Heat recovery for service water heating systems
Well, it's the New Year. I like to start a new year with reflection and learning something new. This month's Code tidbit is courtesy of an Energy Efficiency Inspection seminar I attended recently. One of the items was completely new to me. So, here it is:
§E503.4.6 Heat recovery for service water heating. Condenser heat recovery shall be installed for heating or reheating of service hot water provided the facility operates 24 hours a day, the total installed heat capacity of water-cooled systems exceeds 6,000,000 Btu/h (1758 kW) of heat rejection, and the design service water heating load exceeds 1,000,000 Btu/h (293 kW).
The required heat recovery system shall have the capacity to provide the smaller of:
1. Sixty percent of the peak heat rejection load at design conditions; or
2. The preheating required to raise the peak service hot water draw to 85ºF (29ºC).
1. Facilities that employ condenser heat recovery for space heating or reheat purposes with a heat recovery design exceeding 30 percent of the peak water-cooled condenser load at design conditions.
2. Facilities that provide 60 percent of their service water heating from site solar or site-recovered energy or from other sources.
So, what's it mean? First, it applies to facilities in operation 24 hours a day. This isn't defined. It is probably not an office building that is occupied maybe 12 hours per day, but most likely intended for 'facilities' with 24/7 occupancy - like a hospital. A facility most likely means one or more buildings. It could be a standalone building or a complex.
A 'facility' with over 6mmBTUH of cooling heat rejection is relatively large, but reasonably common. A domestic hot water system (Service Water) that has loads that exceed 1mm BTUH starts at about 25 gpm or 1500 GPH, excluding diversity and other factors that drive the 'load'. This is quite common.
Where a condenser (like a cooling tower) rejects over 6 mmBTUH, we essentially need to use the cooling tower water to pre-heat the domestic hot water. There are exceptions, but I have never seen these installed. So we will spend a chunk of pumping energy to preheat the domestichot water to 85 degrees. It's probable that studies will show the pump energy may out weight the benefits of thermal energy conserved by using this energy recovery.
That leads us to what kind of stuff is put into cooling tower water? Remember the Belvedere hotel in Philly? Recall how Legionella was first discovered? If so, you know we often add a whole bunch chemicals (aka, biocides) to cooling tower water. In some circumstances, we are required pump this water through a heat exchanger to preheat the domestic hot water. If the heater fails, biocides will be pumped directly into the domestic water. What does the Plumbing Code say about that?
§P608.16.3 Heat exchangers. Heat exchangers utilizing an essentially toxic transfer fluid shall be separated from the potable water by double-wall construction. An air gap open to the atmosphere shall be provided between the two walls. Heat exchangers utilizing an essentially nontoxic transfer fluid shall be permitted to be of single-wall construction.
As long as maintenance staff respond when one of the walls of the heat exchanger fails, we are good to go. On a side item, where boiler water is used to heat an indirect fired domestic water heater, make sure the 'facility' is using FDA approved chemicals in the boiler water. If they are not, a double walled heat exchanger may be required.
Happy New Year -