water Tube Boilers. Syska Hennessy Group, New York, NY, 2007, p. 15515-5.

[2] Brain, Marshall. "HowStuffWorks: How Steam Engines Work" HowStuffWorks: Science. N.p., n.d. Web. 23 June 2011. <http://science.howstuffworks.com/transport/engines-equipment/steam2.htm>.

[3] Gruzen Sampton, Sequence of Operation – The New Academic Building of Cooper Union, Morphosis Architects, Los Angeles, CA, Rep. 15959, June 2005. p. 15.