Chapter 1

Units and Unit Systems

UUS 1.1 Refrigeration engineers often use a parameter known as the horsepower per ton (HPT) to evaluate the performance of a refrigeration system. The HPT is defined by,

HPT = (horsepower input to the system)/(heat transfer rate at the evaporator in tons of refrigeration)

Consider a refrigeration system operating with a power input of 12.1 kW while delivering 120,000 Btu/h of cooling capacity at the evaporator. Determine the HPT for this refrigeration system.

ANSWER: HPT = 1.623 hp/ton

UUS 1.2 On a cold winter day, two 1500 W electric baseboard heaters run 10.5 hours to maintain a comfortable temperature of 72 F in a room. The heaters are connected to the off-peak grid of the local power utility. The off-peak rate for electricity is $0.034/kWh. Determine the,

(a) cost to operate these heaters for the day

(b) total energy used in the 10.5 hour period (in kJ and Btu)

ANSWER: $1.07 (a)

56,700 kJ (b)

53,741 Btu (c)