Ductless Heat Pumps Sizing Example

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# Ductless Heat Pumps Sizing Example - PowerPoint PPT Presentation

Ductless Heat Pumps Sizing Example. Gary Nordeen Bill Kingrey, P.E. 11/5/08 AFE Webinar. www.nwBuildings.org/dhp.aspx info@nwBuildings.org. Introduction. This presentation is intended to show how a ductless heat pump is sized for a particular space.

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### Ductless Heat PumpsSizing Example

Gary NordeenBill Kingrey, P.E.11/5/08 AFE Webinar

www.nwBuildings.org/dhp.aspx

info@nwBuildings.org

Introduction

This presentation is intended to show how a ductless heat pump is sized for a particular space.

The example is a maternity room inside a health facility, that needs supplemental cooling.

Heat Loss Calculations

Calculate the heat loss through walls, glass, ceiling/roof, floor and any other building components in the zone to be served, using ASHRAE procedures (chapter 26, Fundamentals)

Heat Gain Calculations
• Survey each zone and estimate heat gains from lights, equipment (computers, copiers, etc), people, building envelope and ventilation
• Calculate both sensible and latent (water vapor) loads
• Use ASHRAE Handbook of Fundamentals Chapter 26
Heat Gain Calculation Example

Assume we are going to provide cooling to a maternity room that is currently heated and ventilated by a central system with heating and ventilation air. We surveyed the room and found the following existing conditions.

Maternity Room Example
• 2 fluorescent lights at 32 watts each - ballast heat included
• 2 People @ 275 BTUH sensible, 105 BTUH latent (water vapor)
• Equipment = 1 computer = 65W
• Vent. Air = 40 CFM outdoor air at ambient temp in summer, 70 in winter
• Envelope (windows, walls and roof) calculated gain = 4,800 BTU
Maternity Room Example Sensible Cooling Loads
• Lights + Equipment + People =

2x32 + 65 + 2 x 275 = 679 watts

679w X 3.412 BTU / watt = 2317 BTUH Sensible

• 40 CFM Ventilation Air at 92dB, 74wB =

40CFM x 1.08 BTUH / CFM x ºF x (92ºF – 75ºF) =734 BTUH

• Total Sensible Cooling Load =

4,800 BTUH (envelope) + 2,317BTUH + 734 BTUH = 7,850 BTUH Sensible

To calculate latent loads, use a Psychometric Chart or software that will determine enthalpy and density of water/air mixtures

Psychometric Calculations
• Or you can use this free online program that will calculate enthalphy, density, relative humidity, etc.

http://www.gorhamschaffler.com/psy.htm

Assumptions:

• Ventilation air is cooled and dehumidified to 75°FDb (dry bulb) at 30%RH (relative humidity)
• Cooling design conditions at the site were 86Db and 68 Wb (wet bulb)
• People = 2 x 105 BTUH = 210 BTUH latent
• Ventilation Air = 40ft³/min at 86Db/68Wb
• 13.55 ft³ of air at that temp = 1 lb of air
• In one hour we will cool 40ft³/min x 60 min/hr/13.55 ft³/lb = 177lbs/hr of air
• Air at 86dB/68wB contains 32.21 BTU/lb (of air)
• Air at 75dB/30% RH = 24.05 BTU/lb
• It will require 32.21- 24.05 = 8.16 BTU for each pound of air cooled
• Total Ventilation Load = 177 lbs/hr x 8.16 BTU/lb

= 1,440 BTU/hr

• Latent Load = People + Ventilation Air

= 210 + 1440 =1,650 BTU/hr

(about 1.5 pints of water per hour)

Selecting Ductless Heat Pump
• Pick a unit that has a capacity greater than or equal to the total cooling load
• Mini-splits express heating and cooling capacity in kW. To convert to BTUH multiply the kW rating by 3412