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HEAT RECLAIM WATER HEATING
Using Waste Heat from Refrigeration Systems to Heat Potable Water
Phil McConnell
Sr. Product Manager
Therma-Stor LLC – Madison, WI
SEMINAR SYNOPSIS
• Reclaiming waste heat from a refrigeration system and using it to heat water is a proven, effective energy saving strategy applicable in many facilities. This session will focus primarily on using waste heat from a refrigeration system to heat potable hot water. After a brief introduction, topics will include determining when to consider using a heat reclaim system, feasibility analysis, system design/component selection, installation details including controls, and performance verification. Different heat reclaim system components will be discussed and compared along with sizing and selection examples. Return on investment calculations will be presented and discussed.
AGENDA
• Introduction
• Criteria for application (when to use)
• Feasibility
• ROI
• System design
• Component options
• Sizing of components
• System design
• Controls
• Performance verification
HEAT RECLAIM INTRODUCTION
REFRIGERATION CYCLE
WHY WATER HEATING?
• Facility has refrigeration and uses hot water• The lower, the better!
• The higher, the better!
• High-grade heat from superheated gas works well for boosting water temperatures
• Desired final water temperature is higher than most other potential uses for reclaimed heat
• Both are good reasons for using desuperheaters for heating water• Potential for additional heat reclaim downstream
• Full condensing
• Refrigerant to air (secondary condenser)
BENEFITS OF HEAT RECLAIM
• Water heating savings
• Reduces fuel consumption needed to heat water
• Natural gas
• Electricity
• Reduces carbon footprint and emissions
• Reduces runtime on water heater
• Increases life cycle
BENEFITS OF HEAT RECLAIM
• Refrigeration benefits
• Increases compressor efficiency
• Approximately one percent per degree of reduction in condensing temperature
• Potential for effective increase in system capacity
• Reduced water use for
• Water cooled systems
• Evaporating condensers
• Reduced water treatment
CRITERIA FOR APPLICATION
When to use
ROI/Feasaiblity
CRITERIA FOR APPLICATION
• Refrigeration considerations
• Hot water considerations
• Logistics
REFRIGERATION CONSIDERATIONS
• Capacity
• Evaporator temperature
• Lower is better
• More high-grade heat
• More compressor runtime
• Refrigerant type
• Pressure drop (restriction) characteristics
• Discharge temperature
• Available superheat (head pressure)
HOT WATER CONSIDERATIONS
• Overall hot water demand
• Coincidental use
• Use water at the same time refrigeration system is running
• Cost of water heating fuel
LOGISTICAL CONSIDERATIONS
• Need to get the water and refrigerant close to each other
• Pipe refrigerant to water
• Plumb water to refrigerant
• Proximity of water heating system to refrigeration system
• Compressor
• Other facility related issues
• Freeze protection
• Structural issues
ROI - OVERVIEW
• How much does it cost to install
• How much can it save?
ROI - OVERVIEW
• How much does it cost to heat water
• Amount of energy used
• Value of energy used
• Fuel source and level of efficiency
• Electricity = 100%
• Gas = efficiency rating less scale/lime
• Don’t consider standby losses
• Calculate Btu’s and value of fuel
• Compare to cost of installation
• Consider additional benefits
ROI – BTU’S USED TO HEAT WATER
Example100 gallons X 8.33 pounds per gallon X 85 degree F rise = 70,806 BTUs divided by 60% efficiency = 118,008 BTUs to heat 100 gallons of water, 85 degrees F, with a 60% water heater efficiency.
Natural Gas118,008 BTUs to heat 100 gallons of water divided by 100,000 BTUs per therm X $1.00 per therm = $1.18 to heat 100 gallons, 85 degreesF, with a 60% water heater efficiency.
Electric78,672 BTUs to heat 100 gallons of water divided by 3413 BTUs per KWH X $.12 per KWH = $2.77 to heat 100 gallons of water, 85 degrees F, with a 90% water heater efficiency.
___ gallons X 8.33 pounds per gallon X ___ degree F rise = ___ BTUsdivided by ____% water heater efficiency = ____ BTUs
ROI – BTU’S USED TO HEAT WATER
• Easiest to estimate water use if there is no data
• Usually not a lot of data on hot water use
• Possible to measure hot water use to learn more
• Easiest to assume water use is constant
• We know it’s not
• Difficult to figure otherwise
• Make sure all parties are comfortable with the assumptions used to calculate the ROI
• Consider using a couple of calculations
• Highest/lowest water flow
• Largest/smallest compressor capacity
ROI – CALCULATION FORM
SYSTEM DESIGN
DESUPERHEATING WATER HEATERS
• Potable vs. non-potable
• Potable water heat exchanger
• Double-wall
• Vented
• UL listed
• Two basic styles
• Tank
• Non-tank
DOUBLE WALL CONSTRUCTION
DOUBLE WALL CONSTRUCTION
DESUPERHEATING WATER HEATERS – TANK STYLE
DESUPERHEATING WATER HEATERS – TANK STYLE
• Water storage tank with integrated heat exchanger
• Advantages
• No water pump required
• Lower power and maintenances
• Reduced scaling/liming potential
• Backup/booster heating optional
• Disadvantages
• Size
• Proximity to compressors
• Not outdoor rated
• Single size, multi-circuit
DESUPERHEATING WATER HEATERS – TANK STYLE
• Sizing
• Consider refrigeration system first
• Limited capacities available
• Too small = too restrictive
• Too large = condensing refrigerant
• Typically based on compressor capacity
• Then consider water heating system
• Try to maximize heat recovery within refrigeration parameters
• Evaluate potential contribution to total load
• ROI calculation
• Logistics
DESUPERHEATING WATER HEATERS – TANK STYLE
DESUPERHEATING WATER HEATERS – TANK STYLE
• Installation details
• Refrigerant
• Hot gas piped to desuperheater
• Warm gas piped to condenser
• Three way valve to control flow based on water temperature
• Water
• Cold water plumbed to inlet
• Warm water plumbed
• Facility
• Water heater
• Recirculation return to mid-port of tank
• Controls
• Aquastat
• Rack controller
DESUPERHEATING WATER HEATERS – TANK STYLE
DESUPERHEATING WATER HEATERS – TANK STYLE
DESUPERHEATING WATER HEATERS – TANK STYLE
• Installation details
• Refrigerant
• Hot gas piped to desuperheater
• Warm gas piped to condenser
• Three way valve to control flow based on water temperature
DESUPERHEATING WATER HEATERS – TANK STYLE
• Water
• Cold water plumbed to inlet
• Warm water plumbed
• Facility
• Water heater
• Recirculation return to mid-port of tank
• Controls
• Aquastat
• Rack controller
• Various sizes , multi-circuit
DESUPERHEATING WATER HEATERS – TANK STYLE
• Controls
• Pressure switch to cycle fan (small units)
• Water bleed valve (multi-circuit)
• Refrigerant 3-way valve is installed on the hot gas line, controlled by an aquastat
• When the water temperature is satisfied, the 3-way heat reclaim valve diverts the hot gas directly to the condenser.
DESUPERHEATING WATER HEATERS – TANK STYLE
DESUPERHEATING WATER HEATERS – NON-TANK
DESUPERHEATING WATER HEATERS – NON-TANK
DESUPERHEATING WATER HEATERS – NON-TANK
• Non-tank type
• Separate heat exchanger and water storage tank
• Advantages
• Small size
• Flexible application
• Outdoor possible (protect from freezing)
DESUPERHEATING WATER HEATERS – NON-TANK
• Disadvantages
• Water pump required
• Increased maintenance and cost
• Subject to scaling and liming
• Reduced efficiency over time
• Increased maintenance
• Multi-circuit units available
DESUPERHEATING WATER HEATERS – NON-TANK
• Sizing
• Multiple sizes available
• Selection based on refrigeration capacity and water flow
• Water heating system sizing
• Pump sizing
• Storage tank sizing
• ROI calculation
• Logistics
DESUPERHEATING WATER HEATERS – NON-TANK
• Installation details
• Refrigerant
• Hot gas piped to desuperheater
• Warm gas piped to condenser
• Three way valve to control flow based on water temperature (option)
DESUPERHEATING WATER HEATERS – NON-TANK
• Water
• Cold water plumbed to storage tank
• Cold water pumped trough desuperheater and back to tank
• Warm water plumbed
• Facility
• Water heater
• Recirculation return to mid-port of tank
• Controls
• Aquastat to control pump or three-way valve
• Pressure switch optional
DESUPERHEATING WATER HEATERS – NON-TANK
DESUPERHEATING WATER HEATERS – NON-TANK
DESUPERHEATING WATER HEATERS – TANK STYLE
• Controls
• A 3-way valve is installed on the hot gas line, controlled by an aquastat
• When the water temperature is satisfied, the 3-way heat reclaim valve diverts the hot gas directly to the condenser
• Pump control
DESUPERHEATING WATER HEATERS – NON-TANK
DESUPERHEATING WATER HEATERS – NON-TANK
DESIGN CONSIDERATIONS
SCALING POTENTIAL
SCALING POTENTIAL
• Scaling is a potential problem in desuperheaters when the secondary fluid is water, because the solubility of limestone (CaCO3) decreases with increasing temperature. The maximum water temperature should ideally not exceed 65-70°C (149-158 F) to avoid scaling problems. If the risk of scaling is increased by the use of hard water, etc., the use of co-current flow should be considered to reduce the risk of excessively high water temperatures.
SCALING POTENTIAL
SCALING AND EFFICIENCY
• Consequences of scaling/liming
• 1/16” scale requires 15% more fuel
• 1/8” scale requires 20% more fuel
• ¼” scale requires 39% more fuel
• 3/8” scale requires 55% more fuel
• ½” scale requires 70% more fuel
• Source: US Bureau of Statistics
VERIFYING PERFORMANCE
BTU METERS
DATA LOGGING
• Hot water use (gallons)
DATA LOGGING
• Hot water temperature and demand
DATA LOGGING
• Water heating system temperatures
DATA LOGGING
• System temperatures with flow
DATA LOGGING
• Gas water heater flue temperatures
DATA LOGGING
QUESTIONS?
THANK YOU!
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